/*
 * Copyright 2019 Google LLC
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "include/core/SkStream.h"
#include "include/core/SkString.h"
#include "include/private/SkChecksum.h"
#include "include/private/SkSpinlock.h"
#include "include/private/SkTFitsIn.h"
#include "include/private/SkThreadID.h"
#include "include/private/SkVx.h"
#include "src/core/SkCpu.h"
#include "src/core/SkOpts.h"
#include "src/core/SkVM.h"

bool gSkVMJITViaDylib{false};

// JIT code isn't MSAN-instrumented, so we won't see when it uses
// uninitialized memory, and we'll not see the writes it makes as properly
// initializing memory.  Instead force the interpreter, which should let
// MSAN see everything our programs do properly.
//
// Similarly, we can't get ASAN's checks unless we let it instrument our interpreter.
#if defined(__has_feature)
    #if __has_feature(memory_sanitizer) || __has_feature(address_sanitizer)
        #undef SKVM_JIT
    #endif
#endif

#if defined(SKVM_JIT)
    #include <dlfcn.h>      // dlopen, dlsym
    #include <sys/mman.h>   // mmap, mprotect
#endif

namespace skvm {

    // Debugging tools, mostly for printing various data structures out to a stream.

    namespace {
        class SkDebugfStream final : public SkWStream {
            size_t fBytesWritten = 0;

            bool write(const void* buffer, size_t size) override {
                SkDebugf("%.*s", size, buffer);
                fBytesWritten += size;
                return true;
            }

            size_t bytesWritten() const override {
                return fBytesWritten;
            }
        };

        struct V { Val id; };
        struct R { Reg id; };
        struct Shift { int bits; };
        struct Splat { int bits; };
        struct Hex   { int bits; };

        static void write(SkWStream* o, const char* s) {
            o->writeText(s);
        }

        static const char* name(Op op) {
            switch (op) {
            #define M(x) case Op::x: return #x;
                SKVM_OPS(M)
            #undef M
            }
            return "unknown op";
        }

        static void write(SkWStream* o, Op op) {
            const char* raw = name(op);
            if (const char* found = strstr(raw, "_imm")) {
                o->write(raw, found-raw);
            } else {
                o->writeText(raw);
            }
        }
        static void write(SkWStream* o, Arg a) {
            write(o, "arg(");
            o->writeDecAsText(a.ix);
            write(o, ")");
        }
        static void write(SkWStream* o, V v) {
            write(o, "v");
            o->writeDecAsText(v.id);
        }
        static void write(SkWStream* o, R r) {
            write(o, "r");
            o->writeDecAsText(r.id);
        }
        static void write(SkWStream* o, Shift s) {
            o->writeDecAsText(s.bits);
        }
        static void write(SkWStream* o, Splat s) {
            float f;
            memcpy(&f, &s.bits, 4);
            o->writeHexAsText(s.bits);
            write(o, " (");
            o->writeScalarAsText(f);
            write(o, ")");
        }
        static void write(SkWStream* o, Hex h) {
            o->writeHexAsText(h.bits);
        }

        template <typename T, typename... Ts>
        static void write(SkWStream* o, T first, Ts... rest) {
            write(o, first);
            write(o, " ");
            write(o, rest...);
        }
    }

    static void dump_builder_program(const std::vector<Builder::Instruction>& program,
                                     SkWStream* o) {
        for (Val id = 0; id < (Val)program.size(); id++) {
            const Builder::Instruction& inst = program[id];
            Op  op = inst.op;
            Val  x = inst.x,
                 y = inst.y,
                 z = inst.z;
            int immy = inst.immy,
                immz = inst.immz;
            write(o, !inst.can_hoist    ? "  " :
                      inst.used_in_loop ? "↑ " :
                                          "↟ ");
            switch (op) {
                case Op::assert_true: write(o, op, V{x}, V{y}); break;

                case Op::store8:  write(o, op, Arg{immy}, V{x}); break;
                case Op::store16: write(o, op, Arg{immy}, V{x}); break;
                case Op::store32: write(o, op, Arg{immy}, V{x}); break;

                case Op::index: write(o, V{id}, "=", op); break;

                case Op::load8:  write(o, V{id}, "=", op, Arg{immy}); break;
                case Op::load16: write(o, V{id}, "=", op, Arg{immy}); break;
                case Op::load32: write(o, V{id}, "=", op, Arg{immy}); break;

                case Op::gather8:  write(o, V{id}, "=", op, Arg{immy}, Hex{immz}, V{x}); break;
                case Op::gather16: write(o, V{id}, "=", op, Arg{immy}, Hex{immz}, V{x}); break;
                case Op::gather32: write(o, V{id}, "=", op, Arg{immy}, Hex{immz}, V{x}); break;

                case Op::uniform8:  write(o, V{id}, "=", op, Arg{immy}, Hex{immz}); break;
                case Op::uniform16: write(o, V{id}, "=", op, Arg{immy}, Hex{immz}); break;
                case Op::uniform32: write(o, V{id}, "=", op, Arg{immy}, Hex{immz}); break;

                case Op::splat:  write(o, V{id}, "=", op, Splat{immy}); break;


                case Op::add_f32: write(o, V{id}, "=", op, V{x}, V{y}      ); break;
                case Op::sub_f32: write(o, V{id}, "=", op, V{x}, V{y}      ); break;
                case Op::mul_f32: write(o, V{id}, "=", op, V{x}, V{y}      ); break;
                case Op::div_f32: write(o, V{id}, "=", op, V{x}, V{y}      ); break;
                case Op::min_f32: write(o, V{id}, "=", op, V{x}, V{y}      ); break;
                case Op::max_f32: write(o, V{id}, "=", op, V{x}, V{y}      ); break;
                case Op::mad_f32: write(o, V{id}, "=", op, V{x}, V{y}, V{z}); break;

                case Op::add_f32_imm: write(o, V{id}, "=", op, V{x}, Splat{immy}); break;
                case Op::sub_f32_imm: write(o, V{id}, "=", op, V{x}, Splat{immy}); break;
                case Op::mul_f32_imm: write(o, V{id}, "=", op, V{x}, Splat{immy}); break;
                case Op::min_f32_imm: write(o, V{id}, "=", op, V{x}, Splat{immy}); break;
                case Op::max_f32_imm: write(o, V{id}, "=", op, V{x}, Splat{immy}); break;

                case Op:: eq_f32: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op::neq_f32: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op:: gt_f32: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op::gte_f32: write(o, V{id}, "=", op, V{x}, V{y}); break;


                case Op::add_i32: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op::sub_i32: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op::mul_i32: write(o, V{id}, "=", op, V{x}, V{y}); break;

                case Op::shl_i32: write(o, V{id}, "=", op, V{x}, Shift{immy}); break;
                case Op::shr_i32: write(o, V{id}, "=", op, V{x}, Shift{immy}); break;
                case Op::sra_i32: write(o, V{id}, "=", op, V{x}, Shift{immy}); break;

                case Op:: eq_i32: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op::neq_i32: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op:: gt_i32: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op::gte_i32: write(o, V{id}, "=", op, V{x}, V{y}); break;

                case Op::add_i16x2: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op::sub_i16x2: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op::mul_i16x2: write(o, V{id}, "=", op, V{x}, V{y}); break;

                case Op::shl_i16x2: write(o, V{id}, "=", op, V{x}, Shift{immy}); break;
                case Op::shr_i16x2: write(o, V{id}, "=", op, V{x}, Shift{immy}); break;
                case Op::sra_i16x2: write(o, V{id}, "=", op, V{x}, Shift{immy}); break;

                case Op:: eq_i16x2: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op::neq_i16x2: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op:: gt_i16x2: write(o, V{id}, "=", op, V{x}, V{y}); break;
                case Op::gte_i16x2: write(o, V{id}, "=", op, V{x}, V{y}); break;

                case Op::bit_and  : write(o, V{id}, "=", op, V{x}, V{y}      ); break;
                case Op::bit_or   : write(o, V{id}, "=", op, V{x}, V{y}      ); break;
                case Op::bit_xor  : write(o, V{id}, "=", op, V{x}, V{y}      ); break;
                case Op::bit_clear: write(o, V{id}, "=", op, V{x}, V{y}      ); break;

                case Op::bit_and_imm: write(o, V{id}, "=", op, V{x}, Hex{immy}); break;
                case Op::bit_or_imm : write(o, V{id}, "=", op, V{x}, Hex{immy}); break;
                case Op::bit_xor_imm: write(o, V{id}, "=", op, V{x}, Hex{immy}); break;

                case Op::select:  write(o, V{id}, "=", op, V{x}, V{y}, V{z}); break;
                case Op::bytes:   write(o, V{id}, "=", op, V{x}, Hex{immy}); break;
                case Op::pack:    write(o, V{id}, "=", op, V{x}, V{y}, Shift{immz}); break;

                case Op::floor:  write(o, V{id}, "=", op, V{x}); break;
                case Op::to_f32: write(o, V{id}, "=", op, V{x}); break;
                case Op::trunc:  write(o, V{id}, "=", op, V{x}); break;
                case Op::round:  write(o, V{id}, "=", op, V{x}); break;
            }

            write(o, "\n");
        }
    }

    void Builder::dump(SkWStream* o) const {
        SkDebugfStream debug;
        if (!o) { o = &debug; }

        o->writeDecAsText(fProgram.size());
        o->writeText(" values:\n");
        dump_builder_program(fProgram, o);
    }

    void Program::dump(SkWStream* o) const {
        SkDebugfStream debug;
        if (!o) { o = &debug; }

        o->writeDecAsText(fRegs);
        o->writeText(" registers, ");
        o->writeDecAsText(fInstructions.size());
        o->writeText(" instructions:\n");
        for (int i = 0; i < (int)fInstructions.size(); i++) {
            if (i == fLoop) { write(o, "loop:\n"); }
            o->writeDecAsText(i);
            o->writeText("\t");
            if (i >= fLoop) { write(o, "    "); }
            const Program::Instruction& inst = fInstructions[i];
            Op   op = inst.op;
            Reg   d = inst.d,
                  x = inst.x,
                  y = inst.y,
                  z = inst.z;
            int immy = inst.immy,
                immz = inst.immz;
            switch (op) {
                case Op::assert_true: write(o, op, R{x}, R{y}); break;

                case Op::store8:  write(o, op, Arg{immy}, R{x}); break;
                case Op::store16: write(o, op, Arg{immy}, R{x}); break;
                case Op::store32: write(o, op, Arg{immy}, R{x}); break;

                case Op::index: write(o, R{d}, "=", op); break;

                case Op::load8:  write(o, R{d}, "=", op, Arg{immy}); break;
                case Op::load16: write(o, R{d}, "=", op, Arg{immy}); break;
                case Op::load32: write(o, R{d}, "=", op, Arg{immy}); break;

                case Op::gather8:  write(o, R{d}, "=", op, Arg{immy}, Hex{immz}, R{x}); break;
                case Op::gather16: write(o, R{d}, "=", op, Arg{immy}, Hex{immz}, R{x}); break;
                case Op::gather32: write(o, R{d}, "=", op, Arg{immy}, Hex{immz}, R{x}); break;

                case Op::uniform8:  write(o, R{d}, "=", op, Arg{immy}, Hex{immz}); break;
                case Op::uniform16: write(o, R{d}, "=", op, Arg{immy}, Hex{immz}); break;
                case Op::uniform32: write(o, R{d}, "=", op, Arg{immy}, Hex{immz}); break;

                case Op::splat:  write(o, R{d}, "=", op, Splat{immy}); break;


                case Op::add_f32: write(o, R{d}, "=", op, R{x}, R{y}      ); break;
                case Op::sub_f32: write(o, R{d}, "=", op, R{x}, R{y}      ); break;
                case Op::mul_f32: write(o, R{d}, "=", op, R{x}, R{y}      ); break;
                case Op::div_f32: write(o, R{d}, "=", op, R{x}, R{y}      ); break;
                case Op::min_f32: write(o, R{d}, "=", op, R{x}, R{y}      ); break;
                case Op::max_f32: write(o, R{d}, "=", op, R{x}, R{y}      ); break;
                case Op::mad_f32: write(o, R{d}, "=", op, R{x}, R{y}, R{z}); break;

                case Op::add_f32_imm: write(o, R{d}, "=", op, R{x}, Splat{immy}); break;
                case Op::sub_f32_imm: write(o, R{d}, "=", op, R{x}, Splat{immy}); break;
                case Op::mul_f32_imm: write(o, R{d}, "=", op, R{x}, Splat{immy}); break;
                case Op::min_f32_imm: write(o, R{d}, "=", op, R{x}, Splat{immy}); break;
                case Op::max_f32_imm: write(o, R{d}, "=", op, R{x}, Splat{immy}); break;

                case Op:: eq_f32: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op::neq_f32: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op:: gt_f32: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op::gte_f32: write(o, R{d}, "=", op, R{x}, R{y}); break;


                case Op::add_i32: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op::sub_i32: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op::mul_i32: write(o, R{d}, "=", op, R{x}, R{y}); break;

                case Op::shl_i32: write(o, R{d}, "=", op, R{x}, Shift{immy}); break;
                case Op::shr_i32: write(o, R{d}, "=", op, R{x}, Shift{immy}); break;
                case Op::sra_i32: write(o, R{d}, "=", op, R{x}, Shift{immy}); break;

                case Op:: eq_i32: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op::neq_i32: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op:: gt_i32: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op::gte_i32: write(o, R{d}, "=", op, R{x}, R{y}); break;


                case Op::add_i16x2: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op::sub_i16x2: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op::mul_i16x2: write(o, R{d}, "=", op, R{x}, R{y}); break;

                case Op::shl_i16x2: write(o, R{d}, "=", op, R{x}, Shift{immy}); break;
                case Op::shr_i16x2: write(o, R{d}, "=", op, R{x}, Shift{immy}); break;
                case Op::sra_i16x2: write(o, R{d}, "=", op, R{x}, Shift{immy}); break;

                case Op:: eq_i16x2: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op::neq_i16x2: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op:: gt_i16x2: write(o, R{d}, "=", op, R{x}, R{y}); break;
                case Op::gte_i16x2: write(o, R{d}, "=", op, R{x}, R{y}); break;


                case Op::bit_and  : write(o, R{d}, "=", op, R{x}, R{y}      ); break;
                case Op::bit_or   : write(o, R{d}, "=", op, R{x}, R{y}      ); break;
                case Op::bit_xor  : write(o, R{d}, "=", op, R{x}, R{y}      ); break;
                case Op::bit_clear: write(o, R{d}, "=", op, R{x}, R{y}      ); break;

                case Op::bit_and_imm: write(o, R{d}, "=", op, R{x}, Hex{immy}); break;
                case Op::bit_or_imm : write(o, R{d}, "=", op, R{x}, Hex{immy}); break;
                case Op::bit_xor_imm: write(o, R{d}, "=", op, R{x}, Hex{immy}); break;

                case Op::select:  write(o, R{d}, "=", op, R{x}, R{y}, R{z}); break;
                case Op::bytes:   write(o, R{d}, "=", op,  R{x}, Hex{immy}); break;
                case Op::pack:    write(o, R{d}, "=", op,   R{x}, R{y}, Shift{immz}); break;

                case Op::floor:  write(o, R{d}, "=", op,  R{x}); break;
                case Op::to_f32: write(o, R{d}, "=", op, R{x}); break;
                case Op::trunc:  write(o, R{d}, "=", op,  R{x}); break;
                case Op::round:  write(o, R{d}, "=", op,  R{x}); break;
            }
            write(o, "\n");
        }
    }

    // Builder -> Program, with liveness and loop hoisting analysis.

