<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Mon, Aug 3, 2015 at 1:37 PM, Hans Wennborg <span dir="ltr"><<a href="mailto:hans@chromium.org" target="_blank">hans@chromium.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span class="">On Sun, Aug 2, 2015 at 5:09 PM, Chandler Carruth <<a href="mailto:chandlerc@google.com">chandlerc@google.com</a>> wrote:<br>
> Would cherrypicking the diagnostics to the 3.7 branch be better or worse?<br>
> (I'm of two minds, curious what others think...)<br>
<br>
</span>The alternative of reverting would have the downside of missing out on<br>
some of the target attribute functionality in 3.7. I haven't been<br>
following closely enough to determine how great a loss that would be,<br>
but as far as I understand, this is still a work in progress, right?<br>
<br>
The alternative of cherry-picking diagnostics has the problem that I<br>
don't think any diagnostics have landed yet :-)<br>
<br>
My inclination is to revert back to safety here. The revert would be<br>
pretty hairy though, as there have been a number of changes to these<br>
files since r239883 landed. I'm still trying to figure this out.</blockquote><div><br></div><div>Here's my view: sometimes features don't make the completeness bar in time for a release. That's normal and not something to be apologetic / troubled about; we'll get the feature in the next release, and we shouldn't try to rush it in. This feature is unusual in that the transitional period has temporarily left us with (arguably) a regression, and that happened to be the state when the 3.7 branch was cut, but our normal philosophies should apply: we have time-based releases, not feature-based ones, and our first response to regressions is to revert to green. So I think reverting r239883 is the way to go. If the diagnostic work is done in time, we can consider unreverting and patching it across.</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="HOEnZb"><div class="h5">
> On Sun, Aug 2, 2015 at 12:00 PM Justin Bogner <<a href="mailto:mail@justinbogner.com">mail@justinbogner.com</a>> wrote:<br>
>><br>
>> Eric Christopher <<a href="mailto:echristo@gmail.com">echristo@gmail.com</a>> writes:<br>
>> > On Thu, Jul 30, 2015 at 10:12 AM Reid Kleckner <<a href="mailto:rnk@google.com">rnk@google.com</a>> wrote:<br>
>> ><br>
>> > I'm opposed to this. Going forward, I would really like target<br>
>> > intrinsics<br>
>> > to be available regardless of the current feature set, so users<br>
>> > don't need<br>
>> > hacks like these.<br>
>> ><br>
>> > Agreed.<br>
>> ><br>
>> ><br>
>> > I see two ways to do this with different tradeoffs:<br>
>> > 1. Diagnose missing target attributes when calling the intel<br>
>> > intrinsics. I<br>
>> > was surprised to find that we don't already do this.<br>
>> ><br>
>> > Sorry. This is on my list of things to do.<br>
>><br>
>> +hans<br>
>><br>
>> I agree with the direction of moving to use target attributes instead of<br>
>> relying on flaky ifdefs, but without any errors or warnings here this is<br>
>> a pretty serious diagnostic regression.<br>
>><br>
>> I think we should revert this on the 3.7 branch. It can stay as is on<br>
>> trunk assuming the diagnostics are coming soon.<br>
>><br>
>> Right now we end up in spaces where we get crashes in the backend<br>
>> instead of a sensible error in far too many situations. Notably:<br>
>><br>
>> <a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_bugs_show-5Fbug.cgi-3Fid-3D24125&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=CnzuN65ENJ1H9py9XLiRvC_UQz6u3oG6GUNn7_wosSM&m=RdspM8_Pm6OLUKwmzkMy0csEp2E-tr3j9snM7PfaBB4&s=A_CJhKtp6KaEjhT-H5ZQ2qKLU3I9_uMr9WoK9RmLO3E&e=" rel="noreferrer" target="_blank">https://llvm.org/bugs/show_bug.cgi?id=24125</a><br>
>> <a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_bugs_show-5Fbug.cgi-3Fid-3D24087&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=CnzuN65ENJ1H9py9XLiRvC_UQz6u3oG6GUNn7_wosSM&m=RdspM8_Pm6OLUKwmzkMy0csEp2E-tr3j9snM7PfaBB4&s=9wzhomdN0ZqgXEtMzJg7TDE-uExIMYx96MlJjjn73Zw&e=" rel="noreferrer" target="_blank">https://llvm.org/bugs/show_bug.cgi?id=24087</a><br>
>> <a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_bugs_show-5Fbug.cgi-3Fid-3D24335&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=CnzuN65ENJ1H9py9XLiRvC_UQz6u3oG6GUNn7_wosSM&m=RdspM8_Pm6OLUKwmzkMy0csEp2E-tr3j9snM7PfaBB4&s=HeT2HWnCGK6E93QKy0rpnG_nXBH0pSZp7bbuUy1Q9yI&e=" rel="noreferrer" target="_blank">https://llvm.org/bugs/show_bug.cgi?id=24335</a><br>
