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The GC RMW instructions all used the same pattern of checking whether the expected features were a subset of the expected features. Unfortunately, this pattern was wrong. It looked this this: ``` auto expected = FeatureSet::GC | FeatureSet::Atomics ... shouldBeTrue(expected <= getModule()->features, ... ``` The problem is that the binary operator `|` caused the feature enums to be converted to int, so `expected` ended up being an int. So the `<=` that was supposed to be overloaded to do a subset check on the features was actually checking whether the integer values of the expected feature set was less than the enabled feature set. This incorrect feature checking let the fuzzer use initial contents with the affected instructions without all the expected features being enabled. Later optimizations could replace these instructions with other instructions that also required shared-everything, but checked for it a different way, causing (correct but late) validation errors. Fix the feature validation validation and remove the overloading of <= to eliminiate this class of bugs in the future.
kripken
approved these changes
Mar 18, 2026
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The GC RMW instructions all used the same pattern of checking whether the expected features were a subset of the expected features. Unfortunately, this pattern was wrong. It looked this this:
The problem is that the binary operator
|caused the feature enums to be converted to int, soexpectedended up being an int. So the<=that was supposed to be overloaded to do a subset check on the features was actually checking whether the integer values of the expected feature set was less than the enabled feature set.This incorrect feature checking let the fuzzer use initial contents with the affected instructions without all the expected features being enabled. Later optimizations could replace these instructions with other instructions that also required shared-everything, but checked for it a different way, causing (correct but late) validation errors.
Fix the feature validation validation and remove the overloading of <= to eliminiate this class of bugs in the future.