Re: glibc-hwcaps for armv7 (neon-vfpv4)

[Florian Weimer <fweimer@redhat.com>]

* Mathieu Malaterre:

> Dear glibc maintainer,
>
> I fail to understand the ld.so man page (Debian/sid version: man-pages
> 6.03) for hwcaps support. Specifically I'd like to install a shared
> lib on a Debian/armhf system (baseline is neon-less) which was build
> with gcc option:
>
>   -march=armv7-a -mfpu=neon-vfpv4
>
> What subfolder should I be using ?

You can use “LD_LIBRARY_PATH=. /bin/true” to see which subdirectories
are probed.  I think either neon/vfp or vfp/neon should be among the
subdirectories.  This functionality has since been removed from upstream
glibc because it sometimes results in hundereds of extra openat system
calls.

> Conversely how should I read the following:
>
> % sudo ldconfig -p | grep hwcap
>         libfoo.so.160 (libc6,x86-64, hwcap: 0x0004000000000000) =>
> /lib/x86_64-linux-gnu/haswell/libfoo.so.160
>         libfoo.so.160 (libc6,x86-64, hwcap: 0x0000000000000004) =>
> /lib/x86_64-linux-gnu/avx512_1/libfoo.so.160
>
> What does "haswell" / "avx512_1" subfolder implies in terms of gcc
> compile options ?

It's complicated.  Nowdays, on x86-64, you can use
glibc-hwcaps/x86-64-v3 and glibc-hwcaps/x86-64-v4, which are designed to
correspond to -march=x86-64-v3 and -march=x86-64-v4, and x86-64-v4 is a
superset of x86-64-v3.

Thanks,
Florian