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Slow optimizations on Dart binary #6042
Description
There is a dart binary that takes extremely long to optimize with this command:
wasm-opt -g -all --closed-world -tnh --type-unfinalizing -O3 --type-ssa --gufa -O3 --type-merging -O1 --type-finalizing
Most passes run in under a second, except that in the first -O3:
- ssa-nomerge takes 90s
- precompute-propagate takes 104s
- heap2local takes 55s
- local-subtyping takes 56s
- code-folding takes 77s
- dae-optimizing takes 361s
- optimize-stack-ir takes 36s
Overall, the first -O3 takes 836s. After that GUFA takes 31s and the second -O3 takes 410s. The -O1 takes 10s, spending most of its time in two executions of coalesce-locals that take 3s each.
Activity
kripken commentedon Oct 23, 2023
code-foldingis odd as it doesn't use LocalGraph, but all the others do and are explained by that.This module has a function with over 100 K locals and over 300 K
local.gets, so LocalGraph has a lot of work to do.Possible optimization ideas:
-O1first removes all those locals, but leaves the param and thelocal.gets, and it is still slow.local.sets for the param lets us skip all the block flowing.unordered_map<Index, ..>which is apparently quite slow.tlively commentedon Oct 24, 2023
Can we replace the unordered_map with index keys with a simple array / vector?
kripken commentedon Oct 24, 2023
Not easily, as we'd need to build an indexing for the
LocalGet*etc. pointers.But, meanwhile it seems the larger issue by far is that we can just skip param flowing entirely, as done in #6046.
A remaining issue is that the sheer number of locals in the function makes us slow nonetheless, but running
-O1first solves that. In theory perhaps-O3should run those passes before the first LocalGraph-using pass, but that would add significant work that usually won't help, I worry.[NFC] LocalGraph: Move definition to logical place (#6045)
[NFC] LocalGraph: Optimize params with no sets (#6046)
[NFC] LocalGraph: Move definition to logical place (WebAssembly#6045)
[NFC] LocalGraph: Optimize params with no sets (WebAssembly#6046)
tlively commentedon Jan 11, 2025
We have made significant performance improvements since I filed this issue, and since I apparently decided I didn't need to attach the file in question, this is no longer actionable.