[release/10.0] JIT: Skip bitcast transformation for promoted locals #120464
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The bitcast transformation produces `BITCAST(LCL_VAR)`, but for SIMDs the local can be promoted and this ends up resulting in invalid IR. We would normally go through the `LCL_FLD` path that DNERs the local, so make sure we still do that.
EgorBo
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Oct 7, 2025
JulieLeeMSFT
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Oct 7, 2025
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Backport of #120435 to release/10.0
/cc @jakobbotsch
Customer Impact
The JIT may internally produce invalid IR for
Unsafe.AsorUnsafe.BitCastthat reinterprets locals fromVector2tolongordouble, provided that local was promoted by the JIT (had its X/Y fields treated as separate variables). The invalid IR can lead to silent bad codegen or crashes in release builds of the runtime. Reported by customer in #120414.Regression
The relevant code exists in .NET 9 as well.
Testing
Unit test added.
Risk
Low. Add additional condition to an optimization sometimes made by the JIT.