[release/10.0] Fix CopyOSContext on amd64 #120329
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The CopyOSContext was skipping floating point registers on amd64. There was an obsolete comment that on some older Windows, the vector registers are not present in the context. I've found to be a problem when porting nativeAOT EH to CoreCLR and to make the context flags correct, I've removed the CONTEXT_FLOATING_POINT from the ContextFlags. But it now turned out that it prevents EH from correctly restoring non-volatile floating point registers after a catch. Since he ContextFlags don't contain the CONTEXT_FLOATING_POINT, the floating point registers were not restored during the resume. This change fixes it by removing the amd64 specific handling and always copying the whole CONTEXT structure. It just masks off the CONTEXT_XSTATE, as the XSTATE is stored out of the basic CONTEXT. Close #118564
jkotas
approved these changes
Oct 2, 2025
JulieLeeMSFT
approved these changes
Oct 2, 2025
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Backport of #120300 to release/10.0
/cc @janvorli
Customer Impact
The issue #118564 causes random and hard to figure out floating point values miscalculations on Windows x64 when JIT uses non-volatile XMM registers across an exception handling code. When a hardware exception (e.g. a NullReferenceException) is thrown and handled, resuming after catch doesn't restore the non-volatile XMM registers, because the CONTEXT was marked as not having any floating point state.
Regression
Testing
Repro test provided by @EgorBo, coreclr pri1 and libraries tests.
Risk
Low.