Re: How to build gcc with AVX512?

["Cameron Fletcher (CF)" <circumflex@disroot.org>]

On 14/11/19 9:12 pm, Xi Ruoyao wrote:
> On 2019-11-14 14:46 +0530, Cameron Fletcher (CF) wrote:
>> On 14/11/19 1:41 pm, Stefan Ring wrote:
>>> On Thu, Nov 14, 2019 at 6:31 AM Cameron Fletcher (CF)
>>> <circumflex@disroot.org> wrote:
>>>> I am trying to build gcc 8.3 with using --with-fpmath=avx but it
>>>> only builds gcc with AVX1.
>>>>
>>>> Can someone help me on how to build gcc with avx512 support?
>>>
>>> Do you mean you want a compiler that understands the avx512
>>> instruction set and can generate code for it?
>>>
>> Yes. I have a cascade lake xeon on which I am building gcc.
> 
> You'll need `-march=cascadelake` to tell GCC your CPU is a Cascade
> Lake, and `-ftree-vectorize` to tell GCC to vectorize some loops into
> AVX/AVX2/AVX512 instructions.

I am trying to build GCC to support  AVX,
not building the application with AVX which I will be doing later.

> 
> If you want to fully utilize AVX512 `-mprefer-vector-width=512` may be
> necessary.  Without this option GCC will use AVX512 instructions (with
> `-march=cascadelake`) but only the 256-bit and 128-bit variants.
> 
> And we shoule notice that using AVX512 may slow down the system (due to
> downclocking).  Daniel Lemire wrote a nice article discussing these
> issues:
> 
> https://lemire.me/blog/2018/09/07/avx-512-when-and-how-to-use-these-new-instructions/
Thanks for this article.