    Program Builder::done(const char* debug_name) {
        // First rewrite the program by issuing instructions as late as possible:
        //    - any side-effect-only (i.e. store) instruction in order as we see them;
        //    - any other instruction only once it's shown to be needed.
        // This elides all dead code and helps minimize value lifetime / register pressure.
        std::vector<Instruction> rewritten;
        rewritten.reserve(fProgram.size());
        std::vector<Val> new_index(fProgram.size(), NA);  // Map old Val index to rewritten index.

        auto rewrite = [&](Val id, auto& recurse) -> Val {
            auto rewrite_input = [&](Val input) -> Val {
                if (input == NA) {
                    return NA;
                }
                if (new_index[input] == NA) {
                    new_index[input] = recurse(input, recurse);
                }
                return new_index[input];
            };

            // The order we rewrite inputs is somewhat arbitrary; we could just go x,y,z.
            // But we try to preserve the original program order as much as possible by
            // rewriting inst's inputs in the order they were themselves originally issued.
            // This makes debugging program dumps a little easier.
            Instruction inst = fProgram[id];
            Val *min = &inst.x,
                *mid = &inst.y,
                *max = &inst.z;
            if (*min > *mid) { std::swap(min, mid); }
            if (*mid > *max) { std::swap(mid, max); }
            if (*min > *mid) { std::swap(min, mid); }
            *min = rewrite_input(*min);
            *mid = rewrite_input(*mid);
            *max = rewrite_input(*max);
            rewritten.push_back(inst);
            return (Val)rewritten.size()-1;
        };

        // Here we go with the actual rewriting, starting with all the store instructions
        // and letting rewrite() work back recursively through their inputs.
        for (Val id = 0; id < (Val)fProgram.size(); id++) {
            if (fProgram[id].op <= Op::store32) {
                rewrite(id, rewrite);
            }
        }
        // We're done with the original order now... everything below will analyze the new program.
        fProgram = std::move(rewritten);


        // We'll want to know when it's safe to recycle registers holding the values
        // produced by each instruction, that is, when no future instruction needs it.
        for (Val id = 0; id < (Val)fProgram.size(); id++) {
            Instruction& inst = fProgram[id];
            // Stores don't really produce values.  Just mark them as dying on issue.
            if (inst.op <= Op::store32) {
                inst.death = id;
            }
            // Extend the lifetime of this instruction's inputs to live until it issues.
            // (We're walking in order, so this is the same as max()ing.)
            if (inst.x != NA) { fProgram[inst.x].death = id; }
            if (inst.y != NA) { fProgram[inst.y].death = id; }
            if (inst.z != NA) { fProgram[inst.z].death = id; }
        }


        // Mark which values don't depend on the loop and can be hoisted.
        for (Val id = 0; id < (Val)fProgram.size(); id++) {
            Builder::Instruction& inst = fProgram[id];

            // Varying loads (and gathers) and stores cannot be hoisted out of the loop.
            if (inst.op <= Op::gather32 && inst.op != Op::assert_true) {
                inst.can_hoist = false;
            }

            // If any of an instruction's inputs can't be hoisted, it can't be hoisted itself.
            if (inst.can_hoist) {
                if (inst.x != NA) { inst.can_hoist &= fProgram[inst.x].can_hoist; }
                if (inst.y != NA) { inst.can_hoist &= fProgram[inst.y].can_hoist; }
                if (inst.z != NA) { inst.can_hoist &= fProgram[inst.z].can_hoist; }
            }

            // We'll want to know if hoisted values are used in the loop;
            // if not, we can recycle their registers like we do loop values.
            if (!inst.can_hoist /*i.e. we're in the loop, so the arguments are used_in_loop*/) {
                if (inst.x != NA) { fProgram[inst.x].used_in_loop = true; }
                if (inst.y != NA) { fProgram[inst.y].used_in_loop = true; }
                if (inst.z != NA) { fProgram[inst.z].used_in_loop = true; }
            }
        }

        char buf[64] = "skvm-jit-";
        if (!debug_name) {
            *SkStrAppendU32(buf+9, this->hash()) = '\0';
            debug_name = buf;
        }

        return {fProgram, fStrides, debug_name};
    }

    // We skip fields only written after Builder::done() (death, can_hoist, used_in_loop) here
    // for equality and hashing.  They'll always have the same default values in Builder::push().
    static bool operator==(const Builder::Instruction& a, const Builder::Instruction& b) {
        return a.op   == b.op
            && a.x    == b.x
            && a.y    == b.y
            && a.z    == b.z
            && a.immy == b.immy
            && a.immz == b.immz;
    }

    uint32_t Builder::InstructionHash::operator()(const Instruction& inst, uint32_t seed) const {
        return SkOpts::hash(&inst, offsetof(Instruction, death), seed);
    }

    uint64_t Builder::hash() const { return (uint64_t)fHashLo | (uint64_t)fHashHi << 32; }

    // Most instructions produce a value and return it by ID,
    // the value-producing instruction's own index in the program vector.
    Val Builder::push(Op op, Val x, Val y, Val z, int immy, int immz) {
        Instruction inst{op, x, y, z, immy, immz,
                         /*death=*/0, /*can_hoist=*/true, /*used_in_loop=*/false};

        // This first InstructionHash{}() call should be free given we're about to use fIndex below.
        fHashLo ^= InstructionHash{}(inst, 0);    // Two hash streams with different seeds.
        fHashHi ^= InstructionHash{}(inst, 1);
        fHashLo = SkChecksum::CheapMix(fHashLo);  // Mix to make sure instruction order matters.
        fHashHi = SkChecksum::CheapMix(fHashHi);

        // Basic common subexpression elimination:
        // if we've already seen this exact Instruction, use it instead of creating a new one.
        if (Val* id = fIndex.find(inst)) {
            return *id;
        }
        Val id = static_cast<Val>(fProgram.size());
        fProgram.push_back(inst);
        fIndex.set(inst, id);
        return id;
    }

    bool Builder::allImm() const { return true; }

    template <typename T, typename... Rest>
    bool Builder::allImm(Val id, T* imm, Rest... rest) const {
        if (fProgram[id].op == Op::splat) {
            static_assert(sizeof(T) == 4);
            memcpy(imm, &fProgram[id].immy, 4);
            return this->allImm(rest...);
        }
        return false;
    }

    Arg Builder::arg(int stride) {
        int ix = (int)fStrides.size();
        fStrides.push_back(stride);
        return {ix};
    }

    void Builder::assert_true(I32 cond, I32 debug) {
    #ifdef SK_DEBUG
        int imm;
        if (this->allImm(cond.id,&imm)) { SkASSERT(imm); return; }
        (void)this->push(Op::assert_true, cond.id,debug.id,NA);
    #endif
    }

    void Builder::store8 (Arg ptr, I32 val) { (void)this->push(Op::store8 , val.id,NA,NA, ptr.ix); }
    void Builder::store16(Arg ptr, I32 val) { (void)this->push(Op::store16, val.id,NA,NA, ptr.ix); }
    void Builder::store32(Arg ptr, I32 val) { (void)this->push(Op::store32, val.id,NA,NA, ptr.ix); }

    I32 Builder::index() { return {this->push(Op::index , NA,NA,NA,0) }; }

    I32 Builder::load8 (Arg ptr) { return {this->push(Op::load8 , NA,NA,NA, ptr.ix) }; }
    I32 Builder::load16(Arg ptr) { return {this->push(Op::load16, NA,NA,NA, ptr.ix) }; }
    I32 Builder::load32(Arg ptr) { return {this->push(Op::load32, NA,NA,NA, ptr.ix) }; }

    I32 Builder::gather8 (Arg ptr, int offset, I32 index) {
        return {this->push(Op::gather8 , index.id,NA,NA, ptr.ix,offset)};
    }
    I32 Builder::gather16(Arg ptr, int offset, I32 index) {
        return {this->push(Op::gather16, index.id,NA,NA, ptr.ix,offset)};
    }
    I32 Builder::gather32(Arg ptr, int offset, I32 index) {
        return {this->push(Op::gather32, index.id,NA,NA, ptr.ix,offset)};
    }

    I32 Builder::uniform8(Arg ptr, int offset) {
        return {this->push(Op::uniform8, NA,NA,NA, ptr.ix, offset)};
    }
    I32 Builder::uniform16(Arg ptr, int offset) {
        return {this->push(Op::uniform16, NA,NA,NA, ptr.ix, offset)};
    }
    I32 Builder::uniform32(Arg ptr, int offset) {
        return {this->push(Op::uniform32, NA,NA,NA, ptr.ix, offset)};
    }

    // The two splat() functions are just syntax sugar over splatting a 4-byte bit pattern.
    I32 Builder::splat(int   n) { return {this->push(Op::splat, NA,NA,NA, n) }; }
    F32 Builder::splat(float f) {
        int bits;
        memcpy(&bits, &f, 4);
        return {this->push(Op::splat, NA,NA,NA, bits)};
    }

    // Be careful peepholing float math!  Transformations you might expect to
    // be legal can fail in the face of NaN/Inf, e.g. 0*x is not always 0.
    // Float peepholes must pass this equivalence test for all ~4B floats:
    //
    //     bool equiv(float x, float y) { return (x == y) || (isnanf(x) && isnanf(y)); }
    //
    //     unsigned bits = 0;
    //     do {
    //        float f;
    //        memcpy(&f, &bits, 4);
    //        if (!equiv(f, ...)) {
    //           abort();
    //        }
    //     } while (++bits != 0);

    F32 Builder::add(F32 x, F32 y) {
        float X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X+Y); }
        if (this->isImm(y.id, 0.0f)) { return x; }   // x+0 == x
        if (this->isImm(x.id, 0.0f)) { return y; }   // 0+y == y
    #if defined(SK_CPU_X86)
        int imm;
        if (this->allImm(y.id, &imm)) { return {this->push(Op::add_f32_imm, x.id,NA,NA, imm)}; }
        if (this->allImm(x.id, &imm)) { return {this->push(Op::add_f32_imm, y.id,NA,NA, imm)}; }
    #endif
        return {this->push(Op::add_f32, x.id, y.id)};
    }

    F32 Builder::sub(F32 x, F32 y) {
        float X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X-Y); }
        if (this->isImm(y.id, 0.0f)) { return x; }   // x-0 == x
    #if defined(SK_CPU_X86)
        int imm;
        if (this->allImm(y.id, &imm)) { return {this->push(Op::sub_f32_imm, x.id,NA,NA, imm)}; }
    #endif
        return {this->push(Op::sub_f32, x.id, y.id)};
    }

    F32 Builder::mul(F32 x, F32 y) {
        float X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X*Y); }
        if (this->isImm(y.id, 1.0f)) { return x; }  // x*1 == x
        if (this->isImm(x.id, 1.0f)) { return y; }  // 1*y == y
    #if defined(SK_CPU_X86)
        int imm;
        if (this->allImm(y.id, &imm)) { return {this->push(Op::mul_f32_imm, x.id,NA,NA, imm)}; }
        if (this->allImm(x.id, &imm)) { return {this->push(Op::mul_f32_imm, y.id,NA,NA, imm)}; }
    #endif
        return {this->push(Op::mul_f32, x.id, y.id)};
    }

    F32 Builder::div(F32 x, F32 y) {
        float X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X/Y); }
        if (this->isImm(y.id, 1.0f)) { return x; }  // x/1 == x
        return {this->push(Op::div_f32, x.id, y.id)};
    }

    F32 Builder::mad(F32 x, F32 y, F32 z) {
        float X,Y,Z;
        if (this->allImm(x.id,&X, y.id,&Y, z.id,&Z)) { return this->splat(X*Y+Z); }
        if (this->isImm(y.id, 1.0f)) { return this->add(x,z); }  // x*1+z == x+z
        if (this->isImm(x.id, 1.0f)) { return this->add(y,z); }  // 1*y+z == y+z
        if (this->isImm(z.id, 0.0f)) { return this->mul(x,y); }  // x*y+0 == x*y
        return {this->push(Op::mad_f32, x.id, y.id, z.id)};
    }

    F32 Builder::min(F32 x, F32 y) {
        float X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(std::min(X,Y)); }
    #if defined(SK_CPU_X86)
        int imm;
        if (this->allImm(y.id, &imm)) { return {this->push(Op::min_f32_imm, x.id,NA,NA, imm)}; }
        if (this->allImm(x.id, &imm)) { return {this->push(Op::min_f32_imm, y.id,NA,NA, imm)}; }
    #endif
        return {this->push(Op::min_f32, x.id, y.id)};
    }
    F32 Builder::max(F32 x, F32 y) {
        float X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(std::max(X,Y)); }
    #if defined(SK_CPU_X86)
        int imm;
        if (this->allImm(y.id, &imm)) { return {this->push(Op::max_f32_imm, x.id,NA,NA, imm)}; }
        if (this->allImm(x.id, &imm)) { return {this->push(Op::max_f32_imm, y.id,NA,NA, imm)}; }
    #endif
        return {this->push(Op::max_f32, x.id, y.id)};
    }

    I32 Builder::add(I32 x, I32 y) { return {this->push(Op::add_i32, x.id, y.id)}; }
    I32 Builder::sub(I32 x, I32 y) { return {this->push(Op::sub_i32, x.id, y.id)}; }
    I32 Builder::mul(I32 x, I32 y) { return {this->push(Op::mul_i32, x.id, y.id)}; }

    I32 Builder::add_16x2(I32 x, I32 y) { return {this->push(Op::add_i16x2, x.id, y.id)}; }
    I32 Builder::sub_16x2(I32 x, I32 y) { return {this->push(Op::sub_i16x2, x.id, y.id)}; }
    I32 Builder::mul_16x2(I32 x, I32 y) { return {this->push(Op::mul_i16x2, x.id, y.id)}; }

