[PATCH] D89198: [X86] Define __LAHF_SAHF__ if feature 'sahf' is set

[Craig Topper via Phabricator via cfe-commits]

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