`wasm-opt -Oz` takes an inordinate amount of time

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#7,319 opened on Feb 24, 2025 11 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 4/5
Estimated time: 3-5 days
Newbie friendliness: 35/100

Issue type: Bug
Clarity: Needs clarification
Activity status: Quiet
Tech stack: wasm
Domain: compilers, performance

Research direction

Start by reproducing the issue through the wasm-opt entry point using the reported optimization levels and enabled features; the test binary is not included in this issue. Compare parallelism and runtime for -Os and -Oz against -O1 and -O2, then verify that any change improves optimization time without losing the reported size results.

Written by the indexing model from the issue text.

Description

Firstly: yay, thank you for fixing the control-flow values issue in the parser! Binaryen can now work on Hoot's binaries. Thank you thank you!

I noticed a performance bug that you may be interested in, for -Oz.

optimization level	bytes	real time	user time
original	5485027	-	-
`-O0`	5835468	0.74s	1.18s
`-O1`	4810648	2.96s	65s
`-O2`	4664753	5.34s	104s
`-Os`	4378817	806s	1081s
`-Oz`	4278988	804s	1103s

This is a 32-logical-cpu system. As you can see, -Os / -Oz don't parallelize very well, and takes a bit too long to get useful results.

I can provide the test file, should that be of interest, though github doesn't seem to want to attach it. Enabled features are --enable-bulk-memory --enable-multivalue --enable-reference-types --enable-gc --enable-tail-call --enable-exception-handling.

Dominant language: WebAssembly
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