wasm-merge: Avoid quadratic time in validation [NFC] #7845
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kripken
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kripken
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tlively
reviewed
Aug 19, 2025
| reportTypeMismatch(valid, "memory", import); | ||
| std::cerr << "index type should match.\n"; | ||
| if (options.validate) { | ||
| // Make sure that the export types match the import types. |
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This seems pretty important, since you could have linking problems even if each module were independently valid. Is there a way to make this validation of the linked types fast enough to run by default?
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We validate by default, so this does run by default.
It is reasonably fast - not significant in time compared to validating an entire huge module, at least.
tlively
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Aug 20, 2025
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The actual merging logic in
wasm-mergeis careful to run in linear time(since #5709). However, validation could actually be quadratic, since we do
it after each module that we merge in. Since we validate by default, this
made
wasm-mergevery slow sometimes.To fix that, validate by default once at the very end. When in pass-debug
mode, which validates after each pass, validate after each module (so we
keep the option for precise debugging of problems).
On a large testcase I am investigating (350 modules, 41 MB wasm after
merge), this makes us go from 5 minutes down to just 19 seconds. With
--no-validation, we drop to 17. (That final number is about how long ittakes to run
wasm-opt350 times to round-trip each of those modules,so it is shows
wasm-merge's merging is very fast.)Actual diff is quite small, read without whitespace to avoid noise.