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JIT ARM32: LSRA does not kill R2R indirection cell reg on calls using it #70552

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JIT ARM32: LSRA does not kill R2R indirection cell reg on calls using it#70552
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arch-arm32area-CodeGen-coreclrCLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI
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@jakobbotsch

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@jakobbotsch
jakobbotsch
opened on Jun 10, 2022

On ARM32 VSD calls and R2R calls pass an indirection cell in R4 which is a callee-saved register. This is unlike other platforms where a volatile register is used instead. Because of this there has to be special handling in getKillSetForCall, but currently we only have such handling for VSD calls:

runtime/src/coreclr/jit/lsrabuild.cpp

Lines 895 to 900 in 9dfbc5b

#ifdef TARGET_ARM
if (call->IsVirtualStub())
{
killMask |= compiler->virtualStubParamInfo->GetRegMask();
}
#else // !TARGET_ARM

We work around this issue in morph by locking in the register requirement on the node itself and marking the constant as no-CSE:

runtime/src/coreclr/jit/morph.cpp

Lines 2113 to 2120 in 9dfbc5b

indirectCellAddress->SetRegNum(REG_R2R_INDIRECT_PARAM);
#ifdef TARGET_ARM
// Issue #xxxx : Don't attempt to CSE this constant on ARM32
//
// This constant has specific register requirements, and LSRA doesn't currently correctly
// handle them when the value is in a CSE'd local.
indirectCellAddress->SetDoNotCSE();
#endif // TARGET_ARM

When I have tried fixing the problem and removing the morph workaround I have seen som large-ish crossgen ARM regressions, so some more investigation is needed.

category:correctness
theme:register-allocator

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