craig.topper added inline comments.
================
Comment at: clang/lib/Basic/Targets/X86.cpp:564
+ if (HasLAHFSAHF)
+ Builder.defineMacro("__LAHF_SAHF__");
if (HasLZCNT)
----------------
MaskRay wrote:
> craig.topper wrote:
> > craig.topper wrote:
> > > It looks like gcc may define this always in 32-bit mode?
> > More confusingly, they define it in 32-bit mode unless -mno-sahf is on the
> > command line. Despite 32-bit mode always having LAHF/SAHF instructions.
> I think this means we fail to set sahf for all 32-bit CPUs. GCC
> gcc/config/i386/i386-options.c sets OPTION_MASK_ISA_SAHF on all 32-bit CPUs
> unless -mno-sahf.
>
> According to
> https://git.noc.ruhr-uni-bochum.de/kostea7x/ghidra/-/commit/239106a356ef8555575a64cc2cfd3fd71b415406?view=parallel
> some pre-2005 Intel and AMD 64-bit CPUs had problems so 'sahf' is disabled
> for some CPUs.
>
The X86Subtarget and X86.td file calls "sahf" as HasLAHFSAHF64. And contains we
have this predicate for codegen uses.
{code}
bool hasLAHFSAHF() const { return HasLAHFSAHF64 || !is64Bit(); }
{code}
Does -mno-sahf prevent gcc from using LAHF/SAHF in 32-bit mode?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D89198/new/
https://reviews.llvm.org/D89198
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