SkSLReturnsInputAlpha.cpp

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/*
 * Copyright 2023 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 "include/private/base/SkTArray.h"
#include "src/sksl/SkSLAnalysis.h"
#include "src/sksl/SkSLDefines.h"
#include "src/sksl/analysis/SkSLProgramUsage.h"
#include "src/sksl/analysis/SkSLProgramVisitor.h"
#include "src/sksl/ir/SkSLConstructor.h"
#include "src/sksl/ir/SkSLConstructorCompound.h"
#include "src/sksl/ir/SkSLConstructorCompoundCast.h"
#include "src/sksl/ir/SkSLConstructorSplat.h"
#include "src/sksl/ir/SkSLExpression.h"
#include "src/sksl/ir/SkSLFunctionDeclaration.h"
#include "src/sksl/ir/SkSLFunctionDefinition.h"
#include "src/sksl/ir/SkSLProgramElement.h"
#include "src/sksl/ir/SkSLReturnStatement.h"
#include "src/sksl/ir/SkSLStatement.h"
#include "src/sksl/ir/SkSLSwizzle.h"
#include "src/sksl/ir/SkSLTernaryExpression.h"
#include "src/sksl/ir/SkSLType.h"
#include "src/sksl/ir/SkSLVariable.h"
#include "src/sksl/ir/SkSLVariableReference.h"
 
#include <memory>
 
using namespace skia_private;
 
namespace SkSL {
namespace {
 
class ReturnsInputAlphaVisitor : public ProgramVisitor {
public:
    ReturnsInputAlphaVisitor(const ProgramUsage& u) : fUsage(u) {}
 
    bool visitProgramElement(const ProgramElement& pe) override {
        const FunctionDeclaration& decl = pe.as<FunctionDefinition>().declaration();
        SkSpan<Variable* const> parameters = decl.parameters();
 
        // We expect a color filter to have a single half4 input.
        if (parameters.size() != 1 ||
            parameters[0]->type().columns() != 4 ||
            !parameters[0]->type().componentType().isFloat()) {
            // This doesn't look like a color filter.
            return true;
        }
        fInputVar = parameters[0];
 
        // If the input variable has been written-to, then returning `input.a` isn't sufficient to
        // guarantee that alpha is preserved.
        ProgramUsage::VariableCounts counts = fUsage.get(*fInputVar);
        if (counts.fWrite != 0) {
            return true;
        }
 
        return INHERITED::visitProgramElement(pe);
    }
 
    bool isInputVar(const Expression& expr) {
        return expr.is<VariableReference>() && expr.as<VariableReference>().variable() == fInputVar;
    }
 
    bool isInputSwizzleEndingWithAlpha(const Expression& expr) {
        if (!expr.is<Swizzle>()) {
            return false;
        }
        const Swizzle& swizzle = expr.as<Swizzle>();
        return this->isInputVar(*swizzle.base()) && swizzle.components().back() == 3;
    }
 
    bool returnsInputAlpha(const Expression& expr) {
        if (this->isInputVar(expr)) {
            // This expression returns the input value as-is.
            return true;
        }
        if (expr.is<Swizzle>()) {
            // It's a swizzle: check for `input.___a`.
            return this->isInputSwizzleEndingWithAlpha(expr);
        }
        if (expr.is<ConstructorSplat>() || expr.is<ConstructorCompound>()) {
            // This is a splat or compound constructor; check for `input.a` as its final component.
            const AnyConstructor& ctor = expr.asAnyConstructor();
            return this->returnsInputAlpha(*ctor.argumentSpan().back());
        }
        if (expr.is<ConstructorCompoundCast>()) {
            // Ignore typecasts between float and half.
            const Expression& arg = *expr.as<ConstructorCompoundCast>().argument();
            return arg.type().componentType().isFloat() && this->returnsInputAlpha(arg);
        }
        if (expr.is<TernaryExpression>()) {
            // Both sides of the ternary must preserve input alpha.
            const TernaryExpression& ternary = expr.as<TernaryExpression>();
            return this->returnsInputAlpha(*ternary.ifTrue()) &&
                   this->returnsInputAlpha(*ternary.ifFalse());
        }
        // We weren't able to pattern-match here.
        return false;
    }
 
    bool visitStatement(const Statement& s) override {
        if (s.is<ReturnStatement>()) {
            return !this->returnsInputAlpha(*s.as<ReturnStatement>().expression());
        }
        return INHERITED::visitStatement(s);
    }
 
    bool visitExpression(const Expression& e) override {
        // No need to recurse into expressions; these can never contain return statements.
        return false;
    }
 
private:
    const ProgramUsage& fUsage;
    const Variable* fInputVar = nullptr;
 
    using INHERITED = ProgramVisitor;
};
 
}  // namespace
 
bool Analysis::ReturnsInputAlpha(const FunctionDefinition& function, const ProgramUsage& usage) {
    ReturnsInputAlphaVisitor visitor{usage};
    return !visitor.visitProgramElement(function);
}
 
}  // namespace SkSL