>><br>
>> Additionally, I'm told this causes issues with configure scripts<br>
>> misdetecting available features, as well as strange compatibility issues<br>
>> like the one that led to this thread.<br>
>><br>
>> This feature is woefully incomplete. We need the warnings/errors for it<br>
>> to be acceptable quality.<br>
>><br>
>> ><br>
>> > 2. We could support some automatic transfer of the target attribute<br>
>> > to the<br>
>> > caller when calling these intrinsics, but I worry that this is too<br>
>> > confusing.<br>
>> ><br>
>> > We could, but it's probably better to leave it as is.<br>
>> ><br>
>> > -eric<br>
>> ><br>
>> ><br>
>> > Implicitly setting a target attribute may block inlining that the<br>
>> > user<br>
>> > expected to happen, for example. Alternatively, there may be a<br>
>> > dynamic<br>
>> > cpuid check in the same function between SSE2 and AVX variants of<br>
>> > the same<br>
>> > algorithm, and now the SSE2 loop will unexpectedly use AVX<br>
>> > instructions.<br>
>> ><br>
>> > So we should probably settle with telling the user to add -msseNN or<br>
>> > __atribute__((target(("sseNN")))).<br>
>> ><br>
>> > On Thu, Jul 30, 2015 at 8:53 AM, Vedant Kumar <<a href="mailto:vsk@apple.com">vsk@apple.com</a>> wrote:<br>
>> ><br>
>> > I've run into some code which no longer compiles because of two<br>
>> > recent<br>
>> > changes:<br>
>> ><br>
>> > 41885d3 Update the intel intrinsic headers to use the target<br>
>> > attribute support.<br>
>> > 695aff1 Use a define for per-file function attributes for the<br>
>> > Intel<br>
>> > intrinsic headers.<br>
>> ><br>
>> > Specifically, one project defines its own SSE4.1 emulation<br>
>> > routines<br>
>> > when the real intrinsics aren't available. This is a problem<br>
>> > because<br>
>> > they've reused the names of the intrinsics. E.g;<br>
>> ><br>
>> > > #ifndef __SSE4_1__<br>
>> > > #define _mm_extract_epi8(a_, ndx) ({ ... })<br>
>> > > static inline __m128i _mm_blendv_epi8(__m128i a, __m128i b,<br>
>> > __m128i<br>
>> > mask) { ... }<br>
>> > > ...<br>
>> > > #endif<br>
>> ><br>
>> > SSE4.1 intrinsics now leak into the project when it's being<br>
>> > compiled<br>
>> > for targets without SSE4.1 support. Compilation fails with<br>
>> > "error:<br>
>> > redefinition ...".<br>
>> ><br>
>> > When these changes were initially being discussed, I think our<br>
>> > stance<br>
>> > was that we shouldn't support code like this [1]. However, we<br>
>> > should<br>
>> > reconsider for the sake of avoiding breakage. AFAICT, we would<br>
>> > need to<br>
>> > revert just two types of changes:<br>
>> ><br>
>> > In lib/Headers/__wmmintrin_aes.h:<br>
>> ><br>
>> > > -#if defined (__SSE4_2__) || defined (__SSE4_1__)<br>
>> > > #include <smmintrin.h><br>
>> > > -#endif<br>
>> ><br>
>> > In lib/Headers/smmintrin.h:<br>
>> ><br>
>> > > -#ifndef __SSE4_1__<br>
>> > > -#error "SSE4.1 instruction set not enabled"<br>
>> > > -#else<br>
>> ><br>
>> > I don't see any downsides to reintroducing these guards. If<br>
>> > everyone's<br>
>> > OK with this, I can mail a patch in. The alternative is to have<br>
>> > clients rewrite their emulation layers like this:<br>
>> ><br>
>> > > #ifdef __SSE4_1__<br>
>> > > #define compat_mm_extract_epi8 _mm_extract_epi8<br>
>> > > static inline __m128i combat_mm_blendv_epi8(__m128i a, __m128i<br>
>> > b,<br>
>> > __m128i mask) __attribute__((__target__(("sse4.1")))) {<br>
>> > > return _mm_blendv_epi8(a, b, mask);<br>
>> > > }<br>
>> > > ...<br>
>> > > #else /* OK, no native SSE 4.1. Define our own. */<br>
>> > > #define compat_mm_extract_epi8(a_, ndx) ({ ... })<br>
>> > > static inline __m128i compat_mm_blendv_epi8(__m128i a, __m128i<br>
>> > b,<br>
>> > __m128i mask) { ... }<br>
>> > > ...<br>
>> > > #endif<br>
>> ><br>
>> > ... and then replace all calls to intrinsics with calls to the<br>
>> > new<br>
>> > compatibility routines. This seems like a lot of tedious work,<br>
>> > and I'd<br>
>> > love to help people avoid it :).<br>
>> ><br>
>> > Let me know what you think!<br>
>> ><br>
>> > vedant<br>
>> ><br>
>> > [1] <a href="http://lists.cs.uiuc.edu/pipermail/cfe-commits/" rel="noreferrer" target="_blank">http://lists.cs.uiuc.edu/pipermail/cfe-commits/</a><br>
>> > Week-of-Mon-20150615/131192.html<br>
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