[cfe-dev] [PROPOSAL] Reintroduce guards for Intel intrinsic headers
Chandler Carruth
chandlerc at google.com
Sun Aug 2 17:09:37 PDT 2015
Would cherrypicking the diagnostics to the 3.7 branch be better or worse?
(I'm of two minds, curious what others think...)
On Sun, Aug 2, 2015 at 12:00 PM Justin Bogner <mail at justinbogner.com> wrote:
> Eric Christopher <echristo at gmail.com> writes:
> > 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.
>
> +hans
>
> I agree with the direction of moving to use target attributes instead of
> relying on flaky ifdefs, but without any errors or warnings here this is
> a pretty serious diagnostic regression.
>
> I think we should revert this on the 3.7 branch. It can stay as is on
> trunk assuming the diagnostics are coming soon.
>
> Right now we end up in spaces where we get crashes in the backend
> instead of a sensible error in far too many situations. Notably:
>
> https://llvm.org/bugs/show_bug.cgi?id=24125
> https://llvm.org/bugs/show_bug.cgi?id=24087
> https://llvm.org/bugs/show_bug.cgi?id=24335
>
> Additionally, I'm told this causes issues with configure scripts
> misdetecting available features, as well as strange compatibility issues
> like the one that led to this thread.
>
> This feature is woefully incomplete. We need the warnings/errors for it
> to be acceptable quality.
>
> >
> > 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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