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<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Forwarding your mail to Elad who is working now on intrinsics. Please take into account a possible delay in Elad’s response, we are
in local holidays this week.<o:p></o:p></span></a></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
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<![if !supportLists]><span style="font-family:"Calibri",sans-serif;color:#2F5496"><span style="mso-list:Ignore">-<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span dir="LTR"></span><b><i><span style="color:#2F5496"> Elena<o:p></o:p></span></i></b></p>
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<p class="MsoNormal"><a name="_____replyseparator"></a><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Reid Kleckner [mailto:rnk@google.com]
<br>
<b>Sent:</b> Thursday, September 29, 2016 22:02<br>
<b>To:</b> Hans Wennborg <hans@chromium.org>; Blank, Guy <guy.blank@intel.com>; Aboud, Amjad <amjad.aboud@intel.com>; Demikhovsky, Elena <elena.demikhovsky@intel.com>; Craig Topper <craig.topper@gmail.com>; Badouh, Asaf <asaf.badouh@intel.com>; Zuckerman, Michael
<michael.zuckerman@intel.com>; Breger, Igor <igor.breger@intel.com><br>
<b>Cc:</b> Nathan Froyd <nfroyd@mozilla.com>; Nico Weber <thakis@chromium.org>; cfe-dev <cfe-dev@lists.llvm.org><br>
<b>Subject:</b> Re: [cfe-dev] clang-cl's <intrin.h>, _tzcnt_u32, and compatibility with MSVC's <intrin.h><o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt">We've been here before:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt"><a href="https://llvm.org/bugs/show_bug.cgi?id=25544">https://llvm.org/bugs/show_bug.cgi?id=25544</a><o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt"><a href="http://llvm.org/viewvc/llvm-project?view=revision&revision=253358">http://llvm.org/viewvc/llvm-project?view=revision&revision=253358</a><o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt"><a href="https://bugs.chromium.org/p/chromium/issues/detail?id=556735">https://bugs.chromium.org/p/chromium/issues/detail?id=556735</a><o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt">Nico's change to avoid including bmiintrin.h if _MSC_VER is set to reduce compile time brought the problem back.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt">Basically, Intel's immintrin.h interface is too big. It's windows.h all over again. It significantly slows down builds of simple projects that include basic STL headers like <string>. Nico
was able to speed up the *overall* build time of Chromium by at least 10% (ask him for specifics) by adding all these '#if !defined(_MSC_VER)' checks to immintrin.h. However, for compatibility, I think we may need intrin.h to include most of that stuff by
default.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt">I added a bunch of Intel folks to basically ask the question: Can we please go back to the old days of mmintrin.h, xmmintrin.h, then emmintrin.h?<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt">If not, we will have to consider adding an evil configuration macro, like WIN32_LEAN_AND_MEAN for windows.h, that opts out of all this extra immintrin.h functionality to speed up compilation.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt">On Thu, Sep 29, 2016 at 11:04 AM, Hans Wennborg <<a href="mailto:hans@chromium.org" target="_blank">hans@chromium.org</a>> wrote:<o:p></o:p></p>
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On Thu, Sep 29, 2016 at 10:50 AM, Nathan Froyd via cfe-dev<br>
<<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<br>
> [While I filed this as <a href="https://llvm.org/bugs/show_bug.cgi?id=30506" target="_blank">
https://llvm.org/bugs/show_bug.cgi?id=30506</a>,<br>
> Paul Robinson suggested in the bug that it might be worth bringing to<br>
> cfe-dev for wider discussion.]<br>
><br>
> Firefox's copy of FFmpeg includes <intrin.h> on MSVC:<br>
><br>
> <a href="https://dxr.mozilla.org/mozilla-central/source/media/ffvpx/libavutil/x86/intmath.h#26" target="_blank">
https://dxr.mozilla.org/mozilla-central/source/media/ffvpx/libavutil/x86/intmath.h#26</a><br>
><br>
> and MSVC's <intrin.h> (after including several other headers) declares<br>
> _tzcnt_u32. clang-cl declares _tzcnt_u32 in <bmiintrin.h>, but<br>
> <bmiintrin.h> is only included from <intrin.h> if an appropriate<br>
> target CPU is detected:<br>
><br>
> <a href="https://github.com/llvm-mirror/clang/blob/master/lib/Headers/immintrin.h#L82" target="_blank">
https://github.com/llvm-mirror/clang/blob/master/lib/Headers/immintrin.h#L82</a><br>
><br>
> even though _tzcnt_u32 (or rather, its underlying implementation<br>
> function, __tzcnt_u32) is explicitly declared to be available<br>
> everywhere:<br>
><br>
> <a href="https://github.com/llvm-mirror/clang/blob/master/lib/Headers/bmiintrin.h#L284" target="_blank">
https://github.com/llvm-mirror/clang/blob/master/lib/Headers/bmiintrin.h#L284</a><br>
><br>
> The net result is that Firefox's copy of FFmpeg doesn't compile with<br>
> clang-cl because of this issue. Upstream has the same code, so I<br>
> assume the same is true there:<br>
><br>
> <a href="https://github.com/FFmpeg/FFmpeg/blob/master/libavutil/x86/intmath.h#L26" target="_blank">
https://github.com/FFmpeg/FFmpeg/blob/master/libavutil/x86/intmath.h#L26</a><br>
><br>
> AFAICT, this behavior was changed by only including <bmiitrin.h> if<br>
> __BMI__ is defined in:<br>
><br>
> <a href="http://reviews.llvm.org/D20291" target="_blank">http://reviews.llvm.org/D20291</a><br>
><br>
> with the desirable goal of reducing compile time. But this change<br>
> also broke things like _tzcnt_u32 being available.<br>
><br>
> What is the right thing to do here? Should <bmiintrin.h> be<br>
> unconditionally included, or should something else be done?<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:46.2pt;text-indent:-17.85pt">I think these intrinsics (it's really just tzcnt though?) that are<br>
useful also for non-BMI targets should probably be moved out of the<br>
BMI-specific header.<br>
<br>
+thakis and rnk if they have any thoughts on this.<o:p></o:p></p>
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