    I32 Builder::shl(I32 x, int bits) {
        if (bits == 0) { return x; }
        int X;
        if (this->allImm(x.id,&X)) { return this->splat(X << bits); }
        return {this->push(Op::shl_i32, x.id,NA,NA, bits)};
    }
    I32 Builder::shr(I32 x, int bits) {
        if (bits == 0) { return x; }
        int X;
        if (this->allImm(x.id,&X)) { return this->splat(unsigned(X) >> bits); }
        return {this->push(Op::shr_i32, x.id,NA,NA, bits)};
    }
    I32 Builder::sra(I32 x, int bits) {
        if (bits == 0) { return x; }
        int X;
        if (this->allImm(x.id,&X)) { return this->splat(X >> bits); }
        return {this->push(Op::sra_i32, x.id,NA,NA, bits)};
    }

    I32 Builder::shl_16x2(I32 x, int bits) { return {this->push(Op::shl_i16x2, x.id,NA,NA, bits)}; }
    I32 Builder::shr_16x2(I32 x, int bits) { return {this->push(Op::shr_i16x2, x.id,NA,NA, bits)}; }
    I32 Builder::sra_16x2(I32 x, int bits) { return {this->push(Op::sra_i16x2, x.id,NA,NA, bits)}; }

    I32 Builder:: eq(F32 x, F32 y) {
        float X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X==Y ? ~0 : 0); }
        return {this->push(Op::eq_f32, x.id, y.id)};
    }
    I32 Builder::neq(F32 x, F32 y) {
        float X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X!=Y ? ~0 : 0); }
        return {this->push(Op::neq_f32, x.id, y.id)};
    }
    I32 Builder::lt(F32 x, F32 y) {
        float X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(Y> X ? ~0 : 0); }
        return {this->push(Op::gt_f32, y.id, x.id)};
    }
    I32 Builder::lte(F32 x, F32 y) {
        float X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(Y>=X ? ~0 : 0); }
        return {this->push(Op::gte_f32, y.id, x.id)};
    }
    I32 Builder::gt(F32 x, F32 y) {
        float X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X> Y ? ~0 : 0); }
        return {this->push(Op::gt_f32, x.id, y.id)};
    }
    I32 Builder::gte(F32 x, F32 y) {
        float X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X>=Y ? ~0 : 0); }
        return {this->push(Op::gte_f32, x.id, y.id)};
    }

    I32 Builder:: eq(I32 x, I32 y) { return {this->push(Op:: eq_i32, x.id, y.id)}; }
    I32 Builder::neq(I32 x, I32 y) { return {this->push(Op::neq_i32, x.id, y.id)}; }
    I32 Builder:: lt(I32 x, I32 y) { return {this->push(Op:: gt_i32, y.id, x.id)}; }
    I32 Builder::lte(I32 x, I32 y) { return {this->push(Op::gte_i32, y.id, x.id)}; }
    I32 Builder:: gt(I32 x, I32 y) { return {this->push(Op:: gt_i32, x.id, y.id)}; }
    I32 Builder::gte(I32 x, I32 y) { return {this->push(Op::gte_i32, x.id, y.id)}; }

    I32 Builder:: eq_16x2(I32 x, I32 y) { return {this->push(Op:: eq_i16x2, x.id, y.id)}; }
    I32 Builder::neq_16x2(I32 x, I32 y) { return {this->push(Op::neq_i16x2, x.id, y.id)}; }
    I32 Builder:: lt_16x2(I32 x, I32 y) { return {this->push(Op:: gt_i16x2, y.id, x.id)}; }
    I32 Builder::lte_16x2(I32 x, I32 y) { return {this->push(Op::gte_i16x2, y.id, x.id)}; }
    I32 Builder:: gt_16x2(I32 x, I32 y) { return {this->push(Op:: gt_i16x2, x.id, y.id)}; }
    I32 Builder::gte_16x2(I32 x, I32 y) { return {this->push(Op::gte_i16x2, x.id, y.id)}; }

    I32 Builder::bit_and(I32 x, I32 y) {
        int X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X&Y); }
    #if defined(SK_CPU_X86)
        int imm;
        if (this->allImm(y.id, &imm)) { return {this->push(Op::bit_and_imm, x.id,NA,NA, imm)}; }
        if (this->allImm(x.id, &imm)) { return {this->push(Op::bit_and_imm, y.id,NA,NA, imm)}; }
    #endif
        return {this->push(Op::bit_and, x.id, y.id)};
    }
    I32 Builder::bit_or(I32 x, I32 y) {
        int X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X|Y); }
    #if defined(SK_CPU_X86)
        int imm;
        if (this->allImm(y.id, &imm)) { return {this->push(Op::bit_or_imm, x.id,NA,NA, imm)}; }
        if (this->allImm(x.id, &imm)) { return {this->push(Op::bit_or_imm, y.id,NA,NA, imm)}; }
    #endif
        return {this->push(Op::bit_or, x.id, y.id)};
    }
    I32 Builder::bit_xor(I32 x, I32 y) {
        int X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X^Y); }
    #if defined(SK_CPU_X86)
        int imm;
        if (this->allImm(y.id, &imm)) { return {this->push(Op::bit_xor_imm, x.id,NA,NA, imm)}; }
        if (this->allImm(x.id, &imm)) { return {this->push(Op::bit_xor_imm, y.id,NA,NA, imm)}; }
    #endif
        return {this->push(Op::bit_xor, x.id, y.id)};
    }
    I32 Builder::bit_clear(I32 x, I32 y) {
        int X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X&~Y); }
    #if defined(SK_CPU_X86)
        int imm;
        if (this->allImm(y.id, &imm)) { return this->bit_and(x, this->splat(~imm)); }
    #endif
        return {this->push(Op::bit_clear, x.id, y.id)};
    }

    I32 Builder::select(I32 x, I32 y, I32 z) {
        int X,Y,Z;
        if (this->allImm(x.id,&X, y.id,&Y, z.id,&Z)) { return this->splat(X?Y:Z); }
        return {this->push(Op::select, x.id, y.id, z.id)};
    }

    I32 Builder::extract(I32 x, int bits, I32 z) {
        int Z;
        if (this->allImm(z.id,&Z) && (~0u>>bits) == (unsigned)Z) { return this->shr(x, bits); }
        return this->bit_and(z, this->shr(x, bits));
    }

    I32 Builder::pack(I32 x, I32 y, int bits) {
        int X,Y;
        if (this->allImm(x.id,&X, y.id,&Y)) { return this->splat(X|(Y<<bits)); }
        return {this->push(Op::pack, x.id,y.id,NA, 0,bits)};
    }

    I32 Builder::bytes(I32 x, int control) {
        return {this->push(Op::bytes, x.id,NA,NA, control)};
    }

    F32 Builder::floor(F32 x) {
        float X;
        if (this->allImm(x.id,&X)) { return this->splat(floorf(X)); }
        return {this->push(Op::floor, x.id)};
    }
    F32 Builder::to_f32(I32 x) {
        int X;
        if (this->allImm(x.id,&X)) { return this->splat((float)X); }
        return {this->push(Op::to_f32, x.id)};
    }
    I32 Builder::trunc(F32 x) {
        float X;
        if (this->allImm(x.id,&X)) { return this->splat((int)X); }
        return {this->push(Op::trunc, x.id)};
    }
    I32 Builder::round(F32 x) {
        float X;
        if (this->allImm(x.id,&X)) { return this->splat((int)lrintf(X)); }
        return {this->push(Op::round, x.id)};
    }

    F32 Builder::unorm(int bits, I32 x) {
        float limit = (1<<bits)-1.0f;
        return mul(to_f32(x), splat(1/limit));
    }
    I32 Builder::unorm(int bits, F32 x) {
        float limit = (1<<bits)-1.0f;
        return round(mul(x, splat(limit)));
    }

    Color Builder::unpack_8888(I32 rgba) {
        return {
            unorm(8, extract(rgba,  0, splat(0xff))),
            unorm(8, extract(rgba,  8, splat(0xff))),
            unorm(8, extract(rgba, 16, splat(0xff))),
            unorm(8, extract(rgba, 24, splat(0xff))),
        };
    }
    Color Builder::unpack_565(I32 bgr) {
        return {
            unorm(5, extract(bgr, 11, splat(0b011'111))),
            unorm(6, extract(bgr,  5, splat(0b111'111))),
            unorm(5, extract(bgr,  0, splat(0b011'111))),
            splat(1.0f),
        };
    }

    void Builder::unpremul(F32* r, F32* g, F32* b, F32 a) {
        skvm::F32 invA = div(splat(1.0f), a),
                  inf  = bit_cast(splat(0x7f800000));
        // If a is 0, so are *r,*g,*b, so set invA to 0 to avoid 0*inf=NaN (instead 0*0 = 0).
        invA = bit_cast(bit_and(lt(invA, inf),
                                bit_cast(invA)));
        *r = mul(*r, invA);
        *g = mul(*g, invA);
        *b = mul(*b, invA);
    }

    void Builder::premul(F32* r, F32* g, F32* b, F32 a) {
        *r = mul(*r, a);
        *g = mul(*g, a);
        *b = mul(*b, a);
    }

    Color Builder::lerp(Color lo, Color hi, F32 t) {
        return {
            lerp(lo.r, hi.r, t),
            lerp(lo.g, hi.g, t),
            lerp(lo.b, hi.b, t),
            lerp(lo.a, hi.a, t),
        };
    }

    // ~~~~ Program::eval() and co. ~~~~ //

    // Handy references for x86-64 instruction encoding:
    // https://wiki.osdev.org/X86-64_Instruction_Encoding
    // https://www-user.tu-chemnitz.de/~heha/viewchm.php/hs/x86.chm/x64.htm
    // https://www-user.tu-chemnitz.de/~heha/viewchm.php/hs/x86.chm/x86.htm
    // http://ref.x86asm.net/coder64.html

    // Used for ModRM / immediate instruction encoding.
    static uint8_t _233(int a, int b, int c) {
        return (a & 3) << 6
             | (b & 7) << 3
             | (c & 7) << 0;
    }

    // ModRM byte encodes the arguments of an opcode.
    enum class Mod { Indirect, OneByteImm, FourByteImm, Direct };
    static uint8_t mod_rm(Mod mod, int reg, int rm) {
        return _233((int)mod, reg, rm);
    }

    static Mod mod(int imm) {
        if (imm == 0)               { return Mod::Indirect; }
        if (SkTFitsIn<int8_t>(imm)) { return Mod::OneByteImm; }
        return Mod::FourByteImm;
    }

    static int imm_bytes(Mod mod) {
        switch (mod) {
            case Mod::Indirect:    return 0;
            case Mod::OneByteImm:  return 1;
            case Mod::FourByteImm: return 4;
            case Mod::Direct: SkUNREACHABLE;
        }
        SkUNREACHABLE;
    }

    // SIB byte encodes a memory address, base + (index * scale).
    static uint8_t sib(Assembler::Scale scale, int index, int base) {
        return _233((int)scale, index, base);
    }

    // The REX prefix is used to extend most old 32-bit instructions to 64-bit.
    static uint8_t rex(bool W,   // If set, operation is 64-bit, otherwise default, usually 32-bit.
                       bool R,   // Extra top bit to select ModRM reg, registers 8-15.
                       bool X,   // Extra top bit for SIB index register.
                       bool B) { // Extra top bit for SIB base or ModRM rm register.
        return 0b01000000   // Fixed 0100 for top four bits.
             | (W << 3)
             | (R << 2)
             | (X << 1)
             | (B << 0);
    }


    // The VEX prefix extends SSE operations to AVX.  Used generally, even with XMM.
    struct VEX {
        int     len;
        uint8_t bytes[3];
    };

    static VEX vex(bool  WE,   // Like REX W for int operations, or opcode extension for float?
                   bool   R,   // Same as REX R.  Pass high bit of dst register, dst>>3.
                   bool   X,   // Same as REX X.
                   bool   B,   // Same as REX B.  Pass y>>3 for 3-arg ops, x>>3 for 2-arg.
                   int  map,   // SSE opcode map selector: 0x0f, 0x380f, 0x3a0f.
                   int vvvv,   // 4-bit second operand register.  Pass our x for 3-arg ops.
                   bool   L,   // Set for 256-bit ymm operations, off for 128-bit xmm.
                   int   pp) { // SSE mandatory prefix: 0x66, 0xf3, 0xf2, else none.

        // Pack x86 opcode map selector to 5-bit VEX encoding.
        map = [map]{
            switch (map) {
                case   0x0f: return 0b00001;
                case 0x380f: return 0b00010;
                case 0x3a0f: return 0b00011;
                // Several more cases only used by XOP / TBM.
            }
            SkUNREACHABLE;
        }();

        // Pack  mandatory SSE opcode prefix byte to 2-bit VEX encoding.
        pp = [pp]{
            switch (pp) {
                case 0x66: return 0b01;
                case 0xf3: return 0b10;
                case 0xf2: return 0b11;
            }
            return 0b00;
        }();

        VEX vex = {0, {0,0,0}};
        if (X == 0 && B == 0 && WE == 0 && map == 0b00001) {
            // With these conditions met, we can optionally compress VEX to 2-byte.
            vex.len = 2;
            vex.bytes[0] = 0xc5;
            vex.bytes[1] = (pp      &  3) << 0
                         | (L       &  1) << 2
                         | (~vvvv   & 15) << 3
                         | (~(int)R &  1) << 7;
        } else {
            // We could use this 3-byte VEX prefix all the time if we like.
            vex.len = 3;
            vex.bytes[0] = 0xc4;
            vex.bytes[1] = (map     & 31) << 0
                         | (~(int)B &  1) << 5
                         | (~(int)X &  1) << 6
                         | (~(int)R &  1) << 7;
            vex.bytes[2] = (pp    &  3) << 0
                         | (L     &  1) << 2
                         | (~vvvv & 15) << 3
                         | (WE    &  1) << 7;
        }
        return vex;
    }

    Assembler::Assembler(void* buf) : fCode((uint8_t*)buf), fCurr(fCode), fSize(0) {}

    size_t Assembler::size() const { return fSize; }

    void Assembler::bytes(const void* p, int n) {
        if (fCurr) {
            memcpy(fCurr, p, n);
            fCurr += n;
        }
        fSize += n;
    }

    void Assembler::byte(uint8_t b) { this->bytes(&b, 1); }
    void Assembler::word(uint32_t w) { this->bytes(&w, 4); }

    void Assembler::align(int mod) {
        while (this->size() % mod) {
            this->byte(0x00);
        }
    }

    void Assembler::int3() {
        this->byte(0xcc);
    }

    void Assembler::vzeroupper() {
        this->byte(0xc5);
        this->byte(0xf8);
        this->byte(0x77);
    }
    void Assembler::ret() { this->byte(0xc3); }

