facebookarchive/prepack

Fuse adjacent JOIN_GENERATORS with same path condition

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#2565 opened on Sep 22, 2018

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 (0 comments) (0 reactions) (0 assignees)JavaScript (14,268 stars) (520 forks)batch import
Instant Renderenhancementhelp wantedlevel 2 (easy)

Description

This...

let x = __abstract("boolean", "(x)");
if (x) console.log("Hello");
if (x) console.log(" World");

currently prepacks to

(function () {
  var _0 = x;

  if (_0) {
    console.log("Hello");
  }

  if (_0) {
    console.log(" World");
  }
})();

but should Prepack to

(function () {
  var _0 = x;

  if (_0) {
    console.log("Hello");
    console.log(" World");
  }
})();

(Seen arising from actual product code.)

Contributor guide

Tech stack
javascript
Domain
performance
Issue type
performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
under 1 hour
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
basic understanding of JavaScriptfamiliarity with Prepack internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Look at the Prepack source code, likely in src/ or lib/ directories, for how JOIN GENERATORS are handled. Search for the specific optimization pattern where adjacent JOIN GENERATORS with the same condition are fused. The issue shows a clear before/after transformation, so the fix involves modifying the logic that emits these generators to merge consecutive ones when the condition is identical. Examine existing tests for similar fusion optimizations to ensure correctness. No linked PRs or comments provide further guidance.