deadCodeElimination() — react Function Reference

Source Code

compiler/packages/babel-plugin-react-compiler/src/Optimization/DeadCodeElimination.ts lines 32–70

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);
      }
    }
  }

  /**
   * Constant propagation and DCE may have deleted or rewritten instructions
   * that reference context variables.
   */
  retainWhere(fn.context, contextVar =>
    state.isIdOrNameUsed(contextVar.identifier),
  );
}