    // Common instruction building for 64-bit opcodes with an immediate argument.
    void Assembler::op(int opcode, int opcode_ext, GP64 dst, int imm) {
        opcode |= 0b0000'0001;   // low bit set for 64-bit operands
        opcode |= 0b1000'0000;   // top bit set for instructions with any immediate

        int imm_bytes = 4;
        if (SkTFitsIn<int8_t>(imm)) {
            imm_bytes = 1;
            opcode |= 0b0000'0010;  // second bit set for 8-bit immediate, else 32-bit.
        }

        this->byte(rex(1,0,0,dst>>3));
        this->byte(opcode);
        this->byte(mod_rm(Mod::Direct, opcode_ext, dst&7));
        this->bytes(&imm, imm_bytes);
    }

    void Assembler::add(GP64 dst, int imm) { this->op(0,0b000, dst,imm); }
    void Assembler::sub(GP64 dst, int imm) { this->op(0,0b101, dst,imm); }
    void Assembler::cmp(GP64 reg, int imm) { this->op(0,0b111, reg,imm); }

    void Assembler::movq(GP64 dst, GP64 src, int off) {
        this->byte(rex(1,dst>>3,0,src>>3));
        this->byte(0x8b);
        this->byte(mod_rm(mod(off), dst&7, src&7));
        this->bytes(&off, imm_bytes(mod(off)));
    }

    void Assembler::op(int prefix, int map, int opcode, Ymm dst, Ymm x, Ymm y, bool W/*=false*/) {
        VEX v = vex(W, dst>>3, 0, y>>3,
                    map, x, 1/*ymm, not xmm*/, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Direct, dst&7, y&7));
    }

    void Assembler::vpaddd (Ymm dst, Ymm x, YmmOrLabel y) { this->op(0x66,  0x0f,0xfe, dst,x,y); }
    void Assembler::vpsubd (Ymm dst, Ymm x, YmmOrLabel y) { this->op(0x66,  0x0f,0xfa, dst,x,y); }
    void Assembler::vpmulld(Ymm dst, Ymm x, Ymm        y) { this->op(0x66,0x380f,0x40, dst,x,y); }

    void Assembler::vpsubw (Ymm dst, Ymm x, Ymm y) { this->op(0x66,0x0f,0xf9, dst,x,y); }
    void Assembler::vpmullw(Ymm dst, Ymm x, Ymm y) { this->op(0x66,0x0f,0xd5, dst,x,y); }

    void Assembler::vpand (Ymm dst, Ymm x, YmmOrLabel y) { this->op(0x66,0x0f,0xdb, dst,x,y); }
    void Assembler::vpor  (Ymm dst, Ymm x, YmmOrLabel y) { this->op(0x66,0x0f,0xeb, dst,x,y); }
    void Assembler::vpxor (Ymm dst, Ymm x, YmmOrLabel y) { this->op(0x66,0x0f,0xef, dst,x,y); }
    void Assembler::vpandn(Ymm dst, Ymm x, Ymm        y) { this->op(0x66,0x0f,0xdf, dst,x,y); }

    void Assembler::vaddps(Ymm dst, Ymm x, YmmOrLabel y) { this->op(0,0x0f,0x58, dst,x,y); }
    void Assembler::vsubps(Ymm dst, Ymm x, YmmOrLabel y) { this->op(0,0x0f,0x5c, dst,x,y); }
    void Assembler::vmulps(Ymm dst, Ymm x, YmmOrLabel y) { this->op(0,0x0f,0x59, dst,x,y); }
    void Assembler::vdivps(Ymm dst, Ymm x, Ymm        y) { this->op(0,0x0f,0x5e, dst,x,y); }
    void Assembler::vminps(Ymm dst, Ymm x, YmmOrLabel y) { this->op(0,0x0f,0x5d, dst,x,y); }
    void Assembler::vmaxps(Ymm dst, Ymm x, YmmOrLabel y) { this->op(0,0x0f,0x5f, dst,x,y); }

    void Assembler::vfmadd132ps(Ymm dst, Ymm x, Ymm y) { this->op(0x66,0x380f,0x98, dst,x,y); }
    void Assembler::vfmadd213ps(Ymm dst, Ymm x, Ymm y) { this->op(0x66,0x380f,0xa8, dst,x,y); }
    void Assembler::vfmadd231ps(Ymm dst, Ymm x, Ymm y) { this->op(0x66,0x380f,0xb8, dst,x,y); }

    void Assembler::vpackusdw(Ymm dst, Ymm x, Ymm y) { this->op(0x66,0x380f,0x2b, dst,x,y); }
    void Assembler::vpackuswb(Ymm dst, Ymm x, Ymm y) { this->op(0x66,  0x0f,0x67, dst,x,y); }

    void Assembler::vpcmpeqd(Ymm dst, Ymm x, Ymm y) { this->op(0x66,0x0f,0x76, dst,x,y); }
    void Assembler::vpcmpgtd(Ymm dst, Ymm x, Ymm y) { this->op(0x66,0x0f,0x66, dst,x,y); }

    void Assembler::vcmpps(Ymm dst, Ymm x, Ymm y, int imm) {
        this->op(0,0x0f,0xc2, dst,x,y);
        this->byte(imm);
    }

    void Assembler::vpblendvb(Ymm dst, Ymm x, Ymm y, Ymm z) {
        int prefix = 0x66,
            map    = 0x3a0f,
            opcode = 0x4c;
        VEX v = vex(0, dst>>3, 0, y>>3,
                    map, x, /*ymm?*/1, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Direct, dst&7, y&7));
        this->byte(z << 4);
    }

    // dst = x op /opcode_ext imm
    void Assembler::op(int prefix, int map, int opcode, int opcode_ext, Ymm dst, Ymm x, int imm) {
        // This is a little weird, but if we pass the opcode_ext as if it were the dst register,
        // the dst register as if x, and the x register as if y, all the bits end up where we want.
        this->op(prefix, map, opcode, (Ymm)opcode_ext,dst,x);
        this->byte(imm);
    }

    void Assembler::vpslld(Ymm dst, Ymm x, int imm) { this->op(0x66,0x0f,0x72,6, dst,x,imm); }
    void Assembler::vpsrld(Ymm dst, Ymm x, int imm) { this->op(0x66,0x0f,0x72,2, dst,x,imm); }
    void Assembler::vpsrad(Ymm dst, Ymm x, int imm) { this->op(0x66,0x0f,0x72,4, dst,x,imm); }

    void Assembler::vpsrlw(Ymm dst, Ymm x, int imm) { this->op(0x66,0x0f,0x71,2, dst,x,imm); }


    void Assembler::vpermq(Ymm dst, Ymm x, int imm) {
        // A bit unusual among the instructions we use, this is 64-bit operation, so we set W.
        bool W = true;
        this->op(0x66,0x3a0f,0x00, dst,x,W);
        this->byte(imm);
    }

    void Assembler::vroundps(Ymm dst, Ymm x, int imm) {
        this->op(0x66,0x3a0f,0x08, dst,x);
        this->byte(imm);
    }

    void Assembler::vmovdqa(Ymm dst, Ymm src) { this->op(0x66,0x0f,0x6f, dst,src); }

    void Assembler::vcvtdq2ps (Ymm dst, Ymm x) { this->op(0,   0x0f,0x5b, dst,x); }
    void Assembler::vcvttps2dq(Ymm dst, Ymm x) { this->op(0xf3,0x0f,0x5b, dst,x); }
    void Assembler::vcvtps2dq (Ymm dst, Ymm x) { this->op(0x66,0x0f,0x5b, dst,x); }

    Assembler::Label Assembler::here() {
        return { (int)this->size(), Label::NotYetSet, {} };
    }

    int Assembler::disp19(Label* l) {
        SkASSERT(l->kind == Label::NotYetSet ||
                 l->kind == Label::ARMDisp19);
        l->kind = Label::ARMDisp19;
        l->references.push_back(here().offset);
        // ARM 19-bit instruction count, from the beginning of this instruction.
        return (l->offset - here().offset) / 4;
    }

    int Assembler::disp32(Label* l) {
        SkASSERT(l->kind == Label::NotYetSet ||
                 l->kind == Label::X86Disp32);
        l->kind = Label::X86Disp32;
        l->references.push_back(here().offset);
        // x86 32-bit byte count, from the end of this instruction.
        return l->offset - (here().offset + 4);
    }

    void Assembler::op(int prefix, int map, int opcode, Ymm dst, Ymm x, Label* l) {
        // IP-relative addressing uses Mod::Indirect with the R/M encoded as-if rbp or r13.
        const int rip = rbp;

        VEX v = vex(0, dst>>3, 0, rip>>3,
                    map, x, /*ymm?*/1, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Indirect, dst&7, rip&7));
        this->word(this->disp32(l));
    }

    void Assembler::op(int prefix, int map, int opcode, Ymm dst, Ymm x, YmmOrLabel y) {
        y.label ? this->op(prefix,map,opcode,dst,x, y.label)
                : this->op(prefix,map,opcode,dst,x, y.ymm  );
    }

    void Assembler::vpshufb(Ymm dst, Ymm x, Label* l) { this->op(0x66,0x380f,0x00, dst,x,l); }
    void Assembler::vptest(Ymm dst, Label* l) { this->op(0x66, 0x380f, 0x17, dst, (Ymm)0, l); }

    void Assembler::vbroadcastss(Ymm dst, Label* l) { this->op(0x66,0x380f,0x18, dst, (Ymm)0, l); }
    void Assembler::vbroadcastss(Ymm dst, Xmm src)  { this->op(0x66,0x380f,0x18, dst, (Ymm)src); }
    void Assembler::vbroadcastss(Ymm dst, GP64 ptr, int off) {
        int prefix = 0x66,
               map = 0x380f,
            opcode = 0x18;
        VEX v = vex(0, dst>>3, 0, ptr>>3,
                    map, 0, /*ymm?*/1, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);

        this->byte(mod_rm(mod(off), dst&7, ptr&7));
        this->bytes(&off, imm_bytes(mod(off)));
    }

    void Assembler::jump(uint8_t condition, Label* l) {
        // These conditional jumps can be either 2 bytes (short) or 6 bytes (near):
        //    7?     one-byte-disp
        //    0F 8? four-byte-disp
        // We always use the near displacement to make updating labels simpler (no resizing).
        this->byte(0x0f);
        this->byte(condition);
        this->word(this->disp32(l));
    }
    void Assembler::je (Label* l) { this->jump(0x84, l); }
    void Assembler::jne(Label* l) { this->jump(0x85, l); }
    void Assembler::jl (Label* l) { this->jump(0x8c, l); }
    void Assembler::jc (Label* l) { this->jump(0x82, l); }

    void Assembler::jmp(Label* l) {
        // Like above in jump(), we could use 8-bit displacement here, but always use 32-bit.
        this->byte(0xe9);
        this->word(this->disp32(l));
    }

    void Assembler::load_store(int prefix, int map, int opcode, Ymm ymm, GP64 ptr) {
        VEX v = vex(0, ymm>>3, 0, ptr>>3,
                    map, 0, /*ymm?*/1, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Indirect, ymm&7, ptr&7));
    }

    void Assembler::vmovups  (Ymm dst, GP64 src) { this->load_store(0   ,  0x0f,0x10, dst,src); }
    void Assembler::vpmovzxwd(Ymm dst, GP64 src) { this->load_store(0x66,0x380f,0x33, dst,src); }
    void Assembler::vpmovzxbd(Ymm dst, GP64 src) { this->load_store(0x66,0x380f,0x31, dst,src); }

    void Assembler::vmovups  (GP64 dst, Ymm src) { this->load_store(0   ,  0x0f,0x11, src,dst); }
    void Assembler::vmovups  (GP64 dst, Xmm src) {
        // Same as vmovups(GP64,YMM) and load_store() except ymm? is 0.
        int prefix = 0,
            map    = 0x0f,
            opcode = 0x11;
        VEX v = vex(0, src>>3, 0, dst>>3,
                    map, 0, /*ymm?*/0, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Indirect, src&7, dst&7));
    }

    void Assembler::vmovq(GP64 dst, Xmm src) {
        int prefix = 0x66,
            map    = 0x0f,
            opcode = 0xd6;
        VEX v = vex(0, src>>3, 0, dst>>3,
                    map, 0, /*ymm?*/0, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Indirect, src&7, dst&7));
    }

    void Assembler::vmovd(GP64 dst, Xmm src) {
        int prefix = 0x66,
            map    = 0x0f,
            opcode = 0x7e;
        VEX v = vex(0, src>>3, 0, dst>>3,
                    map, 0, /*ymm?*/0, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Indirect, src&7, dst&7));
    }

    void Assembler::vmovd_direct(GP64 dst, Xmm src) {
        int prefix = 0x66,
            map    = 0x0f,
            opcode = 0x7e;
        VEX v = vex(0, src>>3, 0, dst>>3,
                    map, 0, /*ymm?*/0, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Direct, src&7, dst&7));
    }

    void Assembler::vmovd(Xmm dst, GP64 src) {
        int prefix = 0x66,
            map    = 0x0f,
            opcode = 0x6e;
        VEX v = vex(0, dst>>3, 0, src>>3,
                    map, 0, /*ymm?*/0, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Indirect, dst&7, src&7));
    }

    void Assembler::vmovd(Xmm dst, Scale scale, GP64 index, GP64 base) {
        int prefix = 0x66,
            map    = 0x0f,
            opcode = 0x6e;
        VEX v = vex(0, dst>>3, index>>3, base>>3,
                    map, 0, /*ymm?*/0, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Indirect, dst&7, rsp));
        this->byte(sib(scale, index&7, base&7));
    }

    void Assembler::vmovd_direct(Xmm dst, GP64 src) {
        int prefix = 0x66,
            map    = 0x0f,
            opcode = 0x6e;
        VEX v = vex(0, dst>>3, 0, src>>3,
                    map, 0, /*ymm?*/0, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Direct, dst&7, src&7));
    }

    void Assembler::movzbl(GP64 dst, GP64 src, int off) {
        if ((dst>>3) || (src>>3)) {
            this->byte(rex(0,dst>>3,0,src>>3));
        }
        this->byte(0x0f);
        this->byte(0xb6);
        this->byte(mod_rm(mod(off), dst&7, src&7));
        this->bytes(&off, imm_bytes(mod(off)));
    }


    void Assembler::movb(GP64 dst, GP64 src) {
        if ((dst>>3) || (src>>3)) {
            this->byte(rex(0,src>>3,0,dst>>3));
        }
        this->byte(0x88);
        this->byte(mod_rm(Mod::Indirect, src&7, dst&7));
    }

    void Assembler::vpinsrw(Xmm dst, Xmm src, GP64 ptr, int imm) {
        int prefix = 0x66,
            map    = 0x0f,
            opcode = 0xc4;
        VEX v = vex(0, dst>>3, 0, ptr>>3,
                    map, src, /*ymm?*/0, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Indirect, dst&7, ptr&7));
        this->byte(imm);
    }

    void Assembler::vpinsrb(Xmm dst, Xmm src, GP64 ptr, int imm) {
        int prefix = 0x66,
            map    = 0x3a0f,
            opcode = 0x20;
        VEX v = vex(0, dst>>3, 0, ptr>>3,
                    map, src, /*ymm?*/0, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Indirect, dst&7, ptr&7));
        this->byte(imm);
    }

    void Assembler::vpextrw(GP64 ptr, Xmm src, int imm) {
        int prefix = 0x66,
            map    = 0x3a0f,
            opcode = 0x15;

