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// Copyright (c) 2018 Google LLC.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// This file implements the SSA rewriting algorithm proposed in
//
// Simple and Efficient Construction of Static Single Assignment Form.
// Braun M., Buchwald S., Hack S., Lei├ča R., Mallon C., Zwinkau A. (2013)
// In: Jhala R., De Bosschere K. (eds)
// Compiler Construction. CC 2013.
// Lecture Notes in Computer Science, vol 7791.
// Springer, Berlin, Heidelberg
//
// https://link.springer.com/chapter/10.1007/978-3-642-37051-9_6
//
// In contrast to common eager algorithms based on dominance and dominance
// frontier information, this algorithm works backwards from load operations.
//
// When a target variable is loaded, it queries the variable's reaching
// definition. If the reaching definition is unknown at the current location,
// it searches backwards in the CFG, inserting Phi instructions at join points
// in the CFG along the way until it finds the desired store instruction.
//
// The algorithm avoids repeated lookups using memoization.
//
// For reducible CFGs, which are a superset of the structured CFGs in SPIRV,
// this algorithm is proven to produce minimal SSA. That is, it inserts the
// minimal number of Phi instructions required to ensure the SSA property, but
// some Phi instructions may be dead
// (https://en.wikipedia.org/wiki/Static_single_assignment_form).
#include "source/opt/ssa_rewrite_pass.h"
#include <memory>
#include <sstream>
#include "source/opcode.h"
#include "source/opt/cfg.h"
#include "source/opt/mem_pass.h"
#include "source/util/make_unique.h"
// Debug logging (0: Off, 1-N: Verbosity level). Replace this with the
// implementation done for
// https://github.com/KhronosGroup/SPIRV-Tools/issues/1351
// #define SSA_REWRITE_DEBUGGING_LEVEL 3
#ifdef SSA_REWRITE_DEBUGGING_LEVEL
#include <ostream>
#else
#define SSA_REWRITE_DEBUGGING_LEVEL 0
#endif
namespace spvtools {
namespace opt {
namespace {
const uint32_t kStoreValIdInIdx = 1;
const uint32_t kVariableInitIdInIdx = 1;
} // namespace
std::string SSARewriter::PhiCandidate::PrettyPrint(const CFG* cfg) const {
std::ostringstream str;
str << "%" << result_id_ << " = Phi[%" << var_id_ << ", BB %" << bb_->id()
<< "](";
if (phi_args_.size() > 0) {
uint32_t arg_ix = 0;
for (uint32_t pred_label : cfg->preds(bb_->id())) {
uint32_t arg_id = phi_args_[arg_ix++];
str << "[%" << arg_id << ", bb(%" << pred_label << ")] ";
}
}
str << ")";
if (copy_of_ != 0) {
str << " [COPY OF " << copy_of_ << "]";
}
str << ((is_complete_) ? " [COMPLETE]" : " [INCOMPLETE]");
return str.str();
}
SSARewriter::PhiCandidate& SSARewriter::CreatePhiCandidate(uint32_t var_id,
BasicBlock* bb) {
// TODO(1841): Handle id overflow.
uint32_t phi_result_id = pass_->context()->TakeNextId();
auto result = phi_candidates_.emplace(
phi_result_id, PhiCandidate(var_id, phi_result_id, bb));
PhiCandidate& phi_candidate = result.first->second;
return phi_candidate;
}
void SSARewriter::ReplacePhiUsersWith(const PhiCandidate& phi_to_remove,
uint32_t repl_id) {
for (uint32_t user_id : phi_to_remove.users()) {
PhiCandidate* user_phi = GetPhiCandidate(user_id);
BasicBlock* bb = pass_->context()->get_instr_block(user_id);
if (user_phi) {
// If the user is a Phi candidate, replace all arguments that refer to
// |phi_to_remove.result_id()| with |repl_id|.
for (uint32_t& arg : user_phi->phi_args()) {
if (arg == phi_to_remove.result_id()) {
arg = repl_id;
}
}
} else if (bb->id() == user_id) {
// The phi candidate is the definition of the variable at basic block
// |bb|. We must change this to the replacement.
