src/tint/lang/core/ir/transform/var_for_dynamic_index.cc

// Copyright 2023 The Tint Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "src/tint/lang/core/ir/transform/var_for_dynamic_index.h"

#include <utility>

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

TINT_INSTANTIATE_TYPEINFO(tint::ir::transform::VarForDynamicIndex);

using namespace tint::number_suffixes;  // NOLINT

namespace tint::ir::transform {

namespace {
// An access that needs replacing.
struct AccessToReplace {
    // The access instruction.
    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 type::Type* dynamic_index_source_type = nullptr;
    // If the access indexes a vector, then the type of that vector
    const 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.
    Value* base = nullptr;
    // The list of constant indices to get from the base to the source object.
    utils::Vector<Value*, 4> indices;

    // A specialization of utils::Hasher for PartialAccess.
    struct Hasher {
        inline std::size_t operator()(const PartialAccess& src) const {
            return utils::Hash(src.base, src.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(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<Constant>();
        ty = const_idx ? ty->Element(const_idx->Value()->ValueAs<u32>()) : ty->Elements().type;
    }
}

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

    std::optional<AccessToReplace> result;
    WalkAccessChain(access, [&](size_t i, ir::Value* index, const type::Type* ty) {
        if (auto* vec = ty->As<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<Constant>()) {
            result = AccessToReplace{access, i, ty};
        }

        return Action::kContinue;
    });

    return result;
}

}  // namespace

VarForDynamicIndex::VarForDynamicIndex() = default;

VarForDynamicIndex::~VarForDynamicIndex() = default;

void VarForDynamicIndex::Run(ir::Module* ir, const DataMap&, DataMap&) const {
    ir::Builder builder(*ir);

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

    // Replace each access instruction that we recorded.
    utils::Hashmap<Value*, Value*, 4> object_to_local;
    utils::Hashmap<PartialAccess, Value*, 4, PartialAccess::Hasher> 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.GetOrCreate(partial_access, [&] {
                auto* intermediate_source = builder.Access(to_replace.dynamic_index_source_type,
                                                           source_object, partial_access.indices);
                intermediate_source->InsertBefore(access);
                return intermediate_source->Result();
            });
        }

        // Declare a local variable and copy the source object to it.
        auto* local = object_to_local.GetOrCreate(source_object, [&] {
            auto* decl =
                builder.Var(ir->Types().ptr(builtin::AddressSpace::kFunction, source_object->Type(),
                                            builtin::Access::kReadWrite));
            decl->SetInitializer(source_object);
            decl->InsertBefore(access);
            return decl->Result();
        });

        // Create a new access instruction using the local variable as the source.
        utils::Vector<Value*, 4> indices{access->Indices().Offset(to_replace.first_dynamic_index)};
        const type::Type* access_type = access->Result()->Type();
        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();
        }

        ir::Instruction* new_access =
            builder.Access(ir->Types().ptr(builtin::AddressSpace::kFunction, access_type,
                                           builtin::Access::kReadWrite),
                           local, indices);
        new_access->InsertBefore(access);

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

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

}  // namespace tint::ir::transform