[mono] Workaround MSVC miscompiling sgen_clz #114786
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After the recent VS upgrade from 17.12.5 to 17.13.2 we started seeing access violations in the mono-aot-cross.exe when targetting wasm. We tracked it down to sgen_clz being miscompiled, we can workaround the compiler bug by switching from ternary condition to if/else.
akoeplinger
requested review from
BrzVlad,
steveisok and
vitek-karas
as code owners
ghost
added
the
| { | ||
| gulong leading_zero_bits; | ||
| return _BitScanReverse (&leading_zero_bits, (gulong)x) ? 31 - leading_zero_bits : 32; | ||
| if (_BitScanReverse (&leading_zero_bits, (gulong)x)) |
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Consider adding an inline comment above this if/else block to explain that the change is a workaround for the MSVC miscompilation issue encountered in VS 17.13.2.
lewing
approved these changes
Apr 17, 2025
steveisok
approved these changes
Apr 17, 2025
jkoritzinsky
approved these changes
Apr 17, 2025
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Author
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/ba-g CI is blocked due to other issues but this is a no-op change so merging to unblock the runtime -> sdk flow |
This was referenced Apr 17, 2025
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/backport to release/9.0-staging |
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/backport to release/8.0-staging |
Contributor
|
Started backporting to release/9.0-staging: https://github.com/dotnet/runtime/actions/runs/14593713790 |
Contributor
|
Started backporting to release/8.0-staging: https://github.com/dotnet/runtime/actions/runs/14593716274 |
This was referenced Apr 22, 2025
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After the recent VS upgrade from 17.12.5 to 17.13.2 we started seeing access violations in the mono-aot-cross.exe when targetting wasm.
We tracked it down to sgen_clz being miscompiled, we can workaround the compiler bug by switching from ternary condition to if/else.
Thanks to @lateralusX for the great help :)