fix: update branch offsets before evaluating compiled function frame size #1699
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the comment describes the underlying issue.
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #1699 +/- ##
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+ Coverage 70.60% 70.63% +0.03%
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Files 180 180
Lines 15338 15338
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+ Hits 10829 10834 +5
+ Misses 4509 4504 -5 ☔ View full report in Codecov by Sentry. 🚀 New features to boost your workflow:
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Robbepop
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Robbepop
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Nov 18, 2025
…size (#1699) * add test case and improve triggering debug assert * fix issue-1698 the comment describes the underlying issue. * remove test because of its massive input file size
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Closes #1698
cc @tjjfvi Thanks again for your detailed bug report with which I was able to isolate the issue.
After finding the bug it was clear to me why fuzzing couldn't find it and why it is fixed on the new branch.
Why didn't fuzzing find it?
How I found it: I scanned through the meta-information of compiled functions of your
repro.wasmfile and found a single HUGE function that had more function local constants than its frame size permits.Fuzzing couldn't find the issue since it only happens for extremely large Wasm functions (>2^15 instructions) where a specific fallback encoding for conditional branch instructions is required. However, fuzzing will never generate Wasm files as huge as needed for this to be found.
What caused the bug?
The bug was that the frame size of a compiled function was computed prior to resolving all the branch offsets. However, for very large functions updating the branch offsets might allocate new function local constants which affect frame sizes.
The new branch removes the fallback encoding for conditional branches and it also removes function local constants - so kinda a double fix.
Usually, I'd add a regression test, however, given that a test testing this would be rather slow and given that this bug cannot occur in this form due to the change in interpreter architecture, I think it is okay to just have the elaborate comment directly in the code.