DeadCodeElimination.ts — react Source File

Source Code

/**
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

import {
  BlockId,
  Environment,
  getHookKind,
  HIRFunction,
  Identifier,
  IdentifierId,
  Instruction,
  InstructionKind,
  InstructionValue,
  ObjectPattern,
} from '../HIR';
import {
  eachInstructionValueOperand,
  eachPatternOperand,
  eachTerminalOperand,
} from '../HIR/visitors';
import {assertExhaustive, retainWhere} from '../Utils/utils';

/*
 * Implements dead-code elimination, eliminating instructions whose values are unused.
 *
 * Note that unreachable blocks are already pruned during HIR construction.
 */
export function deadCodeElimination(fn: HIRFunction): void {
  /**
   * Phase 1: Find/mark all referenced identifiers
   * Usages may be visited AFTER declarations if there are circular phi / data dependencies
   * between blocks, so we wait to sweep until after fixed point iteration is complete
   */
  const state = findReferencedIdentifiers(fn);

  /**
   * Phase 2: Prune / sweep unreferenced identifiers and instructions
   * as possible (subject to HIR structural constraints)
   */
  for (const [, block] of fn.body.blocks) {
    for (const phi of block.phis) {
      if (!state.isIdOrNameUsed(phi.place.identifier)) {
        block.phis.delete(phi);
      }
    }
    retainWhere(block.instructions, instr =>
      state.isIdOrNameUsed(instr.lvalue.identifier),
    );
    // Rewrite retained instructions
    for (let i = 0; i < block.instructions.length; i++) {
      const isBlockValue =
        block.kind !== 'block' && i === block.instructions.length - 1;
      if (!isBlockValue) {
        rewriteInstruction(block.instructions[i], state);
      }
    }
// ... (367 more lines)