src/tint/lang/wgsl/ast/transform/array_length_from_uniform.cc

// Copyright 2021 The Dawn & Tint Authors
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#include "src/tint/lang/wgsl/ast/transform/array_length_from_uniform.h"

#include <memory>
#include <string>
#include <utility>

#include "src/tint/lang/core/fluent_types.h"
#include "src/tint/lang/wgsl/ast/transform/simplify_pointers.h"
#include "src/tint/lang/wgsl/program/clone_context.h"
#include "src/tint/lang/wgsl/program/program_builder.h"
#include "src/tint/lang/wgsl/resolver/resolve.h"
#include "src/tint/lang/wgsl/sem/call.h"
#include "src/tint/lang/wgsl/sem/function.h"
#include "src/tint/lang/wgsl/sem/statement.h"
#include "src/tint/lang/wgsl/sem/variable.h"

TINT_INSTANTIATE_TYPEINFO(tint::ast::transform::ArrayLengthFromUniform);
TINT_INSTANTIATE_TYPEINFO(tint::ast::transform::ArrayLengthFromUniform::Config);
TINT_INSTANTIATE_TYPEINFO(tint::ast::transform::ArrayLengthFromUniform::Result);

using namespace tint::core::fluent_types;  // NOLINT
                                           //
namespace tint::ast::transform {
namespace {

bool ShouldRun(const Program& program) {
    for (auto* fn : program.AST().Functions()) {
        if (auto* sem_fn = program.Sem().Get(fn)) {
            for (auto* builtin : sem_fn->DirectlyCalledBuiltins()) {
                if (builtin->Fn() == wgsl::BuiltinFn::kArrayLength) {
                    return true;
                }
            }
        }
    }
    return false;
}

}  // namespace

ArrayLengthFromUniform::ArrayLengthFromUniform() = default;
ArrayLengthFromUniform::~ArrayLengthFromUniform() = default;

/// PIMPL state for the transform
struct ArrayLengthFromUniform::State {
    /// Constructor
    /// @param program the source program
    /// @param in the input transform data
    /// @param out the output transform data
    explicit State(const Program& program, const DataMap& in, DataMap& out)
        : src(program), inputs(in), outputs(out) {}

    /// Runs the transform
    /// @returns the new program or SkipTransform if the transform is not required
    ApplyResult Run() {
        auto* cfg = inputs.Get<Config>();
        if (cfg == nullptr) {
            b.Diagnostics().AddError(diag::System::Transform, Source{})
                << "missing transform data for "
                << tint::TypeInfo::Of<ArrayLengthFromUniform>().name;
            return resolver::Resolve(b);
        }

        if (!ShouldRun(src)) {
            return SkipTransform;
        }

        const char* kBufferSizeMemberName = "buffer_size";

        // Determine the size of the buffer size array.
        uint32_t max_buffer_size_index = 0;

        IterateArrayLengthOnStorageVar(
            [&](const CallExpression*, const sem::VariableUser*, const sem::GlobalVariable* var) {
                if (auto binding = var->Attributes().binding_point) {
                    auto idx_itr = cfg->bindpoint_to_size_index.find(*binding);
                    if (idx_itr == cfg->bindpoint_to_size_index.end()) {
                        return;
                    }
                    if (idx_itr->second > max_buffer_size_index) {
                        max_buffer_size_index = idx_itr->second;
                    }
                }
            });

        // Get (or create, on first call) the uniform buffer that will receive the
        // size of each storage buffer in the module.
        const Variable* buffer_size_ubo = nullptr;
        auto get_ubo = [&] {
            if (!buffer_size_ubo) {
                // Emit an array<vec4<u32>, N>, where N is 1/4 number of elements.
                // We do this because UBOs require an element stride that is 16-byte
                // aligned.
                auto* buffer_size_struct = b.Structure(
                    b.Sym(), tint::Vector{
                                 b.Member(kBufferSizeMemberName,
                                          b.ty.array(b.ty.vec4(b.ty.u32()),
                                                     u32((max_buffer_size_index / 4) + 1))),
                             });
                buffer_size_ubo =
                    b.GlobalVar(b.Sym(), b.ty.Of(buffer_size_struct), core::AddressSpace::kUniform,
                                b.Group(AInt(cfg->ubo_binding.group)),
                                b.Binding(AInt(cfg->ubo_binding.binding)));
            }
            return buffer_size_ubo;
        };

        std::unordered_set<uint32_t> used_size_indices;

        IterateArrayLengthOnStorageVar([&](const CallExpression* call_expr,
                                           const sem::VariableUser* storage_buffer_sem,
                                           const sem::GlobalVariable* var) {
            auto binding = var->Attributes().binding_point;
            if (!binding) {
                return;
            }
            auto idx_itr = cfg->bindpoint_to_size_index.find(*binding);
            if (idx_itr == cfg->bindpoint_to_size_index.end()) {
                return;
            }

            uint32_t size_index = idx_itr->second;
            used_size_indices.insert(size_index);

            // Load the total storage buffer size from the UBO.
            uint32_t array_index = size_index / 4;
            auto* vec_expr = b.IndexAccessor(
                b.MemberAccessor(get_ubo()->name->symbol, kBufferSizeMemberName), u32(array_index));
            uint32_t vec_index = size_index % 4;
            auto* total_storage_buffer_size = b.IndexAccessor(vec_expr, u32(vec_index));

