google/closure-compiler

Array constructor with length is not eliminated

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#4,046 opened on Jan 22, 2023

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P4featgood first issuehelp wanted

Description

Consider the code:

/** @const {!Array<number>} */
const a = [1];

/** @const {!Array<number>} */
const b = Array(10);
b[0] = 2;

/** @const {!Array<number>} */
const c = [];
c.push(3);

/** @const {!Array<number>} */
const d = Array(10);
d.push(4);

When compiled with

yarn google-closure-compiler -O ADVANCED --js a.js --rewrite_polyfills=false

we get

Array(10)[0]=2;Array(10).push(4);

Arrays created with the Array(len) constructor are not eliminated.

google-closure-compiler version: v20230103

Contributor guide

Tech stack
javascriptjava
Domain
performancebuild system
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
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.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Understanding of JavaScriptFamiliarity with closure compiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Investigate the closure compiler's dead code elimination pass for arrays created with the Array(length) constructor. Look at the source code in the compiler's optimization module, particularly the parts that deal with array literals and constructor calls. The issue shows that Array(10) is not being removed while array literals like [1] and [] are. The fix likely involves adding a pattern for Array constructor calls in the dead code elimination logic. Check the comments for any maintainer hints and consider testing by building the compiler and running the provided example.