        VEX v = vex(0, src>>3, 0, ptr>>3,
                    map, 0, /*ymm?*/0, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Indirect, src&7, ptr&7));
        this->byte(imm);
    }
    void Assembler::vpextrb(GP64 ptr, Xmm src, int imm) {
        int prefix = 0x66,
            map    = 0x3a0f,
            opcode = 0x14;

        VEX v = vex(0, src>>3, 0, ptr>>3,
                    map, 0, /*ymm?*/0, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Indirect, src&7, ptr&7));
        this->byte(imm);
    }

    void Assembler::vgatherdps(Ymm dst, Scale scale, Ymm ix, GP64 base, Ymm mask) {
        // Unlike most instructions, no aliasing is permitted here.
        SkASSERT(dst != ix);
        SkASSERT(dst != mask);
        SkASSERT(mask != ix);

        int prefix = 0x66,
            map    = 0x380f,
            opcode = 0x92;
        VEX v = vex(0, dst>>3, ix>>3, base>>3,
                    map, mask, /*ymm?*/1, prefix);
        this->bytes(v.bytes, v.len);
        this->byte(opcode);
        this->byte(mod_rm(Mod::Indirect, dst&7, rsp));
        this->byte(sib(scale, ix&7, base&7));
    }

    // https://static.docs.arm.com/ddi0596/a/DDI_0596_ARM_a64_instruction_set_architecture.pdf

    static int operator"" _mask(unsigned long long bits) { return (1<<(int)bits)-1; }

    void Assembler::op(uint32_t hi, V m, uint32_t lo, V n, V d) {
        this->word( (hi & 11_mask) << 21
                  | (m  &  5_mask) << 16
                  | (lo &  6_mask) << 10
                  | (n  &  5_mask) <<  5
                  | (d  &  5_mask) <<  0);
    }

    void Assembler::and16b(V d, V n, V m) { this->op(0b0'1'0'01110'00'1, m, 0b00011'1, n, d); }
    void Assembler::orr16b(V d, V n, V m) { this->op(0b0'1'0'01110'10'1, m, 0b00011'1, n, d); }
    void Assembler::eor16b(V d, V n, V m) { this->op(0b0'1'1'01110'00'1, m, 0b00011'1, n, d); }
    void Assembler::bic16b(V d, V n, V m) { this->op(0b0'1'0'01110'01'1, m, 0b00011'1, n, d); }
    void Assembler::bsl16b(V d, V n, V m) { this->op(0b0'1'1'01110'01'1, m, 0b00011'1, n, d); }
    void Assembler::not16b(V d, V n)      { this->op(0b0'1'1'01110'00'10000'00101'10,  n, d); }

    void Assembler::add4s(V d, V n, V m) { this->op(0b0'1'0'01110'10'1, m, 0b10000'1, n, d); }
    void Assembler::sub4s(V d, V n, V m) { this->op(0b0'1'1'01110'10'1, m, 0b10000'1, n, d); }
    void Assembler::mul4s(V d, V n, V m) { this->op(0b0'1'0'01110'10'1, m, 0b10011'1, n, d); }

    void Assembler::cmeq4s(V d, V n, V m) { this->op(0b0'1'1'01110'10'1, m, 0b10001'1, n, d); }
    void Assembler::cmgt4s(V d, V n, V m) { this->op(0b0'1'0'01110'10'1, m, 0b0011'0'1, n, d); }

    void Assembler::sub8h(V d, V n, V m) { this->op(0b0'1'1'01110'01'1, m, 0b10000'1, n, d); }
    void Assembler::mul8h(V d, V n, V m) { this->op(0b0'1'0'01110'01'1, m, 0b10011'1, n, d); }

    void Assembler::fadd4s(V d, V n, V m) { this->op(0b0'1'0'01110'0'0'1, m, 0b11010'1, n, d); }
    void Assembler::fsub4s(V d, V n, V m) { this->op(0b0'1'0'01110'1'0'1, m, 0b11010'1, n, d); }
    void Assembler::fmul4s(V d, V n, V m) { this->op(0b0'1'1'01110'0'0'1, m, 0b11011'1, n, d); }
    void Assembler::fdiv4s(V d, V n, V m) { this->op(0b0'1'1'01110'0'0'1, m, 0b11111'1, n, d); }
    void Assembler::fmin4s(V d, V n, V m) { this->op(0b0'1'0'01110'1'0'1, m, 0b11110'1, n, d); }
    void Assembler::fmax4s(V d, V n, V m) { this->op(0b0'1'0'01110'0'0'1, m, 0b11110'1, n, d); }

    void Assembler::fcmeq4s(V d, V n, V m) { this->op(0b0'1'0'01110'0'0'1, m, 0b1110'0'1, n, d); }
    void Assembler::fcmgt4s(V d, V n, V m) { this->op(0b0'1'1'01110'1'0'1, m, 0b1110'0'1, n, d); }
    void Assembler::fcmge4s(V d, V n, V m) { this->op(0b0'1'1'01110'0'0'1, m, 0b1110'0'1, n, d); }

    void Assembler::fmla4s(V d, V n, V m) { this->op(0b0'1'0'01110'0'0'1, m, 0b11001'1, n, d); }

    void Assembler::tbl(V d, V n, V m) { this->op(0b0'1'001110'00'0, m, 0b0'00'0'00, n, d); }

    void Assembler::op(uint32_t op22, int imm, V n, V d) {
        this->word( (op22 & 22_mask) << 10
                  | imm              << 16   // imm is embedded inside op, bit size depends on op
                  | (n    &  5_mask) <<  5
                  | (d    &  5_mask) <<  0);
    }

    void Assembler::sli4s(V d, V n, int imm) {
        this->op(0b0'1'1'011110'0100'000'01010'1,    ( imm&31), n, d);
    }
    void Assembler::shl4s(V d, V n, int imm) {
        this->op(0b0'1'0'011110'0100'000'01010'1,    ( imm&31), n, d);
    }
    void Assembler::sshr4s(V d, V n, int imm) {
        this->op(0b0'1'0'011110'0100'000'00'0'0'0'1, (-imm&31), n, d);
    }
    void Assembler::ushr4s(V d, V n, int imm) {
        this->op(0b0'1'1'011110'0100'000'00'0'0'0'1, (-imm&31), n, d);
    }
    void Assembler::ushr8h(V d, V n, int imm) {
        this->op(0b0'1'1'011110'0010'000'00'0'0'0'1, (-imm&15), n, d);
    }

    void Assembler::scvtf4s (V d, V n) { this->op(0b0'1'0'01110'0'0'10000'11101'10, n,d); }
    void Assembler::fcvtzs4s(V d, V n) { this->op(0b0'1'0'01110'1'0'10000'1101'1'10, n,d); }
    void Assembler::fcvtns4s(V d, V n) { this->op(0b0'1'0'01110'0'0'10000'1101'0'10, n,d); }

    void Assembler::xtns2h(V d, V n) { this->op(0b0'0'0'01110'01'10000'10010'10, n,d); }
    void Assembler::xtnh2b(V d, V n) { this->op(0b0'0'0'01110'00'10000'10010'10, n,d); }

    void Assembler::uxtlb2h(V d, V n) { this->op(0b0'0'1'011110'0001'000'10100'1, n,d); }
    void Assembler::uxtlh2s(V d, V n) { this->op(0b0'0'1'011110'0010'000'10100'1, n,d); }

    void Assembler::uminv4s(V d, V n) { this->op(0b0'1'1'01110'10'11000'1'1010'10, n,d); }

    void Assembler::brk(int imm16) {
        this->word(0b11010100'001'0000000000000000'000'00
                  | (imm16 & 16_mask) << 5);
    }

    void Assembler::ret(X n) {
        this->word(0b1101011'0'0'10'11111'0000'0'0 << 10
                  | (n & 5_mask) << 5);
    }

    void Assembler::add(X d, X n, int imm12) {
        this->word(0b1'0'0'10001'00   << 22
                  | (imm12 & 12_mask) << 10
                  | (n     &  5_mask) <<  5
                  | (d     &  5_mask) <<  0);
    }
    void Assembler::sub(X d, X n, int imm12) {
        this->word( 0b1'1'0'10001'00  << 22
                  | (imm12 & 12_mask) << 10
                  | (n     &  5_mask) <<  5
                  | (d     &  5_mask) <<  0);
    }
    void Assembler::subs(X d, X n, int imm12) {
        this->word( 0b1'1'1'10001'00  << 22
                  | (imm12 & 12_mask) << 10
                  | (n     &  5_mask) <<  5
                  | (d     &  5_mask) <<  0);
    }

    void Assembler::b(Condition cond, Label* l) {
        const int imm19 = this->disp19(l);
        this->word( 0b0101010'0           << 24
                  | (imm19     & 19_mask) <<  5
                  | ((int)cond &  4_mask) <<  0);
    }
    void Assembler::cbz(X t, Label* l) {
        const int imm19 = this->disp19(l);
        this->word( 0b1'011010'0      << 24
                  | (imm19 & 19_mask) <<  5
                  | (t     &  5_mask) <<  0);
    }
    void Assembler::cbnz(X t, Label* l) {
        const int imm19 = this->disp19(l);
        this->word( 0b1'011010'1      << 24
                  | (imm19 & 19_mask) <<  5
                  | (t     &  5_mask) <<  0);
    }

    void Assembler::ldrq(V dst, X src) { this->op(0b00'111'1'01'11'000000000000, src, dst); }
    void Assembler::ldrs(V dst, X src) { this->op(0b10'111'1'01'01'000000000000, src, dst); }
    void Assembler::ldrb(V dst, X src) { this->op(0b00'111'1'01'01'000000000000, src, dst); }

    void Assembler::strq(V src, X dst) { this->op(0b00'111'1'01'10'000000000000, dst, src); }
    void Assembler::strs(V src, X dst) { this->op(0b10'111'1'01'00'000000000000, dst, src); }
    void Assembler::strb(V src, X dst) { this->op(0b00'111'1'01'00'000000000000, dst, src); }

    void Assembler::fmovs(X dst, V src) {
        this->word(0b0'0'0'11110'00'1'00'110'000000 << 10
                  | (src & 5_mask)                  << 5
                  | (dst & 5_mask)                  << 0);
    }

    void Assembler::ldrq(V dst, Label* l) {
        const int imm19 = this->disp19(l);
        this->word( 0b10'011'1'00     << 24
                  | (imm19 & 19_mask) << 5
                  | (dst   &  5_mask) << 0);
    }

    void Assembler::label(Label* l) {
        if (fCode) {
            // The instructions all currently point to l->offset.
            // We'll want to add a delta to point them to here().
            int delta = here().offset - l->offset;
            l->offset = here().offset;

            if (l->kind == Label::ARMDisp19) {
                for (int ref : l->references) {
                    // ref points to a 32-bit instruction with 19-bit displacement in instructions.
                    uint32_t inst;
                    memcpy(&inst, fCode + ref, 4);

                    // [ 8 bits to preserve] [ 19 bit signed displacement ] [ 5 bits to preserve ]
                    int disp = (int)(inst << 8) >> 13;

                    disp += delta/4;  // delta is in bytes, we want instructions.

                    // Put it all back together, preserving the high 8 bits and low 5.
                    inst = ((disp << 5) &  (19_mask << 5))
                         | ((inst     ) & ~(19_mask << 5));

                    memcpy(fCode + ref, &inst, 4);
                }
            }

            if (l->kind == Label::X86Disp32) {
                for (int ref : l->references) {
                    // ref points to a 32-bit displacement in bytes.
                    int disp;
                    memcpy(&disp, fCode + ref, 4);

                    disp += delta;

                    memcpy(fCode + ref, &disp, 4);
                }
            }
        }
    }

    void Program::eval(int n, void* args[]) const {
        if (const void* b = fJITEntry) {
            void** a = args;
            switch (fStrides.size()) {
                case 0: return ((void(*)(int                        ))b)(n                    );
                case 1: return ((void(*)(int,void*                  ))b)(n,a[0]               );
                case 2: return ((void(*)(int,void*,void*            ))b)(n,a[0],a[1]          );
                case 3: return ((void(*)(int,void*,void*,void*      ))b)(n,a[0],a[1],a[2]     );
                case 4: return ((void(*)(int,void*,void*,void*,void*))b)(n,a[0],a[1],a[2],a[3]);
                case 5: return ((void(*)(int,void*,void*,void*,void*,void*))b)
                                (n,a[0],a[1],a[2],a[3],a[4]);
                default: SkUNREACHABLE;  // TODO
            }
        }

        this->interpret(n, args);
    }

    void Program::interpret(int n, void* args[]) const {
        // We'll operate in SIMT style, knocking off K-size chunks from n while possible.
        constexpr int K = 16;
        using I32 = skvx::Vec<K, int>;
        using F32 = skvx::Vec<K, float>;
        using U32 = skvx::Vec<K, uint32_t>;
        using U16 = skvx::Vec<K, uint16_t>;
        using  U8 = skvx::Vec<K, uint8_t>;

        using I16x2 = skvx::Vec<2*K,  int16_t>;
        using U16x2 = skvx::Vec<2*K, uint16_t>;

        union Slot {
            F32   f32;
            I32   i32;
            U32   u32;
            I16x2 i16x2;
            U16x2 u16x2;
        };

        Slot                     few_regs[16];
        std::unique_ptr<char[]> many_regs;

        Slot* regs = few_regs;

        if (fRegs > (int)SK_ARRAY_COUNT(few_regs)) {
            // Annoyingly we can't trust that malloc() or new will work with Slot because
            // the skvx::Vec types may have alignment greater than what they provide.
            // We'll overallocate one extra register so we can align manually.
            many_regs.reset(new char[ sizeof(Slot) * (fRegs + 1) ]);

            uintptr_t addr = (uintptr_t)many_regs.get();
            addr += alignof(Slot) -
                     (addr & (alignof(Slot) - 1));
            SkASSERT((addr & (alignof(Slot) - 1)) == 0);
            regs = (Slot*)addr;
        }


        auto r = [&](Reg id) -> Slot& {
            SkASSERT(0 <= id && id < fRegs);
            return regs[id];
        };
        auto arg = [&](int ix) {
            SkASSERT(0 <= ix && ix < (int)fStrides.size());
            return args[ix];
        };

        // Step each argument pointer ahead by its stride a number of times.
        auto step_args = [&](int times) {
            for (int i = 0; i < (int)fStrides.size(); i++) {
                args[i] = (void*)( (char*)args[i] + times * fStrides[i] );
            }
        };

        int start = 0,
            stride;
        for ( ; n > 0; start = fLoop, n -= stride, step_args(stride)) {
            stride = n >= K ? K : 1;

            for (int i = start; i < (int)fInstructions.size(); i++) {
                Instruction inst = fInstructions[i];