WriteVariable(phi_to_remove.var_id(), bb, repl_id);
} else {
// For regular loads, traverse the |load_replacement_| table looking for
// instances of |phi_to_remove|.
for (auto& it : load_replacement_) {
if (it.second == phi_to_remove.result_id()) {
it.second = repl_id;
}
}
}
}
}
uint32_t SSARewriter::TryRemoveTrivialPhi(PhiCandidate* phi_candidate) {
uint32_t same_id = 0;
for (uint32_t arg_id : phi_candidate->phi_args()) {
if (arg_id == same_id || arg_id == phi_candidate->result_id()) {
// This is a self-reference operand or a reference to the same value ID.
continue;
}
if (same_id != 0) {
// This Phi candidate merges at least two values. Therefore, it is not
// trivial.
assert(phi_candidate->copy_of() == 0 &&
"Phi candidate transitioning from copy to non-copy.");
return phi_candidate->result_id();
}
same_id = arg_id;
}
// The previous logic has determined that this Phi candidate |phi_candidate|
// is trivial. It is essentially the copy operation phi_candidate->phi_result
// = Phi(same, same, same, ...). Since it is not necessary, we can re-route
// all the users of |phi_candidate->phi_result| to all its users, and remove
// |phi_candidate|.
// Mark the Phi candidate as a trivial copy of |same_id|, so it won't be
// generated.
phi_candidate->MarkCopyOf(same_id);
assert(same_id != 0 && "Completed Phis cannot have %0 in their arguments");
// Since |phi_candidate| always produces |same_id|, replace all the users of
// |phi_candidate| with |same_id|.
ReplacePhiUsersWith(*phi_candidate, same_id);
return same_id;
}
uint32_t SSARewriter::AddPhiOperands(PhiCandidate* phi_candidate) {
assert(phi_candidate->phi_args().size() == 0 &&
"Phi candidate already has arguments");
bool found_0_arg = false;
for (uint32_t pred : pass_->cfg()->preds(phi_candidate->bb()->id())) {
BasicBlock* pred_bb = pass_->cfg()->block(pred);
// If |pred_bb| is not sealed, use %0 to indicate that
// |phi_candidate| needs to be completed after the whole CFG has
// been processed.
//
// Note that we cannot call GetReachingDef() in these cases
// because this would generate an empty Phi candidate in
// |pred_bb|. When |pred_bb| is later processed, a new definition
// for |phi_candidate->var_id_| will be lost because
// |phi_candidate| will still be reached by the empty Phi.
//
// Consider:
//
// BB %23:
// %38 = Phi[%i](%int_0[%1], %39[%25])
//
// ...
//
// BB %25: [Starts unsealed]
// %39 = Phi[%i]()
// %34 = ...
// OpStore %i %34 -> Currdef(%i) at %25 is %34
// OpBranch %23
//
// When we first create the Phi in %38, we add an operandless Phi in
// %39 to hold the unknown reaching def for %i.
//
// But then, when we go to complete %39 at the end. The reaching def
// for %i in %25's predecessor is %38 itself. So we miss the fact
// that %25 has a def for %i that should be used.
//
// By making the argument %0, we make |phi_candidate| incomplete,
// which will cause it to be completed after the whole CFG has
// been scanned.
uint32_t arg_id = IsBlockSealed(pred_bb)
? GetReachingDef(phi_candidate->var_id(), pred_bb)
: 0;
phi_candidate->phi_args().push_back(arg_id);
if (arg_id == 0) {
found_0_arg = true;
} else {
// If this argument is another Phi candidate, add |phi_candidate| to the
// list of users for the defining Phi.
PhiCandidate* defining_phi = GetPhiCandidate(arg_id);
if (defining_phi && defining_phi != phi_candidate) {
defining_phi->AddUser(phi_candidate->result_id());
}
}
}
// If we could not fill-in all the arguments of this Phi, mark it incomplete
// so it gets completed after the whole CFG has been processed.
if (found_0_arg) {
phi_candidate->MarkIncomplete();
incomplete_phis_.push(phi_candidate);
return phi_candidate->result_id();
}
// Try to remove |phi_candidate|, if it's trivial.
uint32_t repl_id = TryRemoveTrivialPhi(phi_candidate);
if (repl_id == phi_candidate->result_id()) {
// |phi_candidate| is complete and not trivial. Add it to the
// list of Phi candidates to generate.
phi_candidate->MarkComplete();
phis_to_generate_.push_back(phi_candidate);
}
return repl_id;
}
uint32_t SSARewriter::GetReachingDef(uint32_t var_id, BasicBlock* bb) {
// If |var_id| has a definition in |bb|, return it.
const auto& bb_it = defs_at_block_.find(bb);
if (bb_it != defs_at_block_.end()) {
const auto& current_defs = bb_it->second;
const auto& var_it = current_defs.find(var_id);
if (var_it != current_defs.end()) {
return var_it->second;
}
}
// Otherwise, look up the value for |var_id| in |bb|'s predecessors.
uint32_t val_id = 0;
auto& predecessors = pass_->cfg()->preds(bb->id());
if (predecessors.size() == 1) {
// If |bb| has exactly one predecessor, we look for |var_id|'s definition
// there.
val_id = GetReachingDef(var_id, pass_->cfg()->block(predecessors[0]));
} else if (predecessors.size() > 1) {
// If there is more than one predecessor, this is a join block which may
// require a Phi instruction. This will act as |var_id|'s current
// definition to break potential cycles.
