On 17 Feb 2018, at 00:12, Craig Topper via Phabricator
<revi...@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
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