                // d = op(x,y/imm,z/imm)
                Reg   d = inst.d,
                      x = inst.x,
                      y = inst.y,
                      z = inst.z;
                int immy = inst.immy,
                    immz = inst.immz;

                // Ops that interact with memory need to know whether we're stride=1 or K,
                // but all non-memory ops can run the same code no matter the stride.
                switch (2*(int)inst.op + (stride == K ? 1 : 0)) {
                    default: SkUNREACHABLE;

                #define STRIDE_1(op) case 2*(int)op
                #define STRIDE_K(op) case 2*(int)op + 1
                    STRIDE_1(Op::store8 ): memcpy(arg(immy), &r(x).i32, 1); break;
                    STRIDE_1(Op::store16): memcpy(arg(immy), &r(x).i32, 2); break;
                    STRIDE_1(Op::store32): memcpy(arg(immy), &r(x).i32, 4); break;

                    STRIDE_K(Op::store8 ): skvx::cast<uint8_t> (r(x).i32).store(arg(immy)); break;
                    STRIDE_K(Op::store16): skvx::cast<uint16_t>(r(x).i32).store(arg(immy)); break;
                    STRIDE_K(Op::store32):                     (r(x).i32).store(arg(immy)); break;

                    STRIDE_1(Op::load8 ): r(d).i32 = 0; memcpy(&r(d).i32, arg(immy), 1); break;
                    STRIDE_1(Op::load16): r(d).i32 = 0; memcpy(&r(d).i32, arg(immy), 2); break;
                    STRIDE_1(Op::load32): r(d).i32 = 0; memcpy(&r(d).i32, arg(immy), 4); break;

                    STRIDE_K(Op::load8 ): r(d).i32= skvx::cast<int>(U8 ::Load(arg(immy))); break;
                    STRIDE_K(Op::load16): r(d).i32= skvx::cast<int>(U16::Load(arg(immy))); break;
                    STRIDE_K(Op::load32): r(d).i32=                 I32::Load(arg(immy)) ; break;

                    // The pointer we base our gather on is loaded indirectly from a uniform:
                    //     - arg(immy) is the uniform holding our gather base pointer somewhere;
                    //     - (const uint8_t*)arg(immy) + immz points to the gather base pointer;
                    //     - *(const T**)foo loads the gather base and casts it to the right type.
                    // After all that we have an ordinary (uniform) pointer `ptr` to load from,
                    // and we then gather from it using the varying indices in r(x).
                    STRIDE_1(Op::gather8):
                        for (int i = 0; i < K; i++) {
                            auto ptr = *(const uint8_t**)((const uint8_t*)arg(immy) + immz);
                            r(d).i32[i] = (i==0) ? ptr[ r(x).i32[i] ] : 0;
                        } break;
                    STRIDE_1(Op::gather16):
                        for (int i = 0; i < K; i++) {
                            auto ptr = *(const uint16_t**)((const uint8_t*)arg(immy) + immz);
                            r(d).i32[i] = (i==0) ? ptr[ r(x).i32[i] ] : 0;
                        } break;
                    STRIDE_1(Op::gather32):
                        for (int i = 0; i < K; i++) {
                            auto ptr = *(const int**)((const uint8_t*)arg(immy) + immz);
                            r(d).i32[i] = (i==0) ? ptr[ r(x).i32[i] ] : 0;
                        } break;

                    STRIDE_K(Op::gather8):
                        for (int i = 0; i < K; i++) {
                            auto ptr = *(const uint8_t**)((const uint8_t*)arg(immy) + immz);
                            r(d).i32[i] = ptr[ r(x).i32[i] ];
                        } break;
                    STRIDE_K(Op::gather16):
                        for (int i = 0; i < K; i++) {
                            auto ptr = *(const uint16_t**)((const uint8_t*)arg(immy) + immz);
                            r(d).i32[i] = ptr[ r(x).i32[i] ];
                        } break;
                    STRIDE_K(Op::gather32):
                        for (int i = 0; i < K; i++) {
                            auto ptr = *(const int**)((const uint8_t*)arg(immy) + immz);
                            r(d).i32[i] = ptr[ r(x).i32[i] ];
                        } break;

                #undef STRIDE_1
                #undef STRIDE_K

                    // Ops that don't interact with memory should never care about the stride.
                #define CASE(op) case 2*(int)op: /*fallthrough*/ case 2*(int)op+1

                    CASE(Op::assert_true):
                    #ifdef SK_DEBUG
                        if (!all(r(x).i32)) {
                            this->dump();
                            SkDebugf("inst %d, register %d\n", i, y);
                            for (int i = 0; i < K; i++) {
                                SkDebugf("\t%2d: %08x (%g)\n", i, r(y).i32[i], r(y).f32[i]);
                            }
                        }
                        SkASSERT(all(r(x).i32));
                    #endif
                    break;

                    CASE(Op::index): static_assert(K == 16, "");
                                     r(d).i32 = n - I32{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
                                     break;

                    CASE(Op::uniform8):
                        r(d).i32 = *(const uint8_t* )( (const char*)arg(immy) + immz );
                        break;
                    CASE(Op::uniform16):
                        r(d).i32 = *(const uint16_t*)( (const char*)arg(immy) + immz );
                        break;
                    CASE(Op::uniform32):
                        r(d).i32 = *(const int*     )( (const char*)arg(immy) + immz );
                        break;

                    CASE(Op::splat): r(d).i32 = immy; break;

                    CASE(Op::add_f32): r(d).f32 = r(x).f32 + r(y).f32; break;
                    CASE(Op::sub_f32): r(d).f32 = r(x).f32 - r(y).f32; break;
                    CASE(Op::mul_f32): r(d).f32 = r(x).f32 * r(y).f32; break;
                    CASE(Op::div_f32): r(d).f32 = r(x).f32 / r(y).f32; break;
                    CASE(Op::min_f32): r(d).f32 = min(r(x).f32, r(y).f32); break;
                    CASE(Op::max_f32): r(d).f32 = max(r(x).f32, r(y).f32); break;

                    CASE(Op::add_f32_imm): {
                        Slot tmp;
                        tmp.i32 = immy;
                        r(d).f32 = r(x).f32 + tmp.f32;
                    } break;
                    CASE(Op::sub_f32_imm): {
                        Slot tmp;
                        tmp.i32 = immy;
                        r(d).f32 = r(x).f32 - tmp.f32;
                    } break;
                    CASE(Op::mul_f32_imm): {
                        Slot tmp;
                        tmp.i32 = immy;
                        r(d).f32 = r(x).f32 * tmp.f32;
                    } break;
                    CASE(Op::min_f32_imm): {
                        Slot tmp;
                        tmp.i32 = immy;
                        r(d).f32 = min(r(x).f32, tmp.f32);
                    } break;
                    CASE(Op::max_f32_imm): {
                        Slot tmp;
                        tmp.i32 = immy;
                        r(d).f32 = max(r(x).f32, tmp.f32);
                    } break;

                    CASE(Op::mad_f32): r(d).f32 = r(x).f32 * r(y).f32 + r(z).f32; break;

                    CASE(Op::add_i32): r(d).i32 = r(x).i32 + r(y).i32; break;
                    CASE(Op::sub_i32): r(d).i32 = r(x).i32 - r(y).i32; break;
                    CASE(Op::mul_i32): r(d).i32 = r(x).i32 * r(y).i32; break;

                    CASE(Op::add_i16x2): r(d).i16x2 = r(x).i16x2 + r(y).i16x2; break;
                    CASE(Op::sub_i16x2): r(d).i16x2 = r(x).i16x2 - r(y).i16x2; break;
                    CASE(Op::mul_i16x2): r(d).i16x2 = r(x).i16x2 * r(y).i16x2; break;

                    CASE(Op::shl_i32): r(d).i32 = r(x).i32 << immy; break;
                    CASE(Op::sra_i32): r(d).i32 = r(x).i32 >> immy; break;
                    CASE(Op::shr_i32): r(d).u32 = r(x).u32 >> immy; break;

                    CASE(Op::shl_i16x2): r(d).i16x2 = r(x).i16x2 << immy; break;
                    CASE(Op::sra_i16x2): r(d).i16x2 = r(x).i16x2 >> immy; break;
                    CASE(Op::shr_i16x2): r(d).u16x2 = r(x).u16x2 >> immy; break;

                    CASE(Op:: eq_f32): r(d).i32 = r(x).f32 == r(y).f32; break;
                    CASE(Op::neq_f32): r(d).i32 = r(x).f32 != r(y).f32; break;
                    CASE(Op:: gt_f32): r(d).i32 = r(x).f32 >  r(y).f32; break;
                    CASE(Op::gte_f32): r(d).i32 = r(x).f32 >= r(y).f32; break;

                    CASE(Op:: eq_i32): r(d).i32 = r(x).i32 == r(y).i32; break;
                    CASE(Op::neq_i32): r(d).i32 = r(x).i32 != r(y).i32; break;
                    CASE(Op:: gt_i32): r(d).i32 = r(x).i32 >  r(y).i32; break;
                    CASE(Op::gte_i32): r(d).i32 = r(x).i32 >= r(y).i32; break;

                    CASE(Op:: eq_i16x2): r(d).i16x2 = r(x).i16x2 == r(y).i16x2; break;
                    CASE(Op::neq_i16x2): r(d).i16x2 = r(x).i16x2 != r(y).i16x2; break;
                    CASE(Op:: gt_i16x2): r(d).i16x2 = r(x).i16x2 >  r(y).i16x2; break;
                    CASE(Op::gte_i16x2): r(d).i16x2 = r(x).i16x2 >= r(y).i16x2; break;

                    CASE(Op::bit_and  ): r(d).i32 = r(x).i32 &  r(y).i32; break;
                    CASE(Op::bit_or   ): r(d).i32 = r(x).i32 |  r(y).i32; break;
                    CASE(Op::bit_xor  ): r(d).i32 = r(x).i32 ^  r(y).i32; break;
                    CASE(Op::bit_clear): r(d).i32 = r(x).i32 & ~r(y).i32; break;

                    CASE(Op::bit_and_imm): r(d).i32 = r(x).i32 & immy; break;
                    CASE(Op::bit_or_imm ): r(d).i32 = r(x).i32 | immy; break;
                    CASE(Op::bit_xor_imm): r(d).i32 = r(x).i32 ^ immy; break;

                    CASE(Op::select): r(d).i32 = skvx::if_then_else(r(x).i32, r(y).i32, r(z).i32);
                                      break;

                    CASE(Op::pack):    r(d).u32 = r(x).u32 | (r(y).u32 << immz); break;

                    CASE(Op::bytes): {
                        const U32 table[] = {
                            0,
                            (r(x).u32      ) & 0xff,
                            (r(x).u32 >>  8) & 0xff,
                            (r(x).u32 >> 16) & 0xff,
                            (r(x).u32 >> 24) & 0xff,
                        };
                        r(d).u32 = table[(immy >>  0) & 0xf] <<  0
                                 | table[(immy >>  4) & 0xf] <<  8
                                 | table[(immy >>  8) & 0xf] << 16
                                 | table[(immy >> 12) & 0xf] << 24;
                    } break;

                    CASE(Op::floor):  r(d).f32 = skvx::floor(r(x).f32); break;
                    CASE(Op::to_f32): r(d).f32 = skvx::cast<float>(r(x).i32); break;
                    CASE(Op::trunc):  r(d).i32 = skvx::cast<int>  (r(x).f32); break;
                    CASE(Op::round):  r(d).i32 = skvx::cast<int>  (r(x).f32 + 0.5f); break;
                #undef CASE
                }
            }
        }
    }

    bool Program::hasJIT() const {
        return fJITEntry != nullptr;
    }

    void Program::dropJIT() {
    #if defined(SKVM_JIT)
        if (fDylib) {
            dlclose(fDylib);
        } else if (fJITEntry) {
            munmap(fJITEntry, fJITSize);
        }
    #else
        SkASSERT(!this->hasJIT());
    #endif

        fJITEntry = nullptr;
        fJITSize  = 0;
        fDylib    = nullptr;
    }

    Program::~Program() { this->dropJIT(); }

    Program::Program(Program&& other) {
        fInstructions    = std::move(other.fInstructions);
        fRegs            = other.fRegs;
        fLoop            = other.fLoop;
        fStrides         = std::move(other.fStrides);

        std::swap(fJITEntry, other.fJITEntry);
        std::swap(fJITSize , other.fJITSize);
        std::swap(fDylib   , other.fDylib);
    }

    Program& Program::operator=(Program&& other) {
        fInstructions    = std::move(other.fInstructions);
        fRegs            = other.fRegs;
        fLoop            = other.fLoop;
        fStrides         = std::move(other.fStrides);

        std::swap(fJITEntry, other.fJITEntry);
        std::swap(fJITSize , other.fJITSize);
        std::swap(fDylib   , other.fDylib);
        return *this;
    }

    Program::Program() {}

    Program::Program(const std::vector<Builder::Instruction>& instructions,
                     const std::vector<int>& strides,
                     const char* debug_name)
        : fStrides(strides)
    {
        this->setupInterpreter(instructions);
    #if 1 && defined(SKVM_JIT)
        this->setupJIT(instructions, debug_name);
    #endif
    }

    // Translate Builder::Instructions to Program::Instructions used by the interpreter.
    void Program::setupInterpreter(const std::vector<Builder::Instruction>& instructions) {
        // Register each instruction is assigned to.
        std::vector<Reg> reg(instructions.size());

        // This next bit is a bit more complicated than strictly necessary;
        // we could just assign every instruction to its own register.
        //
        // But recycling registers is fairly cheap, and good practice for the
        // JITs where minimizing register pressure really is important.
        //
        // Since we have effectively infinite registers, we hoist any value we can.
        // (The JIT may choose a more complex policy to reduce register pressure.)
        auto hoisted = [&](Val id) { return instructions[id].can_hoist; };

        fRegs = 0;
        std::vector<Reg> avail;

        // Assign this value to a register, recycling them where we can.
        auto assign_register = [&](Val id) {
            const Builder::Instruction& inst = instructions[id];

            // If this is a real input and it's lifetime ends at this instruction,
            // we can recycle the register it's occupying.
            auto maybe_recycle_register = [&](Val input) {
                if (input != NA
                        && instructions[input].death == id
                        && !(hoisted(input) && instructions[input].used_in_loop)) {
                    avail.push_back(reg[input]);
                }
            };

            // Take care to not recycle the same register twice.
            if (true                                ) { maybe_recycle_register(inst.x); }
            if (inst.y != inst.x                    ) { maybe_recycle_register(inst.y); }
            if (inst.z != inst.x && inst.z != inst.y) { maybe_recycle_register(inst.z); }

            // Instructions that die at themselves (stores) don't need a register.
            if (inst.death != id) {
                // Allocate a register if we have to, preferring to reuse anything available.
                if (avail.empty()) {
                    reg[id] = fRegs++;
                } else {
                    reg[id] = avail.back();
                    avail.pop_back();
                }
            }
        };

        // Assign a register to each hoisted instruction, then each non-hoisted loop instruction.
        for (Val id = 0; id < (Val)instructions.size(); id++) {
            if ( hoisted(id)) { assign_register(id); }
        }
        for (Val id = 0; id < (Val)instructions.size(); id++) {
            if (!hoisted(id)) { assign_register(id); }
        }

        // Translate Builder::Instructions to Program::Instructions by mapping values to
        // registers.  This will be two passes, first hoisted instructions, then inside the loop.

