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/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";

bool ShouldRun(const Program& program) {
    for (auto* fn : program.AST().Functions()) {
        if (fn->PipelineStage() == PipelineStage::kVertex) {
            return true;
        }
    }
    return false;
}

}  // namespace

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

Transform::ApplyResult OffsetFirstIndex::Apply(const Program& src,
                                               const DataMap& inputs,
                                               DataMap&) const {
    if (!ShouldRun(src)) {
        return SkipTransform;
    }

    const Config* cfg = inputs.Get<Config>();
    if (!cfg) {
        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;

    bool has_vertex_index = false;
    bool has_instance_index = false;

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

    if (!has_vertex_index && !has_instance_index) {
        return SkipTransform;
    }

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

    const ast::Variable* push_constants_var = nullptr;

    // Find first push_constant.
    for (auto* global : src.AST().GlobalVariables()) {
        if (auto* var = global->As<ast::Var>()) {
            auto* v = src.Sem().Get(var);
            if (v->AddressSpace() == core::AddressSpace::kPushConstant) {
                push_constants_var = var;
                auto* str = v->Type()->UnwrapRef()->As<sem::Struct>();
                if (!str) {
                    TINT_ICE() << "expected var<push_constant> type to be struct. Was "
                                  "AddBlockAttribute run?";
                }
                for (auto* member : str->Members()) {
                    members.Push(ctx.CloneWithoutTransform(member->Declaration()));
                }
            }
        }
    }

    // Add push constant members and calculate byte offsets
    if (has_vertex_index) {
        members.Push(b.Member(kFirstVertexName, b.ty.u32(),
                              Vector{b.MemberOffset(AInt(*cfg->first_vertex_offset))}));
    }
    if (has_instance_index) {
        members.Push(b.Member(kFirstInstanceName, b.ty.u32(),
                              Vector{b.MemberOffset(AInt(*cfg->first_instance_offset))}));
    }

    auto new_struct = b.Structure(b.Symbols().New("PushConstants"), std::move(members));

    Symbol buffer_name;

    // If this is the first use of push constants, create a global to hold them.
    if (!push_constants_var) {
        b.Enable(wgsl::Extension::kChromiumExperimentalPushConstant);

        buffer_name = b.Symbols().New("push_constants");
        b.GlobalVar(buffer_name, b.ty.Of(new_struct), core::AddressSpace::kPushConstant);
    } else {
        buffer_name = ctx.Clone(push_constants_var->name->symbol);
    }

    // Replace all variable users of the old struct with the new struct.
    if (push_constants_var) {
        ctx.ReplaceAll([&](const ast::Variable* var) -> const ast::Variable* {
            if (var->type == push_constants_var->type) {
                if (var->As<ast::Parameter>()) {
                    return ctx.dst->Param(ctx.Clone(var->name->symbol), b.ty.Of(new_struct),
                                          ctx.Clone(var->attributes));
                } else {
                    return ctx.dst->Var(ctx.Clone(var->name->symbol), b.ty.Of(new_struct),
                                        ctx.Clone(var->attributes),
                                        core::AddressSpace::kPushConstant);
                }
            }
            return nullptr;
        });
    }

    // 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(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