Cherry-pick #3934 onto 0.35.x release branch. #4012
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The `fpcmp` helper in the x64 backend uses `put_in_xmm_mem` for one of its operands, which allows the compiler to merge a load with the compare instruction (`ucomiss` or `ucomisd`). Unfortunately, as we saw in bytecodealliance#2576 for the integer-compare case, this does not work with our lowering algorithm because compares can be lowered more than once (unlike all other instructions) to reproduce the flags where needed. Merging a load into an op that executes more than once is invalid in general (the two loads may observe different values, which violates the original program semantics because there was only one load originally). This does not result in a miscompilation, but instead will cause a panic at regalloc time because the register that should have been defined by the separate load is never written (the load is never emitted separately). I think this (very subtle, easy to miss) condition was unfortunately not ported over when we moved the logic in bytecodealliance#3682. The existing fcmp-of-load test in `cmp-mem-bug` (from bytecodealliance#2576) does not seem to trigger it, for a reason I haven't fully deduced. I just added the verbatim function body (happens to come from `clang.wasm`) that triggers the bug as a test. Discovered while bringing up regalloc2 support. It's pretty unlikely to hit by chance, which is why I think none of our fuzzing has hit it yet.
fitzgen
approved these changes
Apr 11, 2022
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This cherry-picks the fix to an instruction-lowering bug that causes a regalloc panic from #3934 onto our 0.35.x branch. The original bug was merged prior to the 0.35.0 release, and we missed that when making the fix (we had thought it was unreleased). This fix will be subsequently released as a new point release (0.35.3).
Addresses #4010.