        // The loop begins at the fLoop'th Instruction.
        fLoop = 0;
        fInstructions.reserve(instructions.size());

        // Add a dummy mapping for the N/A sentinel Val to any arbitrary register
        // so lookups don't have to know which arguments are used by which Ops.
        auto lookup_register = [&](Val id) {
            return id == NA ? (Reg)0
                            : reg[id];
        };

        auto push_instruction = [&](Val id, const Builder::Instruction& inst) {
            Program::Instruction pinst{
                inst.op,
                lookup_register(id),
                lookup_register(inst.x),
               {lookup_register(inst.y)},
               {lookup_register(inst.z)},
            };
            if (inst.y == NA) { pinst.immy = inst.immy; }
            if (inst.z == NA) { pinst.immz = inst.immz; }
            fInstructions.push_back(pinst);
        };

        for (Val id = 0; id < (Val)instructions.size(); id++) {
            const Builder::Instruction& inst = instructions[id];
            if (hoisted(id)) {
                push_instruction(id, inst);
                fLoop++;
            }
        }
        for (Val id = 0; id < (Val)instructions.size(); id++) {
            const Builder::Instruction& inst = instructions[id];
            if (!hoisted(id)) {
                push_instruction(id, inst);
            }
        }
    }

#if defined(SKVM_JIT)

    // Just so happens that we can translate the immediate control for our bytes() op
    // to a single 128-bit mask that can be consumed by both AVX2 vpshufb and NEON tbl!
    static void bytes_control(int imm, int mask[4]) {
        auto nibble_to_vpshufb = [](uint8_t n) -> uint8_t {
            // 0 -> 0xff,    Fill with zero
            // 1 -> 0x00,    Select byte 0
            // 2 -> 0x01,         "      1
            // 3 -> 0x02,         "      2
            // 4 -> 0x03,         "      3
            return n - 1;
        };
        uint8_t control[] = {
            nibble_to_vpshufb( (imm >>  0) & 0xf ),
            nibble_to_vpshufb( (imm >>  4) & 0xf ),
            nibble_to_vpshufb( (imm >>  8) & 0xf ),
            nibble_to_vpshufb( (imm >> 12) & 0xf ),
        };
        for (int i = 0; i < 4; i++) {
            mask[i] = (int)control[0] <<  0
                    | (int)control[1] <<  8
                    | (int)control[2] << 16
                    | (int)control[3] << 24;

            // Update each byte that refers to a byte index by 4 to
            // point into the next 32-bit lane, but leave any 0xff
            // that fills with zero alone.
            control[0] += control[0] == 0xff ? 0 : 4;
            control[1] += control[1] == 0xff ? 0 : 4;
            control[2] += control[2] == 0xff ? 0 : 4;
            control[3] += control[3] == 0xff ? 0 : 4;
        }
    }

    bool Program::jit(const std::vector<Builder::Instruction>& instructions,
                      const bool try_hoisting,
                      Assembler* a) const {
        using A = Assembler;

        auto debug_dump = [&] {
        #if 0
            SkDebugfStream stream;
            this->dump(&stream);
            return true;
        #else
            return false;
        #endif
        };

    #if defined(__x86_64__)
        if (!SkCpu::Supports(SkCpu::HSW)) {
            return false;
        }
        A::GP64 N        = A::rdi,
                scratch  = A::rax,
                scratch2 = A::r11,
                arg[]    = { A::rsi, A::rdx, A::rcx, A::r8, A::r9 };

        // All 16 ymm registers are available to use.
        using Reg = A::Ymm;
        uint32_t avail = 0xffff;

    #elif defined(__aarch64__)
        A::X N       = A::x0,
             scratch = A::x8,
             arg[]   = { A::x1, A::x2, A::x3, A::x4, A::x5, A::x6, A::x7 };

        // We can use v0-v7 and v16-v31 freely; we'd need to preserve v8-v15.
        using Reg = A::V;
        uint32_t avail = 0xffff00ff;
    #endif

        if (SK_ARRAY_COUNT(arg) < fStrides.size()) {
            return false;
        }

        auto hoisted = [&](Val id) { return try_hoisting && instructions[id].can_hoist; };

        std::vector<Reg> r(instructions.size());

        struct LabelAndReg {
            A::Label label;
            Reg      reg;
        };
        SkTHashMap<int, LabelAndReg> constants,    // All constants share the same pool.
                                     bytes_masks;  // These vary per-lane.
        LabelAndReg                  iota;         // Exists _only_ to vary per-lane.

        auto warmup = [&](Val id) {
            const Builder::Instruction& inst = instructions[id];

            switch (inst.op) {
                default: break;

                case Op::bytes: if (!bytes_masks.find(inst.immy)) {
                                    bytes_masks.set(inst.immy, {});
                                    if (try_hoisting) {
                                        // vpshufb can always work with the mask from memory,
                                        // but it helps to hoist the mask to a register for tbl.
                                    #if defined(__aarch64__)
                                        LabelAndReg* entry = bytes_masks.find(inst.immy);
                                        if (int found = __builtin_ffs(avail)) {
                                            entry->reg = (Reg)(found-1);
                                            avail ^= 1 << entry->reg;
                                            a->ldrq(entry->reg, &entry->label);
                                        } else {
                                            return false;
                                        }
                                    #endif
                                    }
                                }
                                break;
            }
            return true;
        };

        auto emit = [&](Val id, bool scalar) {
            const Builder::Instruction& inst = instructions[id];

            Op op = inst.op;
            Val x = inst.x,
                y = inst.y,
                z = inst.z;
            int immy = inst.immy,
                immz = inst.immz;

            // Most (but not all) ops create an output value and need a register to hold it, dst.
            // We track each instruction's dst in r[] so we can thread it through as an input
            // to any future instructions needing that value.
            //
            // And some ops may need a temporary register, tmp.  Some need both tmp and dst.
            //
            // tmp and dst are very similar and can and will often be assigned the same register,
            // but tmp may never alias any of the instructions's inputs, while dst may when this
            // instruction consumes that input, i.e. if the input reaches its end of life here.
            //
            // We'll assign both registers lazily to keep register pressure as low as possible.
            bool tmp_is_set = false,
                 dst_is_set = false;
            Reg tmp_reg = (Reg)0;  // This initial value won't matter... anything legal is fine.

            bool ok = true;   // Set to false if we need to assign a register and none's available.

            // First lock in how to choose tmp if we need to based on the registers
            // available before this instruction, not including any of its input registers.
            auto tmp = [&,avail/*important, closing over avail's current value*/]{
                if (!tmp_is_set) {
                    tmp_is_set = true;
                    if (int found = __builtin_ffs(avail)) {
                        // This is a temporary register just for this op,
                        // so we leave it marked available for future ops.
                        tmp_reg = (Reg)(found - 1);
                    } else {
                        // We needed a tmp register but couldn't find one available. :'(
                        // This will cause emit() to return false, in turn causing jit() to fail.
                        if (debug_dump()) {
                            SkDebugf("\nCould not find a register to hold tmp\n");
                        }
                        ok = false;
                    }
                }
                return tmp_reg;
            };

            // Now make available any registers that are consumed by this instruction.
            // (The register pool we can pick dst from is >= the pool for tmp, adding any of these.)
            auto maybe_recycle_register = [&](Val input) {
                if (input != NA
                        && instructions[input].death == id
                        && !(hoisted(input) && instructions[input].used_in_loop)) {
                    avail |= 1 << r[input];
                }
            };
            maybe_recycle_register(x);
            maybe_recycle_register(y);
            maybe_recycle_register(z);
            // set_dst() and dst() will work read/write with this perhaps-just-updated avail.

            // Some ops may decide dst on their own to best fit the instruction (see Op::mad_f32).
            auto set_dst = [&](Reg reg){
                SkASSERT(dst_is_set == false);
                dst_is_set = true;

                SkASSERT(avail & (1<<reg));
                avail ^= 1<<reg;

                r[id] = reg;
            };

            // Thanks to AVX and NEON's 3-argument instruction sets,
            // most ops can use any register as dst.
            auto dst = [&]{
                if (!dst_is_set) {
                    if (int found = __builtin_ffs(avail)) {
                        set_dst((Reg)(found-1));
                    } else {
                        // Same deal as with tmp... all the registers are occupied.  Time to fail!
                        if (debug_dump()) {
                            SkDebugf("\nCould not find a register to hold value %d\n", id);
                        }
                        ok = false;
                    }
                }
                return r[id];
            };

            // Because we use the same logic to pick an arbitrary dst and to pick tmp,
            // and we know that tmp will never overlap any of the inputs, `dst() == tmp()`
            // is a simple idiom to check that the destination does not overlap any of the inputs.
            // Sometimes we can use this knowledge to do better instruction selection.

            // Ok!  Keep in mind that we haven't assigned tmp or dst yet,
            // just laid out hooks for how to do so if we need them, depending on the instruction.
            //
            // Now let's actually assemble the instruction!
            switch (op) {
                default:
                    if (debug_dump()) {
                        SkDEBUGFAILF("\nOp::%s (%d) not yet implemented\n", name(op), op);
                    }
                    return false;  // TODO: many new ops

            #if defined(__x86_64__)
                case Op::assert_true: {
                    a->vptest (r[x], &constants[0xffffffff].label);
                    A::Label all_true;
                    a->jc(&all_true);
                    a->int3();
                    a->label(&all_true);
                } break;

                case Op::store8: if (scalar) { a->vpextrb  (arg[immy], (A::Xmm)r[x], 0); }
                                 else        { a->vpackusdw(tmp(), r[x], r[x]);
                                               a->vpermq   (tmp(), tmp(), 0xd8);
                                               a->vpackuswb(tmp(), tmp(), tmp());
                                               a->vmovq    (arg[immy], (A::Xmm)tmp()); }
                                               break;

                case Op::store16: if (scalar) { a->vpextrw  (arg[immy], (A::Xmm)r[x], 0); }
                                  else        { a->vpackusdw(tmp(), r[x], r[x]);
                                                a->vpermq   (tmp(), tmp(), 0xd8);
                                                a->vmovups  (arg[immy], (A::Xmm)tmp()); }
                                                break;

                case Op::store32: if (scalar) { a->vmovd  (arg[immy], (A::Xmm)r[x]); }
                                  else        { a->vmovups(arg[immy],         r[x]); }
                                                break;

                case Op::load8:  if (scalar) {
                                     a->vpxor  (dst(), dst(), dst());
                                     a->vpinsrb((A::Xmm)dst(), (A::Xmm)dst(), arg[immy], 0);
                                 } else {
                                     a->vpmovzxbd(dst(), arg[immy]);
                                 } break;

                case Op::load16: if (scalar) {
                                     a->vpxor  (dst(), dst(), dst());
                                     a->vpinsrw((A::Xmm)dst(), (A::Xmm)dst(), arg[immy], 0);
                                 } else {
                                     a->vpmovzxwd(dst(), arg[immy]);
                                 } break;

                case Op::load32: if (scalar) { a->vmovd  ((A::Xmm)dst(), arg[immy]); }
                                 else        { a->vmovups(        dst(), arg[immy]); }
                                 break;

                case Op::gather32:
                if (scalar) {
                    auto base  = scratch,
                         index = scratch2;
                    // Our gather base pointer is immz bytes off of uniform immy.
                    a->movq(base, arg[immy], immz);

                    // Grab our index from lane 0 of the index argument.
                    a->vmovd_direct(index, (A::Xmm)r[x]);

                    // dst = *(base + 4*index)
                    a->vmovd((A::Xmm)dst(), A::FOUR, index, base);
                } else {
                    // We may not let any of dst(), index, or mask use the same register,
                    // so we must allocate registers manually and very carefully.

                    // index is argument x and has already been maybe_recycle_register()'d,
                    // so we explicitly ignore its availability during this op.
                    A::Ymm index = r[x];
                    uint32_t avail_during_gather = avail & ~(1<<index);

                    // Choose dst() to not overlap with index.
                    if (int found = __builtin_ffs(avail_during_gather)) {
                        set_dst((A::Ymm)(found-1));
                        avail_during_gather ^= (1<<dst());
                    } else {
                        ok = false;
                        break;
                    }

                    // Choose (temporary) mask to not overlap with dst() or index.
                    A::Ymm mask;
                    if (int found = __builtin_ffs(avail_during_gather)) {
                        mask = (A::Ymm)(found-1);
                    } else {
                        ok = false;
                        break;
                    }