PhiCandidate& phi_candidate = CreatePhiCandidate(var_id, bb);
// Set the value for |bb| to avoid an infinite recursion.
WriteVariable(var_id, bb, phi_candidate.result_id());
val_id = AddPhiOperands(&phi_candidate);
}
// If we could not find a store for this variable in the path from the root
// of the CFG, the variable is not defined, so we use undef.
if (val_id == 0) {
val_id = pass_->GetUndefVal(var_id);
if (val_id == 0) {
return 0;
}
}
WriteVariable(var_id, bb, val_id);
return val_id;
}
void SSARewriter::SealBlock(BasicBlock* bb) {
auto result = sealed_blocks_.insert(bb);
(void)result;
assert(result.second == true &&
"Tried to seal the same basic block more than once.");
}
void SSARewriter::ProcessStore(Instruction* inst, BasicBlock* bb) {
auto opcode = inst->opcode();
assert((opcode == SpvOpStore || opcode == SpvOpVariable) &&
"Expecting a store or a variable definition instruction.");
uint32_t var_id = 0;
uint32_t val_id = 0;
if (opcode == SpvOpStore) {
(void)pass_->GetPtr(inst, &var_id);
val_id = inst->GetSingleWordInOperand(kStoreValIdInIdx);
} else if (inst->NumInOperands() >= 2) {
var_id = inst->result_id();
val_id = inst->GetSingleWordInOperand(kVariableInitIdInIdx);
}
if (pass_->IsTargetVar(var_id)) {
WriteVariable(var_id, bb, val_id);
#if SSA_REWRITE_DEBUGGING_LEVEL > 1
std::cerr << "\tFound store '%" << var_id << " = %" << val_id << "': "
<< inst->PrettyPrint(SPV_BINARY_TO_TEXT_OPTION_FRIENDLY_NAMES)
<< "\n";
#endif
}
}
bool SSARewriter::ProcessLoad(Instruction* inst, BasicBlock* bb) {
uint32_t var_id = 0;
(void)pass_->GetPtr(inst, &var_id);
if (pass_->IsTargetVar(var_id)) {
// Get the immediate reaching definition for |var_id|.
uint32_t val_id = GetReachingDef(var_id, bb);
if (val_id == 0) {
return false;
}
// Schedule a replacement for the result of this load instruction with
// |val_id|. After all the rewriting decisions are made, every use of
// this load will be replaced with |val_id|.
const uint32_t load_id = inst->result_id();
assert(load_replacement_.count(load_id) == 0);
load_replacement_[load_id] = val_id;
PhiCandidate* defining_phi = GetPhiCandidate(val_id);
if (defining_phi) {
defining_phi->AddUser(load_id);
}
#if SSA_REWRITE_DEBUGGING_LEVEL > 1
std::cerr << "\tFound load: "
<< inst->PrettyPrint(SPV_BINARY_TO_TEXT_OPTION_FRIENDLY_NAMES)
<< " (replacement for %" << load_id << " is %" << val_id << ")\n";
#endif
}
return true;
}
void SSARewriter::PrintPhiCandidates() const {
std::cerr << "\nPhi candidates:\n";
for (const auto& phi_it : phi_candidates_) {
std::cerr << "\tBB %" << phi_it.second.bb()->id() << ": "
<< phi_it.second.PrettyPrint(pass_->cfg()) << "\n";
}
std::cerr << "\n";
}
void SSARewriter::PrintReplacementTable() const {
std::cerr << "\nLoad replacement table\n";
for (const auto& it : load_replacement_) {
std::cerr << "\t%" << it.first << " -> %" << it.second << "\n";
}
std::cerr << "\n";
}
bool SSARewriter::GenerateSSAReplacements(BasicBlock* bb) {
#if SSA_REWRITE_DEBUGGING_LEVEL > 1
std::cerr << "Generating SSA replacements for block: " << bb->id() << "\n";
std::cerr << bb->PrettyPrint(SPV_BINARY_TO_TEXT_OPTION_FRIENDLY_NAMES)
<< "\n";
#endif
for (auto& inst : *bb) {
auto opcode = inst.opcode();
if (opcode == SpvOpStore || opcode == SpvOpVariable) {
ProcessStore(&inst, bb);
} else if (inst.opcode() == SpvOpLoad) {
if (!ProcessLoad(&inst, bb)) {
return false;
}
}
}
// Seal |bb|. This means that all the stores in it have been scanned and it's
// ready to feed them into its successors.
