src/tint/lang/hlsl/writer/raise/extract_ternary_values.cc

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#include "src/tint/lang/hlsl/writer/raise/extract_ternary_values.h"

#include "src/tint/lang/core/ir/block.h"
#include "src/tint/lang/core/ir/builder.h"
#include "src/tint/lang/core/ir/constant.h"
#include "src/tint/lang/core/ir/exit.h"
#include "src/tint/lang/core/ir/instruction.h"
#include "src/tint/lang/core/ir/instruction_result.h"
#include "src/tint/lang/core/ir/loop.h"
#include "src/tint/lang/core/ir/switch.h"
#include "src/tint/lang/core/ir/validator.h"
#include "src/tint/lang/core/type/bool.h"
#include "src/tint/lang/core/type/manager.h"
#include "src/tint/lang/hlsl/ir/ternary.h"
#include "src/tint/utils/containers/vector.h"

namespace tint::hlsl::writer::raise {
namespace {

using namespace tint::core::fluent_types;  // NOLINT

/// PIMPL state for the transform.
struct State {
    /// The IR module.
    core::ir::Module& ir;

    /// The IR builder.
    core::ir::Builder b{ir};

    /// The type manager.
    core::type::Manager& ty{ir.Types()};

    /// Process the module.
    void Process() {
        // Collect the ternary instructions that need to be modified.
        Vector<hlsl::ir::Ternary*, 4> worklist;
        for (auto* inst : ir.Instructions()) {
            // Only when the condition is a constant scalar bool.
            if (auto* ternary = inst->As<hlsl::ir::Ternary>();
                ternary && ternary->Cmp()->Is<core::ir::Constant>() &&
                ternary->Cmp()->Type()->Is<core::type::Bool>()) {
                bool cond = ternary->Cmp()->As<core::ir::Constant>()->Value()->ValueAs<bool>();
                // Check whether the ternary value that will not be selected may have side effects.
                auto* unselected_side = cond ? ternary->False() : ternary->True();
                if (auto* instruction_result = unselected_side->As<core::ir::InstructionResult>()) {
                    auto* instruction = instruction_result->Instruction();
                    if (!instruction->GetSideEffects().Empty()) {
                        worklist.Push(ternary);
                    }
                }
            }
        }

        // Process the collected ternaries.
        for (auto* ternary : worklist) {
            bool cond = ternary->Cmp()->As<core::ir::Constant>()->Value()->ValueAs<bool>();
            // Constant scalar bool values as the ternary cond may result in unintended
            // optimizations in DXC that prevent both sides of the expression from being
            // evaluated. Extracting the unevaluated value that may have side effects to a let
            // prevents this.
            b.InsertBefore(ternary, [&] {
                auto* new_false = cond ? b.Let(ternary->False())->Result() : ternary->False();
                auto* new_true = cond ? ternary->True() : b.Let(ternary->True())->Result();
                ternary->SetOperand(0, new_false);
                ternary->SetOperand(1, new_true);
            });
        }
    }
};

}  // namespace

Result<SuccessType> ExtractTernaryValues(core::ir::Module& ir) {
    AssertValid(ir, kExtractTernaryValuesCapabilities, "before hlsl.ExtractTernaryValues");

    State{ir}.Process();

    return Success;
}

}  // namespace tint::hlsl::writer::raise