PruneInitializationDependencies.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 {CompilerError} from '../CompilerError';
import {
  Environment,
  Identifier,
  IdentifierId,
  InstructionId,
  Place,
  PropertyLiteral,
  ReactiveBlock,
  ReactiveFunction,
  ReactiveInstruction,
  ReactiveScopeBlock,
  ReactiveTerminalStatement,
  getHookKind,
  isUseRefType,
  isUseStateType,
} from '../HIR';
import {eachCallArgument, eachInstructionLValue} from '../HIR/visitors';
import DisjointSet from '../Utils/DisjointSet';
import {assertExhaustive} from '../Utils/utils';
import {ReactiveFunctionVisitor, visitReactiveFunction} from './visitors';

/**
 * This pass is built based on the observation by @jbrown215 that arguments
 * to useState and useRef are only used the first time a component is rendered.
 * Any subsequent times, the arguments will be evaluated but ignored. In this pass,
 * we use this fact to improve the output of the compiler by not recomputing values that
 * are only used as arguments (or inputs to arguments to) useState and useRef.
 *
 * This pass isn't yet stress-tested so it's not enabled by default. It's only enabled
 * to support certain debug modes that detect non-idempotent code, since non-idempotent
 * code can "safely" be used if its only passed to useState and useRef. We plan to rewrite
 * this pass in HIR and enable it as an optimization in the future.
 *
 * Algorithm:
 * We take two passes over the reactive function AST. In the first pass, we gather
 * aliases and build relationships between property accesses--the key thing we need
 * to do here is to find that, e.g., $0.x and $1 refer to the same value if
 * $1 = PropertyLoad $0.x.
 *
 * In the second pass, we traverse the AST in reverse order and track how each place
 * is used. If a place is read from in any Terminal, we mark the place as "Update", meaning
 * it is used whenever the component is updated/re-rendered. If a place is read from in
 * a useState or useRef hook call, we mark it as "Create", since it is only used when the
 * component is created. In other instructions, we propagate the inferred place for the
 * instructions lvalues onto any other instructions that are read.
 *
 * Whenever we finish this reverse pass over a reactive block, we can look at the blocks
 * dependencies and see whether the dependencies are used in an "Update" context or only
 * in a "Create" context. If a dependency is create-only, then we can remove that dependency
 * from the block.
 */

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