[cfe-dev] [PROPOSAL] Reintroduce guards for Intel intrinsic headers

[Eric Christopher]

On Thu, Jul 30, 2015 at 10:12 AM Reid Kleckner <rnk at google.com> wrote:

> I'm opposed to this. Going forward, I would really like target intrinsics
> to be available regardless of the current feature set, so users don't need
> hacks like these.
>
>
Agreed.


> I see two ways to do this with different tradeoffs:
> 1. Diagnose missing target attributes when calling the intel intrinsics. I
> was surprised to find that we don't already do this.
>

Sorry. This is on my list of things to do.


> 2. We could support some automatic transfer of the target attribute to the
> caller when calling these intrinsics, but I worry that this is too
> confusing.
>

We could, but it's probably better to leave it as is.

-eric


>
> Implicitly setting a target attribute may block inlining that the user
> expected to happen, for example. Alternatively, there may be a dynamic
> cpuid check in the same function between SSE2 and AVX variants of the same
> algorithm, and now the SSE2 loop will unexpectedly use AVX instructions.
>
> So we should probably settle with telling the user to add -msseNN or
> __atribute__((target(("sseNN")))).
>
> On Thu, Jul 30, 2015 at 8:53 AM, Vedant Kumar <vsk at apple.com> wrote:
>
>> I've run into some code which no longer compiles because of two recent
>> changes:
>>
>>   41885d3 Update the intel intrinsic headers to use the target attribute
>> support.
>>   695aff1 Use a define for per-file function attributes for the Intel
>> intrinsic headers.
>>
>> Specifically, one project defines its own SSE4.1 emulation routines when
>> the real intrinsics aren't available. This is a problem because they've
>> reused the names of the intrinsics. E.g;
>>
>> > #ifndef __SSE4_1__
>> > #define _mm_extract_epi8(a_, ndx) ({ ... })
>> > static inline __m128i _mm_blendv_epi8(__m128i a, __m128i b, __m128i
>> mask) { ... }
>> > ...
>> > #endif
>>
>> SSE4.1 intrinsics now leak into the project when it's being compiled for
>> targets without SSE4.1 support. Compilation fails with "error: redefinition
>> ...".
>>
>> When these changes were initially being discussed, I think our stance was
>> that we shouldn't support code like this [1]. However, we should reconsider
>> for the sake of avoiding breakage. AFAICT, we would need to revert just two
>> types of changes:
>>
>> In lib/Headers/__wmmintrin_aes.h:
>>
>> > -#if defined (__SSE4_2__) || defined (__SSE4_1__)
>> >  #include <smmintrin.h>
>> > -#endif
>>
>>
>> In lib/Headers/smmintrin.h:
>>
>> > -#ifndef __SSE4_1__
>> > -#error "SSE4.1 instruction set not enabled"
>> > -#else
>>
>> I don't see any downsides to reintroducing these guards. If everyone's OK
>> with this, I can mail a patch in. The alternative is to have clients
>> rewrite their emulation layers like this:
>>
>> > #ifdef __SSE4_1__
>> > #define compat_mm_extract_epi8 _mm_extract_epi8
>> > static inline __m128i combat_mm_blendv_epi8(__m128i a, __m128i b,
>> __m128i mask) __attribute__((__target__(("sse4.1")))) {
>> >   return _mm_blendv_epi8(a, b, mask);
>> > }
>> > ...
>> > #else /* OK, no native SSE 4.1. Define our own. */
>> > #define compat_mm_extract_epi8(a_, ndx) ({ ... })
>> > static inline __m128i compat_mm_blendv_epi8(__m128i a, __m128i b,
>> __m128i mask) { ... }
>> > ...
>> > #endif
>>
>> ... and then replace all calls to intrinsics with calls to the new
>> compatibility routines. This seems like a lot of tedious work, and I'd love
>> to help people avoid it :).
>>
>> Let me know what you think!
>>
>> vedant
>>
>> [1]
>> http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20150615/131192.html
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>>
>
>
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