src/tint/lang/glsl/writer/raise/shader_io.cc

// Copyright 2024 The Dawn & Tint Authors
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#include "src/tint/lang/glsl/writer/raise/shader_io.h"

#include <memory>

#include "src/tint/lang/core/ir/builder.h"
#include "src/tint/lang/core/ir/module.h"
#include "src/tint/lang/core/ir/transform/shader_io.h"
#include "src/tint/lang/core/ir/validator.h"

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

namespace tint::glsl::writer::raise {

namespace {

/// PIMPL state for the parts of the shader IO transform specific to GLSL.
/// For GLSL, we declare a global variable for each input and output. The wrapper entry point then
/// loads from and stores to these variables.
struct StateImpl : core::ir::transform::ShaderIOBackendState {
    /// The input variables.
    Vector<core::ir::Var*, 4> input_vars;
    /// The output variables.
    Vector<core::ir::Var*, 4> output_vars;

    /// The configuration options.
    const ShaderIOConfig& config;

    /// Constructor
    StateImpl(core::ir::Module& mod, core::ir::Function* f, const ShaderIOConfig& cfg)
        : ShaderIOBackendState(mod, f), config(cfg) {}

    /// Destructor
    ~StateImpl() override {}

    /// Retrieve the gl_ string corresponding to a builtin.
    /// @param builtin the builtin
    /// @param address_space the address space (input or output)
    /// @returns the gl_ string corresponding to that builtin
    const char* GLSLBuiltinToString(core::BuiltinValue builtin, core::AddressSpace address_space) {
        switch (builtin) {
            case core::BuiltinValue::kPosition: {
                if (address_space == core::AddressSpace::kOut) {
                    return "gl_Position";
                }
                if (address_space == core::AddressSpace::kIn) {
                    return "gl_FragCoord";
                }
                TINT_UNREACHABLE();
            }
            case core::BuiltinValue::kVertexIndex:
                return "gl_VertexID";
            case core::BuiltinValue::kInstanceIndex:
                return "gl_InstanceID";
            case core::BuiltinValue::kFrontFacing:
                return "gl_FrontFacing";
            case core::BuiltinValue::kFragDepth:
                return "gl_FragDepth";
            case core::BuiltinValue::kLocalInvocationId:
                return "gl_LocalInvocationID";
            case core::BuiltinValue::kLocalInvocationIndex:
                return "gl_LocalInvocationIndex";
            case core::BuiltinValue::kGlobalInvocationId:
                return "gl_GlobalInvocationID";
            case core::BuiltinValue::kNumWorkgroups:
                return "gl_NumWorkGroups";
            case core::BuiltinValue::kWorkgroupId:
                return "gl_WorkGroupID";
            case core::BuiltinValue::kSampleIndex:
                return "gl_SampleID";
            case core::BuiltinValue::kSampleMask: {
                if (address_space == core::AddressSpace::kIn) {
                    return "gl_SampleMaskIn";
                } else {
                    return "gl_SampleMask";
                }
                TINT_UNREACHABLE();
            }
            case core::BuiltinValue::kPointSize:
                return "gl_PointSize";
            default:
                TINT_UNREACHABLE();
        }
    }

    /// Declare a global variable for each IO entry listed in @p entries.
    /// @param vars the list of variables
    /// @param entries the entries to emit
    /// @param addrspace the address to use for the global variables
    /// @param access the access mode to use for the global variables
    /// @param name_suffix the suffix to add to struct and variable names
    void MakeVars(Vector<core::ir::Var*, 4>& vars,
                  Vector<core::type::Manager::StructMemberDesc, 4>& entries,
                  core::AddressSpace addrspace,
                  core::Access access,
                  const char* name_suffix) {
        for (auto io : entries) {
            StringStream name;

            if (io.attributes.builtin) {
                name << GLSLBuiltinToString(*io.attributes.builtin, addrspace);
            } else {
                name << ir.NameOf(func).Name();

                if (io.attributes.location) {
                    name << "_loc" << io.attributes.location.value();
                    if (io.attributes.blend_src.has_value()) {
                        name << "_idx" << io.attributes.blend_src.value();
                    }
                }
                name << name_suffix;
            }

            // Create an IO variable and add it to the root block.
            auto* ptr = ty.ptr(addrspace, io.type, access);
            auto* var = b.Var(name.str(), ptr);
            var->SetAttributes(io.attributes);
            ir.root_block->Append(var);
            vars.Push(var);
        }
    }

