facebookarchive/prepack

Prepack emits if-statements with no consequent or alternate code

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#2220 opened on Jul 6, 2018

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 (3 comments) (0 reactions) (0 assignees)JavaScript (14,268 stars) (520 forks)batch import
Instant Renderenhancementhelp wantedlevel 1 (super easy)priority: lowserializer

Description

This

(function () {
  let n = global.__abstract ? __abstract("number", "23") : 23;
  if (n === 42) {
    let d = Date.now(); // dead pure generator entry
  }
})();

prepacks to

if (23 === 42) {}

which is obviously suboptimal.

During its fixpoint computation, the visitor should not visit the condition until at least one generator entry cannot be omitted.

Contributor guide

Tech stack
javascript
Domain
performance
Issue type
bug
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.
1-3 hours
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
JavaScript knowledgeCompiler architecture basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Investigate the fixpoint computation and visitor code in Prepack. Look for the relevant visitor logic that handles if statements and generator entries. Determine how to delay visiting the condition until at least one generator entry is confirmed not omitted. The issue suggests a specific fix in the visitor condition traversal.