SealBlock(bb);
#if SSA_REWRITE_DEBUGGING_LEVEL > 1
PrintPhiCandidates();
PrintReplacementTable();
std::cerr << "\n\n";
#endif
return true;
}
uint32_t SSARewriter::GetReplacement(std::pair<uint32_t, uint32_t> repl) {
uint32_t val_id = repl.second;
auto it = load_replacement_.find(val_id);
while (it != load_replacement_.end()) {
val_id = it->second;
it = load_replacement_.find(val_id);
}
return val_id;
}
uint32_t SSARewriter::GetPhiArgument(const PhiCandidate* phi_candidate,
uint32_t ix) {
assert(phi_candidate->IsReady() &&
"Tried to get the final argument from an incomplete/trivial Phi");
uint32_t arg_id = phi_candidate->phi_args()[ix];
while (arg_id != 0) {
PhiCandidate* phi_user = GetPhiCandidate(arg_id);
if (phi_user == nullptr || phi_user->IsReady()) {
// If the argument is not a Phi or it's a Phi candidate ready to be
// emitted, return it.
return arg_id;
}
arg_id = phi_user->copy_of();
}
assert(false &&
"No Phi candidates in the copy-of chain are ready to be generated");
return 0;
}
bool SSARewriter::ApplyReplacements() {
bool modified = false;
#if SSA_REWRITE_DEBUGGING_LEVEL > 2
std::cerr << "\n\nApplying replacement decisions to IR\n\n";
PrintPhiCandidates();
PrintReplacementTable();
std::cerr << "\n\n";
#endif
// Add Phi instructions from completed Phi candidates.
std::vector<Instruction*> generated_phis;
for (const PhiCandidate* phi_candidate : phis_to_generate_) {
#if SSA_REWRITE_DEBUGGING_LEVEL > 2
std::cerr << "Phi candidate: " << phi_candidate->PrettyPrint(pass_->cfg())
<< "\n";
#endif
assert(phi_candidate->is_complete() &&
"Tried to instantiate a Phi instruction from an incomplete Phi "
"candidate");
// Build the vector of operands for the new OpPhi instruction.
uint32_t type_id = pass_->GetPointeeTypeId(
pass_->get_def_use_mgr()->GetDef(phi_candidate->var_id()));
std::vector<Operand> phi_operands;
uint32_t arg_ix = 0;
std::unordered_map<uint32_t, uint32_t> already_seen;
for (uint32_t pred_label : pass_->cfg()->preds(phi_candidate->bb()->id())) {
uint32_t op_val_id = GetPhiArgument(phi_candidate, arg_ix++);
if (already_seen.count(pred_label) == 0) {
phi_operands.push_back(
{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {op_val_id}});
phi_operands.push_back(
{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {pred_label}});
already_seen[pred_label] = op_val_id;
} else {
// It is possible that there are two edges from the same parent block.
// Since the OpPhi can have only one entry for each parent, we have to
// make sure the two edges are consistent with each other.
assert(already_seen[pred_label] == op_val_id &&
"Inconsistent value for duplicate edges.");
}
}
// Generate a new OpPhi instruction and insert it in its basic
// block.
std::unique_ptr<Instruction> phi_inst(
new Instruction(pass_->context(), SpvOpPhi, type_id,
phi_candidate->result_id(), phi_operands));
generated_phis.push_back(phi_inst.get());
pass_->get_def_use_mgr()->AnalyzeInstDef(&*phi_inst);
pass_->context()->set_instr_block(&*phi_inst, phi_candidate->bb());
auto insert_it = phi_candidate->bb()->begin();
insert_it.InsertBefore(std::move(phi_inst));
pass_->context()->get_decoration_mgr()->CloneDecorations(
phi_candidate->var_id(), phi_candidate->result_id(),
{SpvDecorationRelaxedPrecision});
modified = true;
}
// Scan uses for all inserted Phi instructions. Do this separately from the
// registration of the Phi instruction itself to avoid trying to analyze uses
// of Phi instructions that have not been registered yet.