    /// @copydoc ShaderIO::BackendState::FinalizeInputs
    Vector<core::ir::FunctionParam*, 4> FinalizeInputs() override {
        MakeVars(input_vars, inputs, core::AddressSpace::kIn, core::Access::kRead, "_Input");
        return tint::Empty;
    }

    /// @copydoc ShaderIO::BackendState::FinalizeOutputs
    const core::type::Type* FinalizeOutputs() override {
        MakeVars(output_vars, outputs, core::AddressSpace::kOut, core::Access::kWrite, "_Output");
        return ty.void_();
    }

    /// @copydoc ShaderIO::BackendState::GetInput
    core::ir::Value* GetInput(core::ir::Builder& builder, uint32_t idx) override {
        auto* from = input_vars[idx]->Result(0);
        auto* value = builder.Load(from)->Result(0);

        // TODO(dsinclair): Enable when `bitcast` is supported
        // auto& builtin = inputs[idx].attributes.builtin;
        // if (builtin.has_value()) {
        //     switch (builtin.value()) {
        //         case core::BuiltinValue::kVertexIndex:
        //         case core::BuiltinValue::kInstanceIndex:
        //         case core::BuiltinValue::kSampleIndex: {
        //             // GLSL uses i32 for these, so bitcast to u32.
        //             value = builder.Bitcast(ty.u32(), value)->Result(0);
        //             break;
        //         }
        //         case core::BuiltinValue::kSampleMask: {
        //             // gl_SampleMask is an array of i32. Retrieve the first element and
        //             // bitcast it to u32.
        //             auto* ptr = ty.ptr(core::AddressSpace::kOut, ty.i32(), core::Access::kWrite);

        //             auto* elem = builder.Load(builder.Access(ptr, value, 0_u));
        //             value = builder.Bitcast(ty.u32(), elem)->Result(0);
        //             break;
        //         }
        //         default:
        //             break;
        //     }
        // }

        return value;
    }

    /// @copydoc ShaderIO::BackendState::SetOutput
    void SetOutput(core::ir::Builder& builder, uint32_t idx, core::ir::Value* value) override {
        // Clamp frag_depth values if necessary.
        if (outputs[idx].attributes.builtin == core::BuiltinValue::kFragDepth) {
            value = ClampFragDepth(builder, value);
        }

        // Store the output to the global variable declared earlier.
        auto* to = output_vars[idx]->Result(0);
        builder.Store(to, value);

        if (outputs[idx].attributes.builtin == core::BuiltinValue::kPosition) {
            // Negate the gl_Position.y value
            auto* y = builder.Swizzle(ty.f32(), output_vars[idx], {1});
            auto* negate = builder.Negation(ty.f32(), y);
            builder.StoreVectorElement(output_vars[idx], 1_u, negate);

            // Recalculate gl_Position.z = ((2.0f * gl_Position.z) - gl_Position.w);
            auto* z = builder.Swizzle(ty.f32(), output_vars[idx], {2});
            auto* w = builder.Swizzle(ty.f32(), output_vars[idx], {3});
            auto* mul = builder.Multiply(ty.f32(), 2_f, z);
            auto* sub = builder.Subtract(ty.f32(), mul, w);
            builder.StoreVectorElement(output_vars[idx], 2_u, sub);
        }
    }

    /// Clamp a frag_depth builtin value if necessary.
    /// @param builder the builder to use for new instructions
    /// @param frag_depth the incoming frag_depth value
    /// @returns the clamped value
    core::ir::Value* ClampFragDepth([[maybe_unused]] core::ir::Builder& builder,
                                    core::ir::Value* frag_depth) {
        // TODO(dsinclair): Add a clamp frag depth implementation. The `depth_offsets.{min,max}` are
        // offsets into a push_constant structure, not the actual values as was done here.
        //
        // This needs to create a the `push_constant` structure (or append to the existing one if it
        // was already created) and add these members.
        return frag_depth;
    }

    /// @copydoc ShaderIO::BackendState::NeedsVertexPointSize
    bool NeedsVertexPointSize() const override { return true; }
};

}  // namespace

Result<SuccessType> ShaderIO(core::ir::Module& ir, const ShaderIOConfig& config) {
    auto result = ValidateAndDumpIfNeeded(ir, "ShaderIO transform");
    if (result != Success) {
        return result;
    }

    core::ir::transform::RunShaderIOBase(ir, [&](core::ir::Module& mod, core::ir::Function* func) {
        return std::make_unique<StateImpl>(mod, func, config);
    });

    return Success;
}

}  // namespace tint::glsl::writer::raise