                    // Our gather base pointer is immz bytes off of uniform immy.
                    auto base = scratch;
                    a->movq(base, arg[immy], immz);
                    a->vpcmpeqd(mask, mask, mask);   // (All lanes enabled.)
                    a->vgatherdps(dst(), A::FOUR, index, base, mask);
                }
                break;

                case Op::uniform8: a->movzbl(scratch, arg[immy], immz);
                                   a->vmovd_direct((A::Xmm)dst(), scratch);
                                   a->vbroadcastss(dst(), (A::Xmm)dst());
                                   break;

                case Op::uniform32: a->vbroadcastss(dst(), arg[immy], immz);
                                    break;

                case Op::index: a->vmovd_direct((A::Xmm)tmp(), N);
                                a->vbroadcastss(tmp(), (A::Xmm)tmp());
                                a->vpsubd(dst(), tmp(), &iota.label);
                                break;

                case Op::splat: if (immy) { a->vbroadcastss(dst(), &constants[immy].label); }
                                else      { a->vpxor(dst(), dst(), dst()); }
                                break;

                case Op::add_f32: a->vaddps(dst(), r[x], r[y]); break;
                case Op::sub_f32: a->vsubps(dst(), r[x], r[y]); break;
                case Op::mul_f32: a->vmulps(dst(), r[x], r[y]); break;
                case Op::div_f32: a->vdivps(dst(), r[x], r[y]); break;
                case Op::min_f32: a->vminps(dst(), r[x], r[y]); break;
                case Op::max_f32: a->vmaxps(dst(), r[x], r[y]); break;

                case Op::mad_f32:
                    if      (avail & (1<<r[x])) { set_dst(r[x]); a->vfmadd132ps(r[x], r[z], r[y]); }
                    else if (avail & (1<<r[y])) { set_dst(r[y]); a->vfmadd213ps(r[y], r[x], r[z]); }
                    else if (avail & (1<<r[z])) { set_dst(r[z]); a->vfmadd231ps(r[z], r[x], r[y]); }
                    else                        {                SkASSERT(dst() == tmp());
                                                                 a->vmovdqa    (dst(),r[x]);
                                                                 a->vfmadd132ps(dst(),r[z], r[y]); }
                                                                 break;

                case Op::add_f32_imm: a->vaddps(dst(), r[x], &constants[immy].label); break;
                case Op::sub_f32_imm: a->vsubps(dst(), r[x], &constants[immy].label); break;
                case Op::mul_f32_imm: a->vmulps(dst(), r[x], &constants[immy].label); break;
                case Op::min_f32_imm: a->vminps(dst(), r[x], &constants[immy].label); break;
                case Op::max_f32_imm: a->vmaxps(dst(), r[x], &constants[immy].label); break;

                case Op::add_i32: a->vpaddd (dst(), r[x], r[y]); break;
                case Op::sub_i32: a->vpsubd (dst(), r[x], r[y]); break;
                case Op::mul_i32: a->vpmulld(dst(), r[x], r[y]); break;

                case Op::sub_i16x2: a->vpsubw (dst(), r[x], r[y]); break;
                case Op::mul_i16x2: a->vpmullw(dst(), r[x], r[y]); break;
                case Op::shr_i16x2: a->vpsrlw (dst(), r[x], immy); break;

                case Op::bit_and  : a->vpand (dst(), r[x], r[y]); break;
                case Op::bit_or   : a->vpor  (dst(), r[x], r[y]); break;
                case Op::bit_xor  : a->vpxor (dst(), r[x], r[y]); break;
                case Op::bit_clear: a->vpandn(dst(), r[y], r[x]); break;  // N.B. Y then X.
                case Op::select   : a->vpblendvb(dst(), r[z], r[y], r[x]); break;

                case Op::bit_and_imm: a->vpand (dst(), r[x], &constants[immy].label); break;
                case Op::bit_or_imm : a->vpor  (dst(), r[x], &constants[immy].label); break;
                case Op::bit_xor_imm: a->vpxor (dst(), r[x], &constants[immy].label); break;

                case Op::shl_i32: a->vpslld(dst(), r[x], immy); break;
                case Op::shr_i32: a->vpsrld(dst(), r[x], immy); break;
                case Op::sra_i32: a->vpsrad(dst(), r[x], immy); break;

                case Op::eq_i32: a->vpcmpeqd(dst(), r[x], r[y]); break;
                case Op::gt_i32: a->vpcmpgtd(dst(), r[x], r[y]); break;

                case Op:: eq_f32: a->vcmpeqps (dst(), r[x], r[y]); break;
                case Op::neq_f32: a->vcmpneqps(dst(), r[x], r[y]); break;
                case Op:: gt_f32: a->vcmpltps (dst(), r[y], r[x]); break;
                case Op::gte_f32: a->vcmpleps (dst(), r[y], r[x]); break;

                case Op::pack: a->vpslld(tmp(),  r[y], immz);
                               a->vpor  (dst(), tmp(), r[x]);
                               break;

                case Op::floor : a->vroundps  (dst(), r[x], Assembler::FLOOR); break;
                case Op::to_f32: a->vcvtdq2ps (dst(), r[x]); break;
                case Op::trunc : a->vcvttps2dq(dst(), r[x]); break;
                case Op::round : a->vcvtps2dq (dst(), r[x]); break;

                case Op::bytes: a->vpshufb(dst(), r[x], &bytes_masks.find(immy)->label);
                                break;

            #elif defined(__aarch64__)
                case Op::assert_true: {
                    a->uminv4s(tmp(), r[x]);   // uminv acts like an all() across the vector.
                    a->fmovs(scratch, tmp());
                    A::Label all_true;
                    a->cbnz(scratch, &all_true);
                    a->brk(0);
                    a->label(&all_true);
                } break;

                case Op::store8: a->xtns2h(tmp(), r[x]);
                                 a->xtnh2b(tmp(), tmp());
                   if (scalar) { a->strb  (tmp(), arg[immy]); }
                   else        { a->strs  (tmp(), arg[immy]); }
                                 break;
                // TODO: another case where it'd be okay to alias r[x] and tmp if r[x] dies here.

                case Op::store32: if (scalar) { a->strs(r[x], arg[immy]); }
                                  else        { a->strq(r[x], arg[immy]); }
                                                break;

                case Op::load8: if (scalar) { a->ldrb(tmp(), arg[immy]); }
                                else        { a->ldrs(tmp(), arg[immy]); }
                                              a->uxtlb2h(tmp(), tmp());
                                              a->uxtlh2s(dst(), tmp());
                                              break;

                case Op::load32: if (scalar) { a->ldrs(dst(), arg[immy]); }
                                 else        { a->ldrq(dst(), arg[immy]); }
                                               break;

                case Op::splat: if (immy) { a->ldrq(dst(), &constants[immy].label); }
                                else      { a->eor16b(dst(), dst(), dst()); }
                                break;
                                // TODO: If we hoist these, pack 4 values in each register
                                // and use vector/lane operations, cutting the register
                                // pressure cost of hoisting by 4?

                case Op::add_f32: a->fadd4s(dst(), r[x], r[y]); break;
                case Op::sub_f32: a->fsub4s(dst(), r[x], r[y]); break;
                case Op::mul_f32: a->fmul4s(dst(), r[x], r[y]); break;
                case Op::div_f32: a->fdiv4s(dst(), r[x], r[y]); break;
                case Op::min_f32: a->fmin4s(dst(), r[x], r[y]); break;
                case Op::max_f32: a->fmax4s(dst(), r[x], r[y]); break;

                case Op::mad_f32: // fmla4s is z += x*y
                    if (avail & (1<<r[z])) { set_dst(r[z]); a->fmla4s( r[z],  r[x],  r[y]);   }
                    else {                                  a->orr16b(tmp(),  r[z],  r[z]);
                                                            a->fmla4s(tmp(),  r[x],  r[y]);
                                       if(dst() != tmp()) { a->orr16b(dst(), tmp(), tmp()); } }
                                                            break;

                // We should not see _imm ops on ARM.
                case Op::mul_f32_imm: SkUNREACHABLE; break;

                case Op:: gt_f32: a->fcmgt4s (dst(), r[x], r[y]); break;
                case Op::gte_f32: a->fcmge4s (dst(), r[x], r[y]); break;
                case Op:: eq_f32: a->fcmeq4s (dst(), r[x], r[y]); break;
                case Op::neq_f32: a->fcmeq4s (tmp(), r[x], r[y]);
                                  a->not16b  (dst(), tmp());      break;


                case Op::add_i32: a->add4s(dst(), r[x], r[y]); break;
                case Op::sub_i32: a->sub4s(dst(), r[x], r[y]); break;
                case Op::mul_i32: a->mul4s(dst(), r[x], r[y]); break;

                case Op::sub_i16x2: a->sub8h (dst(), r[x], r[y]); break;
                case Op::mul_i16x2: a->mul8h (dst(), r[x], r[y]); break;
                case Op::shr_i16x2: a->ushr8h(dst(), r[x], immy); break;

                case Op::bit_and  : a->and16b(dst(), r[x], r[y]); break;
                case Op::bit_or   : a->orr16b(dst(), r[x], r[y]); break;
                case Op::bit_xor  : a->eor16b(dst(), r[x], r[y]); break;
                case Op::bit_clear: a->bic16b(dst(), r[x], r[y]); break;

                case Op::select: // bsl16b is x = x ? y : z
                    if (avail & (1<<r[x])) { set_dst(r[x]); a->bsl16b( r[x],  r[y],  r[z]); }
                    else {                                  a->orr16b(tmp(),  r[x],  r[x]);
                                                            a->bsl16b(tmp(),  r[y],  r[z]);
                                       if(dst() != tmp()) { a->orr16b(dst(), tmp(), tmp()); } }
                                                            break;

                case Op::shl_i32: a-> shl4s(dst(), r[x], immy); break;
                case Op::shr_i32: a->ushr4s(dst(), r[x], immy); break;
                case Op::sra_i32: a->sshr4s(dst(), r[x], immy); break;

                case Op::eq_i32: a->cmeq4s(dst(), r[x], r[y]); break;
                case Op::gt_i32: a->cmgt4s(dst(), r[x], r[y]); break;

                case Op::pack:
                    if (avail & (1<<r[x])) { set_dst(r[x]); a->sli4s ( r[x],  r[y],  immz); }
                    else                   {                a->shl4s (tmp(),  r[y],  immz);
                                                            a->orr16b(dst(), tmp(),  r[x]); }
                                                            break;

                case Op::to_f32: a->scvtf4s (dst(), r[x]); break;
                case Op::trunc:  a->fcvtzs4s(dst(), r[x]); break;
                case Op::round:  a->fcvtns4s(dst(), r[x]); break;

                case Op::bytes:
                    if (try_hoisting) { a->tbl (dst(), r[x], bytes_masks.find(immy)->reg); }
                    else              { a->ldrq(tmp(), &bytes_masks.find(immy)->label);
                                        a->tbl (dst(), r[x], tmp()); }
                                        break;
            #endif
            }

            // Calls to tmp() or dst() might have flipped this false from its default true state.
            return ok;
        };


        #if defined(__x86_64__)
            const int K = 8;
            auto jump_if_less = [&](A::Label* l) { a->jl (l); };
            auto jump         = [&](A::Label* l) { a->jmp(l); };

            auto add = [&](A::GP64 gp, int imm) { a->add(gp, imm); };
            auto sub = [&](A::GP64 gp, int imm) { a->sub(gp, imm); };

            auto exit = [&]{ a->vzeroupper(); a->ret(); };
        #elif defined(__aarch64__)
            const int K = 4;
            auto jump_if_less = [&](A::Label* l) { a->blt(l); };
            auto jump         = [&](A::Label* l) { a->b  (l); };

            auto add = [&](A::X gp, int imm) { a->add(gp, gp, imm); };
            auto sub = [&](A::X gp, int imm) { a->sub(gp, gp, imm); };

            auto exit = [&]{ a->ret(A::x30); };
        #endif

        A::Label body,
                 tail,
                 done;

        for (Val id = 0; id < (Val)instructions.size(); id++) {
            if (!warmup(id)) {
                return false;
            }
            if (hoisted(id) && !emit(id, /*scalar=*/false)) {
                return false;
            }
        }

        a->label(&body);
        {
            a->cmp(N, K);
            jump_if_less(&tail);
            for (Val id = 0; id < (Val)instructions.size(); id++) {
                if (!hoisted(id) && !emit(id, /*scalar=*/false)) {
                    return false;
                }
            }
            for (int i = 0; i < (int)fStrides.size(); i++) {
                if (fStrides[i]) {
                    add(arg[i], K*fStrides[i]);
                }
            }
            sub(N, K);
            jump(&body);
        }

        a->label(&tail);
        {
            a->cmp(N, 1);
            jump_if_less(&done);
            for (Val id = 0; id < (Val)instructions.size(); id++) {
                if (!hoisted(id) && !emit(id, /*scalar=*/true)) {
                    return false;
                }
            }
            for (int i = 0; i < (int)fStrides.size(); i++) {
                if (fStrides[i]) {
                    add(arg[i], 1*fStrides[i]);
                }
            }
            sub(N, 1);
            jump(&tail);
        }

        a->label(&done);
        {
            exit();
        }

        // Except for explicit aligned load and store instructions, AVX allows
        // memory operands to be unaligned.  So even though we're creating 16
        // byte patterns on ARM or 32-byte patterns on x86, we only need to
        // align to 4 bytes, the element size and alignment requirement.

        constants.foreach([&](int imm, LabelAndReg* entry) {
            a->align(4);
            a->label(&entry->label);
            for (int i = 0; i < K; i++) {
                a->word(imm);
            }
        });

        bytes_masks.foreach([&](int imm, LabelAndReg* entry) {
            // One 16-byte pattern for ARM tbl, that same pattern twice for x86-64 vpshufb.
            a->align(4);
            a->label(&entry->label);
            int mask[4];
            bytes_control(imm, mask);
            a->bytes(mask, sizeof(mask));
        #if defined(__x86_64__)
            a->bytes(mask, sizeof(mask));
        #endif
        });

        if (!iota.label.references.empty()) {
            a->align(4);
            a->label(&iota.label);
            for (int i = 0; i < K; i++) {
                a->word(i);
            }
        }

        return true;
    }

    void Program::setupJIT(const std::vector<Builder::Instruction>& instructions,
                           const char* debug_name) {
        // Assemble with no buffer to determine a.size(), the number of bytes we'll assemble.
        Assembler a{nullptr};

        // First try allowing code hoisting (faster code)
        // then again without if that fails (lower register pressure).
        bool try_hoisting = true;
        if (!this->jit(instructions, try_hoisting, &a)) {
            try_hoisting = false;
            if (!this->jit(instructions, try_hoisting, &a)) {
                return;
            }
        }

        // Allocate space that we can remap as executable.
        const size_t page = sysconf(_SC_PAGESIZE);
        fJITSize = ((a.size() + page - 1) / page) * page;  // mprotect works at page granularity.
        fJITEntry = mmap(nullptr,fJITSize, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1,0);

        // Assemble the program for real.
        a = Assembler{fJITEntry};
        SkAssertResult(this->jit(instructions, try_hoisting, &a));
        SkASSERT(a.size() <= fJITSize);

        // Remap as executable, and flush caches on platforms that need that.
        mprotect(fJITEntry, fJITSize, PROT_READ|PROT_EXEC);
        __builtin___clear_cache((char*)fJITEntry,
                                (char*)fJITEntry + fJITSize);

        // For profiling and debugging, it's helpful to have this code loaded
        // dynamically rather than just jumping info fJITEntry.
        if (gSkVMJITViaDylib) {
            // Dump the raw program binary.
            SkString path = SkStringPrintf("/tmp/%s.XXXXXX", debug_name);
            int fd = mkstemp(path.writable_str());
            ::write(fd, fJITEntry, a.size());
            close(fd);

            this->dropJIT();  // (unmap and null out fJITEntry.)

            // Convert it in-place to a dynamic library with a single symbol "skvm_jit":
            SkString cmd = SkStringPrintf(
                    "echo '.global _skvm_jit\n_skvm_jit: .incbin \"%s\"'"
                    " | clang -x assembler -shared - -o %s",
                    path.c_str(), path.c_str());
            system(cmd.c_str());

            // Load that dynamic library and look up skvm_jit().
            fDylib = dlopen(path.c_str(), RTLD_NOW|RTLD_LOCAL);
            fJITEntry = dlsym(fDylib, "skvm_jit");
        }
    }
#endif

}  // namespace skvm