src/tint/transform/expand_compound_assignment.cc

// Copyright 2022 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/transform/expand_compound_assignment.h"

#include <utility>

#include "src/tint/ast/compound_assignment_statement.h"
#include "src/tint/ast/increment_decrement_statement.h"
#include "src/tint/program_builder.h"
#include "src/tint/sem/block_statement.h"
#include "src/tint/sem/expression.h"
#include "src/tint/sem/for_loop_statement.h"
#include "src/tint/sem/statement.h"
#include "src/tint/transform/utils/hoist_to_decl_before.h"

TINT_INSTANTIATE_TYPEINFO(tint::transform::ExpandCompoundAssignment);

namespace tint::transform {

ExpandCompoundAssignment::ExpandCompoundAssignment() = default;

ExpandCompoundAssignment::~ExpandCompoundAssignment() = default;

bool ExpandCompoundAssignment::ShouldRun(const Program* program,
                                         const DataMap&) const {
  for (auto* node : program->ASTNodes().Objects()) {
    if (node->IsAnyOf<ast::CompoundAssignmentStatement,
                      ast::IncrementDecrementStatement>()) {
      return true;
    }
  }
  return false;
}

/// Internal class used to collect statement expansions during the transform.
class State {
 private:
  /// The clone context.
  CloneContext& ctx;

  /// The program builder.
  ProgramBuilder& b;

  /// The HoistToDeclBefore helper instance.
  HoistToDeclBefore hoist_to_decl_before;

 public:
  /// Constructor
  /// @param context the clone context
  explicit State(CloneContext& context)
      : ctx(context), b(*ctx.dst), hoist_to_decl_before(ctx) {}

  /// Replace `stmt` with a regular assignment statement of the form:
  ///     lhs = lhs op rhs
  /// The LHS expression will only be evaluated once, and any side effects will
  /// be hoisted to `let` declarations above the assignment statement.
  /// @param stmt the statement to replace
  /// @param lhs the lhs expression from the source statement
  /// @param rhs the rhs expression in the destination module
  /// @param op the binary operator
  void Expand(const ast::Statement* stmt,
              const ast::Expression* lhs,
              const ast::Expression* rhs,
              ast::BinaryOp op) {
    // Helper function to create the new LHS expression. This will be called
    // twice when building the non-compound assignment statement, so must
    // not produce expressions that cause side effects.
    std::function<const ast::Expression*()> new_lhs;

    // Helper function to create a variable that is a pointer to `expr`.
    auto hoist_pointer_to = [&](const ast::Expression* expr) {
      auto name = b.Sym();
      auto* ptr = b.AddressOf(ctx.Clone(expr));
      auto* decl = b.Decl(b.Let(name, nullptr, ptr));
      hoist_to_decl_before.InsertBefore(ctx.src->Sem().Get(stmt), decl);
      return name;
    };

    // Helper function to hoist `expr` to a let declaration.
    auto hoist_expr_to_let = [&](const ast::Expression* expr) {
      auto name = b.Sym();
      auto* decl = b.Decl(b.Let(name, nullptr, ctx.Clone(expr)));
      hoist_to_decl_before.InsertBefore(ctx.src->Sem().Get(stmt), decl);
      return name;
    };

    // Helper function that returns `true` if the type of `expr` is a vector.
    auto is_vec = [&](const ast::Expression* expr) {
      return ctx.src->Sem().Get(expr)->Type()->UnwrapRef()->Is<sem::Vector>();
    };

    // Hoist the LHS expression subtree into local constants to produce a new
    // LHS that we can evaluate twice.
    // We need to special case compound assignments to vector components since
    // we cannot take the address of a vector component.
    auto* index_accessor = lhs->As<ast::IndexAccessorExpression>();
    auto* member_accessor = lhs->As<ast::MemberAccessorExpression>();
    if (lhs->Is<ast::IdentifierExpression>() ||
        (member_accessor &&
         member_accessor->structure->Is<ast::IdentifierExpression>())) {
      // This is the simple case with no side effects, so we can just use the
      // original LHS expression directly.
      // Before:
      //     foo.bar += rhs;
      // After:
      //     foo.bar = foo.bar + rhs;
      new_lhs = [&]() { return ctx.Clone(lhs); };
    } else if (index_accessor && is_vec(index_accessor->object)) {
      // This is the case for vector component via an array accessor. We need
      // to capture a pointer to the vector and also the index value.
      // Before:
      //     v[idx()] += rhs;
      // After:
      //     let vec_ptr = &v;
      //     let index = idx();
      //     (*vec_ptr)[index] = (*vec_ptr)[index] + rhs;
      auto lhs_ptr = hoist_pointer_to(index_accessor->object);
      auto index = hoist_expr_to_let(index_accessor->index);
      new_lhs = [&, lhs_ptr, index]() {
        return b.IndexAccessor(b.Deref(lhs_ptr), index);
      };
    } else if (member_accessor && is_vec(member_accessor->structure)) {
      // This is the case for vector component via a member accessor. We just
      // need to capture a pointer to the vector.
      // Before:
      //     a[idx()].y += rhs;
      // After:
      //     let vec_ptr = &a[idx()];
      //     (*vec_ptr).y = (*vec_ptr).y + rhs;
      auto lhs_ptr = hoist_pointer_to(member_accessor->structure);
      new_lhs = [&, lhs_ptr]() {
        return b.MemberAccessor(b.Deref(lhs_ptr),
                                ctx.Clone(member_accessor->member));
      };
    } else {
      // For all other statements that may have side-effecting expressions, we
      // just need to capture a pointer to the whole LHS.
      // Before:
      //     a[idx()] += rhs;
      // After:
      //     let lhs_ptr = &a[idx()];
      //     (*lhs_ptr) = (*lhs_ptr) + rhs;
      auto lhs_ptr = hoist_pointer_to(lhs);
      new_lhs = [&, lhs_ptr]() { return b.Deref(lhs_ptr); };
    }

    // Replace the statement with a regular assignment statement.
    auto* value = b.create<ast::BinaryExpression>(op, new_lhs(), rhs);
    ctx.Replace(stmt, b.Assign(new_lhs(), value));
  }

  /// Finalize the transformation and clone the module.
  void Finalize() {
    hoist_to_decl_before.Apply();
    ctx.Clone();
  }
};

void ExpandCompoundAssignment::Run(CloneContext& ctx,
                                   const DataMap&,
                                   DataMap&) const {
  State state(ctx);
  for (auto* node : ctx.src->ASTNodes().Objects()) {
    if (auto* assign = node->As<ast::CompoundAssignmentStatement>()) {
      state.Expand(assign, assign->lhs, ctx.Clone(assign->rhs), assign->op);
    } else if (auto* inc_dec = node->As<ast::IncrementDecrementStatement>()) {
      // For increment/decrement statements, `i++` becomes `i = i + 1`.
      // TODO(jrprice): Simplify this when we have untyped literals.
      auto* sem_lhs = ctx.src->Sem().Get(inc_dec->lhs);
      const ast::IntLiteralExpression* one =
          sem_lhs->Type()->UnwrapRef()->is_signed_integer_scalar()
              ? ctx.dst->Expr(1)->As<ast::IntLiteralExpression>()
              : ctx.dst->Expr(1u)->As<ast::IntLiteralExpression>();
      auto op =
          inc_dec->increment ? ast::BinaryOp::kAdd : ast::BinaryOp::kSubtract;
      state.Expand(inc_dec, inc_dec->lhs, one, op);
    }
  }
  state.Finalize();
}

}  // namespace tint::transform