src/sksl/transform/SkSLFindAndDeclareBuiltinVariables.cpp

/*
 * Copyright 2021 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/SkSpan.h"
#include "include/core/SkTypes.h"
#include "src/core/SkTHash.h"
#include "src/sksl/SkSLBuiltinTypes.h"
#include "src/sksl/SkSLCompiler.h"
#include "src/sksl/SkSLContext.h"
#include "src/sksl/SkSLProgramSettings.h"
#include "src/sksl/analysis/SkSLProgramUsage.h"
#include "src/sksl/ir/SkSLFunctionDeclaration.h"
#include "src/sksl/ir/SkSLFunctionDefinition.h"
#include "src/sksl/ir/SkSLIRNode.h"
#include "src/sksl/ir/SkSLInterfaceBlock.h"
#include "src/sksl/ir/SkSLLayout.h"
#include "src/sksl/ir/SkSLProgram.h"
#include "src/sksl/ir/SkSLProgramElement.h"
#include "src/sksl/ir/SkSLSymbol.h"
#include "src/sksl/ir/SkSLSymbolTable.h"
#include "src/sksl/ir/SkSLType.h"
#include "src/sksl/ir/SkSLVarDeclarations.h"
#include "src/sksl/ir/SkSLVariable.h"
#include "src/sksl/transform/SkSLTransform.h"

#include <algorithm>
#include <memory>
#include <string_view>
#include <vector>

namespace SkSL {
namespace Transform {
namespace {

class BuiltinVariableScanner {
public:
    BuiltinVariableScanner(const Context& context, const SymbolTable& symbols)
            : fContext(context)
            , fSymbols(symbols) {}

    void addDeclaringElement(const ProgramElement* decl) {
        // Make sure we only add a built-in variable once. We only have a small handful of built-in
        // variables to declare, so linear search here is good enough.
        if (std::find(fNewElements.begin(), fNewElements.end(), decl) == fNewElements.end()) {
            fNewElements.push_back(decl);
        }
    }

    void addDeclaringElement(const Symbol* symbol) {
        if (!symbol || !symbol->is<Variable>()) {
            return;
        }
        const Variable& var = symbol->as<Variable>();
        if (const GlobalVarDeclaration* decl = var.globalVarDeclaration()) {
            this->addDeclaringElement(decl);
        } else if (const InterfaceBlock* block = var.interfaceBlock()) {
            this->addDeclaringElement(block);
        } else {
            // Double-check that this variable isn't associated with a global or an interface block.
            // (Locals and parameters will come along naturally as part of the associated function.)
            SkASSERTF(var.storage() != VariableStorage::kGlobal &&
                      var.storage() != VariableStorage::kInterfaceBlock,
                      "%.*s", (int)var.name().size(), var.name().data());
        }
    }

    void addImplicitFragColorWrite(SkSpan<const std::unique_ptr<ProgramElement>> elements) {
        for (const std::unique_ptr<ProgramElement>& pe : elements) {
            if (!pe->is<FunctionDefinition>()) {
                continue;
            }
            const FunctionDefinition& funcDef = pe->as<FunctionDefinition>();
            if (funcDef.declaration().isMain()) {
                if (funcDef.declaration().returnType().matches(*fContext.fTypes.fHalf4)) {
                    // We synthesize writes to sk_FragColor if main() returns a color, even if it's
                    // otherwise unreferenced.
                    this->addDeclaringElement(fSymbols.findBuiltinSymbol(Compiler::FRAGCOLOR_NAME));
                }
                // Now that main() has been found, we can stop scanning.
                break;
            }
        }
    }

    static std::string_view GlobalVarBuiltinName(const ProgramElement& elem) {
        return elem.as<GlobalVarDeclaration>().varDeclaration().var()->name();
    }

    static std::string_view InterfaceBlockName(const ProgramElement& elem) {
        return elem.as<InterfaceBlock>().instanceName();
    }

    void sortNewElements() {
        std::sort(fNewElements.begin(),
                  fNewElements.end(),
                  [](const ProgramElement* a, const ProgramElement* b) {
                      if (a->kind() != b->kind()) {
                          return a->kind() < b->kind();
                      }
                      switch (a->kind()) {
                          case ProgramElement::Kind::kGlobalVar:
                              SkASSERT(a == b ||
                                       GlobalVarBuiltinName(*a) != GlobalVarBuiltinName(*b));
                              return GlobalVarBuiltinName(*a) < GlobalVarBuiltinName(*b);

                          case ProgramElement::Kind::kInterfaceBlock:
                              SkASSERT(a == b || InterfaceBlockName(*a) != InterfaceBlockName(*b));
                              return InterfaceBlockName(*a) < InterfaceBlockName(*b);

                          default:
                              SkUNREACHABLE;
                      }
                  });
    }

    const Context& fContext;
    const SymbolTable& fSymbols;
    std::vector<const ProgramElement*> fNewElements;
};

}  // namespace

void FindAndDeclareBuiltinVariables(Program& program) {
    using Interface = Program::Interface;
    const Context& context = *program.fContext;
    const SymbolTable& symbols = *program.fSymbols;
    BuiltinVariableScanner scanner(context, symbols);

    if (ProgramConfig::IsFragment(program.fConfig->fKind)) {
        // Find main() in the program and check its return type.
        // If it's half4, we treat that as an implicit write to sk_FragColor and add a reference.
        scanner.addImplicitFragColorWrite(program.fOwnedElements);
    }

    // Scan all the variables used by the program and declare any built-ins.
    for (const auto& [var, counts] : program.fUsage->fVariableCounts) {
        if (var->isBuiltin()) {
            scanner.addDeclaringElement(var);

            switch (var->layout().fBuiltin) {
                // Set the RTFlip program input if we find sk_FragCoord or sk_Clockwise.
                case SK_FRAGCOORD_BUILTIN:
                    if (!context.fConfig->fSettings.fForceNoRTFlip) {
                        program.fInterface.fRTFlipUniform |= Interface::kRTFlip_FragCoord;
                    }
                    break;

                case SK_CLOCKWISE_BUILTIN:
                    if (!context.fConfig->fSettings.fForceNoRTFlip) {
                        program.fInterface.fRTFlipUniform |= Interface::kRTFlip_Clockwise;
                    }
                    break;

                // Set the UseLastFragColor program input if we find sk_LastFragColor.
                // Metal and Dawn define this as a program input, rather than a global variable.
                case SK_LASTFRAGCOLOR_BUILTIN:
                    program.fInterface.fUseLastFragColor = true;
                    break;

                // Set secondary color output if we find sk_SecondaryFragColor.
                case SK_SECONDARYFRAGCOLOR_BUILTIN:
                    program.fInterface.fOutputSecondaryColor = true;
                    break;
            }
        }
    }

    // Sort the referenced builtin functions into a consistent order; otherwise our output will
    // become non-deterministic. The exact order isn't particularly important.
    scanner.sortNewElements();

    // Add all the newly-declared elements to the program, and update ProgramUsage to match.
    program.fSharedElements.insert(program.fSharedElements.begin(),
                                   scanner.fNewElements.begin(),
                                   scanner.fNewElements.end());

    for (const ProgramElement* element : scanner.fNewElements) {
        program.fUsage->add(*element);
    }
}

}  // namespace Transform
}  // namespace SkSL