[cfe-dev] The intrinsics headers (especially avx512) are too big. What to do about it?

[C Bergström via cfe-dev]

This old discussion may cover some of this as well? I also thought I
remember something more recent around this..
http://clang-developers.42468.n3.nabble.com/PROPOSAL-Reintroduce-guards-for-Intel-intrinsic-headers-td4046979.html

On Sat, May 14, 2016 at 8:59 AM, Sean Silva via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> Sorry if this is a stupid question, but do the windows intrinsic headers
> actually contain the same contents as clang's? (e.g. maybe the windows ones
> don't cover all the ISA's that clang's do).
>
> -- Sean Silva
>
> On Thu, May 12, 2016 at 9:16 AM, Nico Weber via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
>>
>> Hi,
>>
>> on Windows, C++ system headers like e.g. <string> end up pulling in
>> intrin.h. clang's intrinsic headers are very large.
>>
>> If you take a cc file containing just `#include <string>` and run that
>> through the preprocessor with `cl /P test.cc` and `clang-cl /P test.cc`, the
>> test.I file generated by clang-cl is 1.7MB while the one created by cl.exe
>> is 0.7MB. This is solely due to clang's intrin.h expanding to way more
>> stuff.
>>
>> The biggest offenders are avx512vlintrin.h, avx512fintrin.h,
>> avx512vlbwintrin.h which add up to 657kB already. Before r239883, we only
>> included avx headers if __AVX512F__ etc was defined. This is currently never
>> the case in practice. Later (r243394 r243402 r243406 and more), the avx
>> headers got much bigger.
>>
>> Parsing all this code takes time -- removing the avx512 includes from
>> immintrin.h locally makes compiling a file containing just the <string>
>> header 0.25s faster (!), and building all of v8 gets 6% faster, just from
>> not including the avx512 headers.
>>
>> What can we do about this? Since avx512 is new, maybe they could be not
>> part of immintrin.h? Or we could re-introduce
>>
>>   #if !__has_feature(modules) && defined(__AVX512BW__)
>>
>> include guards in immintrin.h. This would give us a speed win immediately
>> without drawbacks as far as I can see, but in a few years when people start
>> compiling with /arch:avx512 that'd go away again. (Then again, by then,
>> modules are hopefully commonly available. cl.exe doesn't have an
>> /arch:avx512 switch yet, so this is probably several years away from
>> happening.)
>>
>> Comments? Is it feasible to require that people who want to use avx512
>> include a new header instead of immintrin.h? Else, does anyone have a better
>> idea other than reintroducing the #ifdefs, augmented with the module check?
>>
>> Thanks,
>> Nico
>>
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>