            // Calculate actual array length
            //                total_storage_buffer_size - array_offset
            // array_length = ----------------------------------------
            //                             array_stride
            const Expression* total_size = total_storage_buffer_size;
            if (TINT_UNLIKELY(storage_buffer_sem->Type()->Is<core::type::Pointer>())) {
                TINT_ICE() << "storage buffer variable should not be a pointer. These should have "
                              "been removed by the SimplifyPointers transform";
                return;
            }
            auto* storage_buffer_type = storage_buffer_sem->Type()->UnwrapRef();
            const core::type::Array* array_type = nullptr;
            if (auto* str = storage_buffer_type->As<core::type::Struct>()) {
                // The variable is a struct, so subtract the byte offset of the array
                // member.
                auto* array_member_sem = str->Members().Back();
                array_type = array_member_sem->Type()->As<core::type::Array>();
                total_size = b.Sub(total_storage_buffer_size, u32(array_member_sem->Offset()));
            } else if (auto* arr = storage_buffer_type->As<core::type::Array>()) {
                array_type = arr;
            } else {
                TINT_ICE() << "expected form of arrayLength argument to be &array_var or "
                              "&struct_var.array_member";
                return;
            }
            auto* array_length = b.Div(total_size, u32(array_type->Stride()));

            ctx.Replace(call_expr, array_length);
        });

        outputs.Add<Result>(used_size_indices);

        ctx.Clone();
        return resolver::Resolve(b);
    }

  private:
    /// The source program
    const Program& src;
    /// The transform inputs
    const DataMap& inputs;
    /// The transform outputs
    DataMap& outputs;
    /// The target program builder
    ProgramBuilder b;
    /// The clone context
    program::CloneContext ctx = {&b, &src, /* auto_clone_symbols */ true};

    /// Iterate over all arrayLength() builtins that operate on
    /// storage buffer variables.
    /// @param functor of type void(const CallExpression*, const
    /// sem::VariableUser, const sem::GlobalVariable*). It takes in an
    /// CallExpression of the arrayLength call expression node, a
    /// sem::VariableUser of the used storage buffer variable, and the
    /// sem::GlobalVariable for the storage buffer.
    template <typename F>
    void IterateArrayLengthOnStorageVar(F&& functor) {
        auto& sem = src.Sem();

        // Find all calls to the arrayLength() builtin.
        for (auto* node : src.ASTNodes().Objects()) {
            auto* call_expr = node->As<CallExpression>();
            if (!call_expr) {
                continue;
            }

            auto* call = sem.Get(call_expr)->UnwrapMaterialize()->As<sem::Call>();
            auto* builtin = call->Target()->As<sem::BuiltinFn>();
            if (!builtin || builtin->Fn() != wgsl::BuiltinFn::kArrayLength) {
                continue;
            }

            if (auto* call_stmt = call->Stmt()->Declaration()->As<CallStatement>()) {
                if (call_stmt->expr == call_expr) {
                    // arrayLength() is used as a statement.
                    // The argument expression must be side-effect free, so just drop the statement.
                    RemoveStatement(ctx, call_stmt);
                    continue;
                }
            }

            // Get the storage buffer that contains the runtime array.
            // Since we require SimplifyPointers, we can assume that the arrayLength()
            // call has one of two forms:
            //   arrayLength(&struct_var.array_member)
            //   arrayLength(&array_var)
            auto* param = call_expr->args[0]->As<UnaryOpExpression>();
            if (TINT_UNLIKELY(!param || param->op != core::UnaryOp::kAddressOf)) {
                TINT_ICE() << "expected form of arrayLength argument to be &array_var or "
                              "&struct_var.array_member";
                break;
            }
            auto* storage_buffer_expr = param->expr;
            if (auto* accessor = param->expr->As<MemberAccessorExpression>()) {
                storage_buffer_expr = accessor->object;
            }
            auto* storage_buffer_sem = sem.Get<sem::VariableUser>(storage_buffer_expr);
            if (TINT_UNLIKELY(!storage_buffer_sem)) {
                TINT_ICE() << "expected form of arrayLength argument to be &array_var or "
                              "&struct_var.array_member";
                break;
            }

            // Get the index to use for the buffer size array.
            auto* var = tint::As<sem::GlobalVariable>(storage_buffer_sem->Variable());
            if (TINT_UNLIKELY(!var)) {
                TINT_ICE() << "storage buffer is not a global variable";
                break;
            }
            functor(call_expr, storage_buffer_sem, var);
        }
    }
};

Transform::ApplyResult ArrayLengthFromUniform::Apply(const Program& src,
                                                     const DataMap& inputs,
                                                     DataMap& outputs) const {
    return State{src, inputs, outputs}.Run();
}

ArrayLengthFromUniform::Config::Config(BindingPoint ubo_bp) : ubo_binding(ubo_bp) {}
ArrayLengthFromUniform::Config::Config(const Config&) = default;
ArrayLengthFromUniform::Config& ArrayLengthFromUniform::Config::operator=(const Config&) = default;
ArrayLengthFromUniform::Config::~Config() = default;

ArrayLengthFromUniform::Result::Result(std::unordered_set<uint32_t> used_size_indices_in)
    : used_size_indices(std::move(used_size_indices_in)) {}
ArrayLengthFromUniform::Result::Result(const Result&) = default;
ArrayLengthFromUniform::Result::~Result() = default;

}  // namespace tint::ast::transform