Re: SIMDebian: Debian Partial Fork with Radical ISA Baseline

[Guillem Jover <guillem@debian.org>]

On Tue, 2019-04-09 at 06:48:59 +0000, Mo Zhou wrote:
> On Sat, Apr 06, 2019 at 10:55:35PM +0200, Guillem Jover wrote:
> > If what you are interested in though is just a small subset of the
> > archive, another option that would benefit everyone and is perhaps
> > less cumbersome than having to jugle around with multiple archives
> > and package rebuilds/variants, is to make use of libc's hwcaps [H]
> > support, which means the dynamic linker will automatically load the
> > best optimized shared object for the current hardware. This of course
> > can complicate a bit the packaging, and bloat it, but if the performance
> > improvement is substantial, it might be a very good trade-off.
> >   [H] man ld.so "NOTES" / "Hardware capabilities"
> 
> This sounds like a nice feature. However, unfortunately, the "avx2" and
> "avx512" features I wanted didn't show up in the list... IIRC in my
> original post I presented a C++ example with Eigen (a header-only
> library). Reverse deps such as TensorFlow would benefit from this HWCAPS
> feature if ld.so supported amd64's avx2 and avx512.

I guess the man page is just not exhaustive. ld.so should support those
hwcaps, as it handles them mostly as opaque ASCII strings sent by the
kernel. On a system with a CPU supporting those, just check the output
for something like:

  $ LD_SHOW_AUXV=1 /lib/ld-linux.so.2 2>/dev/null|grep AT_HWCAP:

Thanks,
Guillem