How to build gcc with AVX512?

[Xi Ruoyao]

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.

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/