[PATCH x86_64][BZ #20139] Don't allow configure with not supporting AVX512 assembler w/o --disable-avx512.

[Florian Weimer]

On 06/27/2016 08:36 PM, Andrew Senkevich wrote:
> 2016-06-27 20:53 GMT+03:00 Joseph Myers <joseph@codesourcery.com>:
>> On Mon, 27 Jun 2016, Andrew Senkevich wrote:
>>
>>> Hi,
>>>
>>> this patch adds new configure option --enable-avx512 and defaults it for
>>> x86_64.
>>>
>>> To fix BZ #20139 we need don't let to configure with not supporting
>>> AVX512 assembler w/o --disable-avx512.
>>
>> What assembler version is required to support AVX512, and when was that
>> version released?  It might be better to increase the minimum binutils
>> version for building glibc (generally, or for x86_64).
>
> May be it will be the best way.  Needed version is 2.25 and it was
> released at Mon, 5 Jan 2015.

It's a fairly recent version.

But there has been a Debian release with it, and an Ubuntu LTS release. 
Fedora 23 (current is 24) has it, too.  We can provide it for 
downstreams of Fedora.  (Right now, Red Hat Enterprise Linux 7 is still 
suitable for glibc development on x86_64, but is at some 2.23-derived 
release, and a requirement for 2.25 would put an end to this.)

I'm not familiar with the openSUSE/SLES branching model.  openSUSE Leap 
42.1 has 2.25, and is marked as “official release”.  There seem to be 
2.25 binutils built for SLE-12, but I think you have to use them already 
to replace the 2.19-derived binutils in stock SLE-12.

But even if we force the use of AVX512 for future glibc releases, it 
will not stop applications from misbehaving in weird ways when they run 
on older glibc versions.  This is my main worry.

Thanks,
Florian