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// Copyright 2021 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include "src/tint/transform/simplify_pointers.h"
#include <memory>
#include <unordered_map>
#include <utility>
#include <vector>
#include "src/tint/program_builder.h"
#include "src/tint/sem/block_statement.h"
#include "src/tint/sem/function.h"
#include "src/tint/sem/statement.h"
#include "src/tint/sem/variable.h"
#include "src/tint/transform/unshadow.h"
namespace tint::transform {
namespace {
/// PointerOp describes either possible indirection or address-of action on an
/// expression.
struct PointerOp {
/// Positive: Number of times the `expr` was dereferenced (*expr)
/// Negative: Number of times the `expr` was 'addressed-of' (&expr)
/// Zero: no pointer op on `expr`
int indirections = 0;
/// The expression being operated on
const ast::Expression* expr = nullptr;
} // namespace
/// The PIMPL state for the SimplifyPointers transform
struct SimplifyPointers::State {
/// The clone context
CloneContext& ctx;
/// Constructor
/// @param context the clone context
explicit State(CloneContext& context) : ctx(context) {}
/// Traverses the expression `expr` looking for non-literal array indexing
/// expressions that would affect the computed address of a pointer
/// expression. The function-like argument `cb` is called for each found.
/// @param expr the expression to traverse
/// @param cb a function-like object with the signature
/// `void(const ast::Expression*)`, which is called for each array index
/// expression
template <typename F>
static void CollectSavedArrayIndices(const ast::Expression* expr, F&& cb) {
if (auto* a = expr->As<ast::IndexAccessorExpression>()) {
CollectSavedArrayIndices(a->object, cb);
if (!a->index->Is<ast::LiteralExpression>()) {
if (auto* m = expr->As<ast::MemberAccessorExpression>()) {
CollectSavedArrayIndices(m->structure, cb);
if (auto* u = expr->As<ast::UnaryOpExpression>()) {
CollectSavedArrayIndices(u->expr, cb);
// Note: Other ast::Expression types can be safely ignored as they cannot be
// used to generate a reference or pointer.
// See
/// Reduce walks the expression chain, collapsing all address-of and
/// indirection ops into a PointerOp.
/// @param in the expression to walk
/// @returns the reduced PointerOp
PointerOp Reduce(const ast::Expression* in) const {
PointerOp op{0, in};
while (true) {
if (auto* unary = op.expr->As<ast::UnaryOpExpression>()) {
switch (unary->op) {
case ast::UnaryOp::kIndirection:
op.expr = unary->expr;
case ast::UnaryOp::kAddressOf:
op.expr = unary->expr;
if (auto* user = ctx.src->Sem().Get<sem::VariableUser>(op.expr)) {
auto* var = user->Variable();
if (var->Is<sem::LocalVariable>() && //
var->Declaration()->is_const && //
var->Type()->Is<sem::Pointer>()) {
op.expr = var->Declaration()->constructor;
return op;
/// Performs the transformation
void Run() {
// A map of saved expressions to their saved variable name
std::unordered_map<const ast::Expression*, Symbol> saved_vars;
// Register the ast::Expression transform handler.
// This performs two different transformations:
// * Identifiers that resolve to the pointer-typed `let` declarations are
// replaced with the recursively inlined initializer expression for the
// `let` declaration.
// * Sub-expressions inside the pointer-typed `let` initializer expression
// that have been hoisted to a saved variable are replaced with the saved
// variable identifier.
ctx.ReplaceAll([&](const ast::Expression* expr) -> const ast::Expression* {
// Look to see if we need to swap this Expression with a saved variable.
auto it = saved_vars.find(expr);
if (it != saved_vars.end()) {
return ctx.dst->Expr(it->second);
// Reduce the expression, folding away chains of address-of / indirections
auto op = Reduce(expr);
// Clone the reduced root expression
expr = ctx.CloneWithoutTransform(op.expr);
// And reapply the minimum number of address-of / indirections
for (int i = 0; i < op.indirections; i++) {
expr = ctx.dst->Deref(expr);
for (int i = 0; i > op.indirections; i--) {
expr = ctx.dst->AddressOf(expr);
return expr;
// Find all the pointer-typed `let` declarations.
// Note that these must be function-scoped, as module-scoped `let`s are not
// permitted.
for (auto* node : ctx.src->ASTNodes().Objects()) {
if (auto* let = node->As<ast::VariableDeclStatement>()) {
if (!let->variable->is_const) {
continue; // Not a `let` declaration. Ignore.
auto* var = ctx.src->Sem().Get(let->variable);
if (!var->Type()->Is<sem::Pointer>()) {
continue; // Not a pointer type. Ignore.
// We're dealing with a pointer-typed `let` declaration.
// Scan the initializer expression for array index expressions that need
// to be hoist to temporary "saved" variables.
std::vector<const ast::VariableDeclStatement*> saved;
var->Declaration()->constructor, [&](const ast::Expression* idx_expr) {
// We have a sub-expression that needs to be saved.
// Create a new variable
auto saved_name = ctx.dst->Symbols().New(
ctx.src->Symbols().NameFor(var->Declaration()->symbol) + "_save");
auto* decl =
ctx.dst->Decl(ctx.dst->Let(saved_name, nullptr, ctx.Clone(idx_expr)));
// Record the substitution of `idx_expr` to the saved variable
// with the symbol `saved_name`. This will be used by the
// ReplaceAll() handler above.
saved_vars.emplace(idx_expr, saved_name);
// Find the place to insert the saved declarations.
// Special care needs to be made for lets declared as the initializer
// part of for-loops. In this case the block will hold the for-loop
// statement, not the let.
if (!saved.empty()) {
auto* stmt = ctx.src->Sem().Get(let);
auto* block = stmt->Block();
// Find the statement owned by the block (either the let decl or a
// for-loop)
while (block != stmt->Parent()) {
stmt = stmt->Parent();
// Declare the stored variables just before stmt. Order here is
// important as order-of-operations needs to be preserved.
// CollectSavedArrayIndices() visits the LHS of an index accessor
// before the index expression.
for (auto* decl : saved) {
// Note that repeated calls to InsertBefore() with the same `before`
// argument will result in nodes to inserted in the order the
// calls are made (last call is inserted last).
ctx.InsertBefore(block->Declaration()->statements, stmt->Declaration(),
// As the original `let` declaration will be fully inlined, there's no
// need for the original declaration to exist. Remove it.
RemoveStatement(ctx, let);
SimplifyPointers::SimplifyPointers() = default;
SimplifyPointers::~SimplifyPointers() = default;
void SimplifyPointers::Run(CloneContext& ctx, const DataMap&, DataMap&) const {
} // namespace tint::transform