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Remove intrusive_list (NFC) #2299

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Remove intrusive_list (NFC) #2299

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@keithw keithw commented Sep 17, 2023

This PR replaces the intrusive_list class with a std::list of unique_ptr's, trying to keep everything else mostly the same.

@keithw keithw requested a review from sbc100 September 17, 2023 02:00
@keithw keithw force-pushed the remove-intrusive-list branch 3 times, most recently from ebc4b36 to deb0a58 Compare September 18, 2023 05:54
// * No initializer lists
// * Asserts instead of exceptions
// * Some functions are not implemented (merge, remove, remove_if, reverse,
// unique, sort, non-member comparison operators)
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Do you know the rationale for creating this class in the first place? Does the reasoning for it no longer hold?

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It's not super clear (#542), but generally the reason to prefer an intrusive list vs. std::list is about performance (an intrusive_list only needs one allocation to make a list node with both the contained object and the prev/next pointers). And indeed, I just benchmarked this PR (with wasm-validate clang.wasm) and std::list is a significant slowdown for large modules. :-(

main branch:

time ./bin/wasm-validate ~/w2c2/examples/clang/clang.wasm

real	0m1.887s
user	0m1.451s
sys	0m0.436s

this PR:

time ./bin/wasm-validate ~/w2c2/examples/clang/clang.wasm

real	0m2.967s
user	0m2.531s
sys	0m0.436s

I wasn't expecting it to be this bad -- let's hold off on this for now until I find a better way.

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I believe I originally switched from a vector to an intrusive list, since I thought we'd want to be able to do insertions into the list. I'm not sure that was a good decision, tbh. It may make sense to see whether switching it back to a vector would work.

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@keithw keithw Sep 19, 2023

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Well, here's a version with std::vector for the ExprList and std::deque for the ModuleFieldList (since we sometimes iterate over the ModuleFieldList while also appending to it...). Still unfortunately a performance regression, but not as bad as before:

$ time ./bin/wasm-validate ~/w2c2/examples/clang/clang.wasm 

real	0m2.456s
user	0m1.972s
sys	0m0.484s

@keithw keithw force-pushed the remove-intrusive-list branch 2 times, most recently from deb0a58 to 0eec053 Compare September 19, 2023 05:42
@keithw keithw marked this pull request as draft September 20, 2023 17:47
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keithw commented Sep 20, 2023

Reverting to draft -- I think changing ModuleFieldList away from intrusive_list in a way that doesn't introduce a performance regression will take a more involved change. The Module structure has internal pointers pointing inside the ModuleFieldList, and likes to iterate over the ModuleFieldList while appending to it, so a naive replacement with vector isn't safe (and even deque fails -- in the fuzzer only, not in any of the asan spec tests, which spooks me a bit).

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3 participants