for (Instruction* phi_inst : generated_phis) {
pass_->get_def_use_mgr()->AnalyzeInstUse(&*phi_inst);
}
#if SSA_REWRITE_DEBUGGING_LEVEL > 1
std::cerr << "\n\nReplacing the result of load instructions with the "
"corresponding SSA id\n\n";
#endif
// Apply replacements from the load replacement table.
for (auto& repl : load_replacement_) {
uint32_t load_id = repl.first;
uint32_t val_id = GetReplacement(repl);
Instruction* load_inst =
pass_->context()->get_def_use_mgr()->GetDef(load_id);
#if SSA_REWRITE_DEBUGGING_LEVEL > 2
std::cerr << "\t"
<< load_inst->PrettyPrint(
SPV_BINARY_TO_TEXT_OPTION_FRIENDLY_NAMES)
<< " (%" << load_id << " -> %" << val_id << ")\n";
#endif
// Remove the load instruction and replace all the uses of this load's
// result with |val_id|. Kill any names or decorates using the load's
// result before replacing to prevent incorrect replacement in those
// instructions.
pass_->context()->KillNamesAndDecorates(load_id);
pass_->context()->ReplaceAllUsesWith(load_id, val_id);
pass_->context()->KillInst(load_inst);
modified = true;
}
return modified;
}
void SSARewriter::FinalizePhiCandidate(PhiCandidate* phi_candidate) {
assert(phi_candidate->phi_args().size() > 0 &&
"Phi candidate should have arguments");
uint32_t ix = 0;
for (uint32_t pred : pass_->cfg()->preds(phi_candidate->bb()->id())) {
BasicBlock* pred_bb = pass_->cfg()->block(pred);
uint32_t& arg_id = phi_candidate->phi_args()[ix++];
if (arg_id == 0) {
// If |pred_bb| is still not sealed, it means it's unreachable. In this
// case, we just use Undef as an argument.
arg_id = IsBlockSealed(pred_bb)
? GetReachingDef(phi_candidate->var_id(), pred_bb)
: pass_->GetUndefVal(phi_candidate->var_id());
}
}
// This candidate is now completed.
phi_candidate->MarkComplete();
// If |phi_candidate| is not trivial, add it to the list of Phis to generate.
if (TryRemoveTrivialPhi(phi_candidate) == phi_candidate->result_id()) {
// If we could not remove |phi_candidate|, it means that it is complete
// and not trivial. Add it to the list of Phis to generate.
assert(!phi_candidate->copy_of() && "A completed Phi cannot be trivial.");
phis_to_generate_.push_back(phi_candidate);
}
}
void SSARewriter::FinalizePhiCandidates() {
#if SSA_REWRITE_DEBUGGING_LEVEL > 1
std::cerr << "Finalizing Phi candidates:\n\n";
PrintPhiCandidates();
std::cerr << "\n";
#endif
// Now, complete the collected candidates.
while (incomplete_phis_.size() > 0) {
PhiCandidate* phi_candidate = incomplete_phis_.front();
incomplete_phis_.pop();
FinalizePhiCandidate(phi_candidate);
}
}
Pass::Status SSARewriter::RewriteFunctionIntoSSA(Function* fp) {
#if SSA_REWRITE_DEBUGGING_LEVEL > 0
std::cerr << "Function before SSA rewrite:\n"
<< fp->PrettyPrint(0) << "\n\n\n";
#endif
// Collect variables that can be converted into SSA IDs.
pass_->CollectTargetVars(fp);
// Generate all the SSA replacements and Phi candidates. This will
// generate incomplete and trivial Phis.
bool succeeded = pass_->cfg()->WhileEachBlockInReversePostOrder(
fp->entry().get(), [this](BasicBlock* bb) {
if (!GenerateSSAReplacements(bb)) {
return false;
}
return true;
});
if (!succeeded) {
return Pass::Status::Failure;
}
// Remove trivial Phis and add arguments to incomplete Phis.
FinalizePhiCandidates();
// Finally, apply all the replacements in the IR.
bool modified = ApplyReplacements();
#if SSA_REWRITE_DEBUGGING_LEVEL > 0
std::cerr << "\n\n\nFunction after SSA rewrite:\n"
<< fp->PrettyPrint(0) << "\n";
#endif
return modified ? Pass::Status::SuccessWithChange
: Pass::Status::SuccessWithoutChange;
}
Pass::Status SSARewritePass::Process() {
Status status = Status::SuccessWithoutChange;
for (auto& fn : *get_module()) {
status =
CombineStatus(status, SSARewriter(this).RewriteFunctionIntoSSA(&fn));
if (status == Status::Failure) {
break;
}
}
return status;
}
} // namespace opt
} // namespace spvtools