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

// Copyright 2024 The Dawn & Tint Authors
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#include "src/tint/lang/wgsl/ast/transform/offset_first_index.h"

#include <memory>
#include <unordered_map>
#include <utility>

#include "src/tint/lang/core/builtin_value.h"
#include "src/tint/lang/core/fluent_types.h"
#include "src/tint/lang/wgsl/ast/transform/push_constant_helper.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/function.h"
#include "src/tint/lang/wgsl/sem/member_accessor_expression.h"
#include "src/tint/lang/wgsl/sem/struct.h"
#include "src/tint/lang/wgsl/sem/variable.h"

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

TINT_INSTANTIATE_TYPEINFO(tint::ast::transform::OffsetFirstIndex);
TINT_INSTANTIATE_TYPEINFO(tint::ast::transform::OffsetFirstIndex::Config);

namespace tint::ast::transform {
namespace {

// Push constant names
constexpr char kFirstVertexName[] = "first_vertex";
constexpr char kFirstInstanceName[] = "first_instance";

}  // namespace

OffsetFirstIndex::OffsetFirstIndex() = default;
OffsetFirstIndex::~OffsetFirstIndex() = default;

Transform::ApplyResult OffsetFirstIndex::Apply(const Program& src,
                                               const DataMap& inputs,
                                               DataMap&) const {
    const Config* cfg = inputs.Get<Config>();
    if (!cfg) {
        return SkipTransform;
    }
    if (!cfg->first_vertex_offset.has_value() && !cfg->first_instance_offset.has_value()) {
        return SkipTransform;
    }

    ProgramBuilder b;
    program::CloneContext ctx{&b, &src, /* auto_clone_symbols */ true};

    // Map of builtin usages
    std::unordered_map<const sem::Variable*, const char*> builtin_vars;
    std::unordered_map<const core::type::StructMember*, const char*> builtin_members;

    // Traverse the AST scanning for builtin accesses via variables (includes
    // parameters) or structure member accesses.
    for (auto* node : src.ASTNodes().Objects()) {
        if (auto* var = node->As<Variable>()) {
            for (auto* attr : var->attributes) {
                if (auto* builtin_attr = attr->As<BuiltinAttribute>()) {
                    core::BuiltinValue builtin = builtin_attr->builtin;
                    if (builtin == core::BuiltinValue::kVertexIndex && cfg &&
                        cfg->first_vertex_offset.has_value()) {
                        auto* sem_var = src.Sem().Get(var);
                        builtin_vars.emplace(sem_var, kFirstVertexName);
                    }
                    if (builtin == core::BuiltinValue::kInstanceIndex && cfg &&
                        cfg->first_instance_offset.has_value()) {
                        auto* sem_var = src.Sem().Get(var);
                        builtin_vars.emplace(sem_var, kFirstInstanceName);
                    }
                }
            }
        }
        if (auto* member = node->As<StructMember>()) {
            for (auto* attr : member->attributes) {
                if (auto* builtin_attr = attr->As<BuiltinAttribute>()) {
                    core::BuiltinValue builtin = builtin_attr->builtin;
                    if (builtin == core::BuiltinValue::kVertexIndex && cfg &&
                        cfg->first_vertex_offset.has_value()) {
                        auto* sem_mem = src.Sem().Get(member);
                        builtin_members.emplace(sem_mem, kFirstVertexName);
                    }
                    if (builtin == core::BuiltinValue::kInstanceIndex && cfg &&
                        cfg->first_instance_offset.has_value()) {
                        auto* sem_mem = src.Sem().Get(member);
                        builtin_members.emplace(sem_mem, kFirstInstanceName);
                    }
                }
            }
        }
    }

    Vector<const ast::StructMember*, 8> members;

    PushConstantHelper helper(ctx);

    // Add push constant members and calculate byte offsets
    if (cfg->first_vertex_offset.has_value()) {
        helper.InsertMember(kFirstVertexName, b.ty.u32(), *cfg->first_vertex_offset);
    }
    if (cfg->first_instance_offset.has_value()) {
        helper.InsertMember(kFirstInstanceName, b.ty.u32(), *cfg->first_instance_offset);
    }

    Symbol buffer_name = helper.Run();

    // Fix up all references to the builtins with the offsets
    ctx.ReplaceAll([&](const Expression* expr) -> const Expression* {
        if (auto* sem = ctx.src->Sem().GetVal(expr)) {
            if (auto* user = sem->UnwrapLoad()->As<sem::VariableUser>()) {
                auto it = builtin_vars.find(user->Variable());
                if (it != builtin_vars.end()) {
                    return ctx.dst->Add(b.Bitcast(b.ty.u32(), ctx.CloneWithoutTransform(expr)),
                                        ctx.dst->MemberAccessor(buffer_name, it->second));
                }
            }
            if (auto* access = sem->As<sem::StructMemberAccess>()) {
                auto it = builtin_members.find(access->Member());
                if (it != builtin_members.end()) {
                    return ctx.dst->Add(b.Bitcast(b.ty.u32(), ctx.CloneWithoutTransform(expr)),
                                        ctx.dst->MemberAccessor(buffer_name, it->second));
                }
            }
        }
        // Not interested in this expression. Just clone.
        return nullptr;
    });

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

OffsetFirstIndex::Config::Config() = default;

OffsetFirstIndex::Config::Config(std::optional<int32_t> first_vertex_off,
                                 std::optional<int32_t> first_instance_off)
    : first_vertex_offset(first_vertex_off), first_instance_offset(first_instance_off) {}

OffsetFirstIndex::Config::~Config() = default;

}  // namespace tint::ast::transform