src/tint/lang/spirv/writer/raise/var_for_dynamic_index.cc

// Copyright 2023 The Dawn & Tint Authors
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#include "src/tint/lang/spirv/writer/raise/var_for_dynamic_index.h"

#include <utility>

#include "src/tint/lang/core/fluent_types.h"
#include "src/tint/lang/core/ir/builder.h"
#include "src/tint/lang/core/ir/module.h"
#include "src/tint/lang/core/ir/validator.h"
#include "src/tint/lang/core/type/array.h"
#include "src/tint/lang/core/type/matrix.h"
#include "src/tint/lang/core/type/pointer.h"
#include "src/tint/lang/core/type/vector.h"
#include "src/tint/utils/containers/hashmap.h"

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

namespace tint::spirv::writer::raise {

namespace {
// An access that needs replacing.
struct AccessToReplace {
    // The access instruction.
    core::ir::Access* access = nullptr;
    // The index of the first dynamic index.
    size_t first_dynamic_index = 0;
    // The object type that corresponds to the source of the first dynamic index.
    const core::type::Type* dynamic_index_source_type = nullptr;
    // If the access indexes a vector, then the type of that vector
    const core::type::Vector* vector_access_type = nullptr;
};

// A partial access chain that uses constant indices to get to an object that will be
// dynamically indexed.
struct PartialAccess {
    // The base object.
    core::ir::Value* base = nullptr;
    // The list of constant indices to get from the base to the source object.
    Vector<core::ir::Value*, 4> indices;

    /// @returns the hash code of the PartialAccess
    tint::HashCode HashCode() const { return Hash(base, indices); }

    // An equality helper for PartialAccess.
    bool operator==(const PartialAccess& other) const {
        return base == other.base && indices == other.indices;
    }
};

enum class Action { kStop, kContinue };

template <typename CALLBACK>
void WalkAccessChain(core::ir::Access* access, CALLBACK&& callback) {
    auto indices = access->Indices();
    auto* ty = access->Object()->Type();
    for (size_t i = 0; i < indices.Length(); i++) {
        if (callback(i, indices[i], ty) == Action::kStop) {
            break;
        }
        auto* const_idx = indices[i]->As<core::ir::Constant>();
        ty = const_idx ? ty->Element(const_idx->Value()->ValueAs<u32>()) : ty->Elements().type;
    }
}

std::optional<AccessToReplace> ShouldReplace(core::ir::Access* access) {
    if (access->Result(0)->Type()->Is<core::type::Pointer>()) {
        // No need to modify accesses into pointer types.
        return {};
    }

    std::optional<AccessToReplace> result;
    WalkAccessChain(access, [&](size_t i, core::ir::Value* index, const core::type::Type* ty) {
        if (auto* vec = ty->As<core::type::Vector>()) {
            // If we haven't found a dynamic index before the vector, then the transform doesn't
            // need to hoist the access into a var as a vector value can be dynamically indexed.
            // If we have found a dynamic index before the vector, then make a note that we're
            // indexing a vector as we can't obtain a pointer to a vector element, so this needs to
            // be handled specially.
            if (result) {
                result->vector_access_type = vec;
            }
            return Action::kStop;
        }

        // Check if this is the first dynamic index.
        if (!result && !index->Is<core::ir::Constant>()) {
            result = AccessToReplace{access, i, ty};
        }

        return Action::kContinue;
    });

    return result;
}

void Run(core::ir::Module& ir) {
    core::ir::Builder builder(ir);

    // Find the access instructions that need replacing.
    Vector<AccessToReplace, 4> worklist;
    for (auto* inst : ir.Instructions()) {
        if (auto* access = inst->As<core::ir::Access>()) {
            if (auto to_replace = ShouldReplace(access)) {
                worklist.Push(to_replace.value());
            }
        }
    }

    // Replace each access instruction that we recorded.
    Hashmap<core::ir::Value*, core::ir::Value*, 4> object_to_var;
    Hashmap<PartialAccess, core::ir::Value*, 4> source_object_to_value;
    for (const auto& to_replace : worklist) {
        auto* access = to_replace.access;
        auto* source_object = access->Object();

