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JIT: Fix emitter::emitSplit to not create zero-sized method fragment #107568

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Sep 20, 2024
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JIT: Fix emitter::emitSplit to not create zero-sized method fragment #107568

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d40e42f
Fix emitSplit logic
amanasifkhalid Sep 9, 2024
0fbeb49
Remove TODO
amanasifkhalid Sep 19, 2024
e6f57af
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amanasifkhalid Sep 19, 2024
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17 changes: 15 additions & 2 deletions src/coreclr/jit/emit.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3071,8 +3071,21 @@ void emitter::emitSplit(emitLocation* startLoc,

} // end for loop

splitIfNecessary();
assert(curSize < UW_MAX_FRAGMENT_SIZE_BYTES);
if ((igLastCandidate != nullptr) && (curSize == candidateSize))
{
// TODO: This scenario may be possible (but very unlikely) when using the default fragment size.
// Fixing this would likely require padding out the last fragment to not be zero-sized, and doing the split.
// For now, assume this only occurs under stress modes, or when using DOTNET_JitSplitFunctionSize.
assert(maxSplitSize != UW_MAX_FRAGMENT_SIZE_BYTES);
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I don't think this comment is quite right. Yes, it is unlikely. But we're talking about a single empty fragment at the end. If there was a non-empty fragment, then curSize != candidateSize. So it seems like the fix works for any split size, including the default, and doesn't require this assert or TODO. (It might require other commenting/documentation, but that's a separate question.)

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But we're talking about a single empty fragment at the end.

Yeah, on second thought, I think you're right. If we have a 512KB-sized IG, followed by an empty IG that happens to be zero-sized, then the last fragment should be exactly 512KB in size, which shouldn't break any unwind data invariants. I'll remove the TODO.

JITDUMP("emitSplit: can't split at last candidate IG%02u because it would create a zero-sized fragment\n",
igLastCandidate->igNum);
}
else
{
splitIfNecessary();
}

assert((curSize > 0) && (curSize < UW_MAX_FRAGMENT_SIZE_BYTES));
}

/*****************************************************************************
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