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

[Sean Silva via cfe-dev]

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
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20160513/952c8d8f/attachment.html>