[PATCH] D43394: [X86] Add 'sahf' CPU feature, and emit __LAHFSAHF__ for it

[Dimitry Andric via cfe-commits]

On 17 Feb 2018, at 00:12, Craig Topper via Phabricator <reviews at reviews.llvm.org> wrote:
> 
> craig.topper added inline comments.
> 
> 
> ================
> Comment at: lib/Basic/Targets/X86.cpp:1049
> 
> +  if (HasLAHFSAHF)
> +    Builder.defineMacro("__LAHFSAHF__");
> ----------------
> dim wrote:
>> craig.topper wrote:
>>> Does gcc define this? It's such a low level instruction I have a hard time believing this define would be useful.
>> I tried gcc 6, 7 and 8, and while they do expose stuff like `__POPCNT__`, I see no `__LAHFSAHF__`.  I am supposing Jonathan's original intent with this was to make it easily testable in source, so you can insert the right assembly for the target CPU.  The same could really be said for things like `__RDSEED__`, and some other defines...
> Most of the defines indicate the availability of intrinsics. At least that was their original intent. They used to control what intrinsic header were included in x86intrin.h or immintrin.h. Though now we include everything except in MSVC compatible mode and allow the target attribute to provide per function control.
> 
> I'd prefer not to add this one if gcc doesn't have it.

Jonathan, can you comment on your reasons for adding the __LAHFSAHF__ define?  Was this just a nice-to-have, or do you require it for something related to https://bugs.llvm.org/show_bug.cgi?id=36028 ?

> 
> 
> Repository:
>  rC Clang
> 
> https://reviews.llvm.org/D43394