        // If the access starts with at least one constant index, extract the source of the first
        // dynamic access to avoid copying the whole object.
        if (to_replace.first_dynamic_index > 0) {
            PartialAccess partial_access = {
                access->Object(), access->Indices().Truncate(to_replace.first_dynamic_index)};
            source_object =
                source_object_to_value.GetOrAdd(partial_access, [&]() -> core::ir::Value* {
                    // If the source is a constant, then the partial access will also produce a
                    // constant. Extract the constant::Value and use that as the new source object.
                    if (source_object->Is<core::ir::Constant>()) {
                        for (const auto& i : partial_access.indices) {
                            auto idx = i->As<core::ir::Constant>()->Value()->ValueAs<uint32_t>();
                            source_object = builder.Constant(
                                source_object->As<core::ir::Constant>()->Value()->Index(idx));
                        }
                        return source_object;
                    }

                    // Extract a non-constant intermediate source using an access instruction that
                    // we insert immediately after the definition of the root source object.
                    auto* intermediate_source =
                        builder.Access(to_replace.dynamic_index_source_type, source_object,
                                       partial_access.indices);
                    builder.InsertAfter(source_object,
                                        [&] { builder.Append(intermediate_source); });
                    return intermediate_source->Result(0);
                });
        }

        // Declare a variable and copy the source object to it.
        auto* var = object_to_var.GetOrAdd(source_object, [&] {
            // If the source object is a constant we use a module-scope variable, as it could be
            // indexed by multiple functions. Otherwise, we declare a function-scope variable
            // immediately after the definition of the source object.
            core::ir::Var* decl = nullptr;
            if (source_object->Is<core::ir::Constant>()) {
                decl = builder.Var(ir.Types().ptr(core::AddressSpace::kPrivate,
                                                  source_object->Type(), core::Access::kReadWrite));
                ir.root_block->Append(decl);
            } else {
                builder.InsertAfter(source_object, [&] {
                    decl = builder.Var(ir.Types().ptr(core::AddressSpace::kFunction,
                                                      source_object->Type(),
                                                      core::Access::kReadWrite));

                    // If we ever support value declarations at module-scope, we will need to modify
                    // the partial access logic above since `access` instructions cannot be used in
                    // the root block.
                    TINT_ASSERT(decl->Block() != ir.root_block);
                });
            }

            decl->SetInitializer(source_object);
            return decl->Result(0);
        });

        // Create a new access instruction using the new variable as the source.
        Vector<core::ir::Value*, 4> indices{
            access->Indices().Offset(to_replace.first_dynamic_index)};
        const core::type::Type* access_type = access->Result(0)->Type();
        core::ir::Value* vector_index = nullptr;
        if (to_replace.vector_access_type) {
            // The old access indexed the element of a vector.
            // Its not valid to obtain the address of an element of a vector, so we need to access
            // up to the vector, then use LoadVectorElement to load the element.
            // As a vector element is always a scalar, we know the last index of the access is the
            // index on the vector. Pop that index to obtain the index to pass to
            // LoadVectorElement(), and perform the rest of the access chain.
            access_type = to_replace.vector_access_type;
            vector_index = indices.Pop();
        }

        auto addrspace = var->Type()->As<core::type::Pointer>()->AddressSpace();
        core::ir::Instruction* new_access = builder.Access(
            ir.Types().ptr(addrspace, access_type, core::Access::kReadWrite), var, indices);
        new_access->InsertBefore(access);

        core::ir::Instruction* load = nullptr;
        if (to_replace.vector_access_type) {
            load = builder.LoadVectorElement(new_access->Result(0), vector_index);
        } else {
            load = builder.Load(new_access);
        }

        // Replace all uses of the old access instruction with the loaded result.
        access->Result(0)->ReplaceAllUsesWith(load->Result(0));
        access->ReplaceWith(load);
        access->Destroy();
    }
}

}  // namespace

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

    Run(ir);

    return Success;
}

}  // namespace tint::spirv::writer::raise