[PATCH] D95891: [ARM] Do not emit ldrexd/strexd on Cortex-M chips

[Ayke via Phabricator via llvm-commits]

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The ldrexd/strexd instructions are not supported on M-class chips, see for example https://developer.arm.com/documentation/dui0489/e/arm-and-thumb-instructions/memory-access-instructions/ldrex-and-strex which says:

> All these 32-bit Thumb instructions are available in ARMv6T2 and
> above, except that LDREXD and STREXD are not available in the ARMv7-M
> architecture.

Looking at the ARMv8-M architecture, it appears that these instructions aren't supported either. The Architecture Reference Manual lists ldrex/strex but not ldrexd/strexd: https://developer.arm.com/documentation/ddi0553/bn/

Godbolt example on LLVM 11.0.0, which incorrectly emits ldrexd/strexd instructions: https://llvm.godbolt.org/z/5qqPnE


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D95891

Files:
  llvm/lib/Target/ARM/ARMISelLowering.cpp
  llvm/test/CodeGen/ARM/atomic-64bit.ll

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