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Re: [PATCH 1/N] x86_64 vectorization support: vectorized math functions addition to Glibc
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: "Carlos O'Donell" <carlos at redhat dot com>
- Cc: "Joseph S. Myers" <joseph at codesourcery dot com>, "Zamyatin, Igor" <igor dot zamyatin at intel dot com>, Andrew Senkevich <andrew dot n dot senkevich at gmail dot com>, libc-alpha <libc-alpha at sourceware dot org>, "Melik-Adamyan, Areg" <areg dot melik-adamyan at intel dot com>, "jakub at redhat dot com" <jakub at redhat dot com>
- Date: Fri, 12 Sep 2014 12:05:01 -0700
- Subject: Re: [PATCH 1/N] x86_64 vectorization support: vectorized math functions addition to Glibc
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On Fri, Sep 12, 2014 at 8:10 AM, Carlos O'Donell <carlos@redhat.com> wrote:
> On 09/12/2014 11:01 AM, H.J. Lu wrote:
>> On Thu, Sep 11, 2014 at 5:10 PM, Carlos O'Donell <carlos@redhat.com> wrote:
>>> On 09/11/2014 04:57 PM, H.J. Lu wrote:
>>>>> That doesn't answer my question. Maybe glibc 2.21 provides such versions
>>>>> for all x86 ISAs there are at present, up to AVX512 - and then a new
>>>>> extension AVX1024 appears. When GCC 7 is used with glibc 2.21 headers and
>>>>> -mavx1024, it must not try to generate calls to the AVX1024 functions,
>>>>> because glibc 2.21 doesn't have such functions. But maybe glibc 2.26 adds
>>>>> the AVX1024 functions. So something needs to be different in the headers
>>>>> of 2.26 to inform GCC 7 that AVX1024 versions of the functions are
>>>>> available. And I think that means the directive that communicates
>>>>> function availability to the compiler needs to identify the set of ISAs
>>>>> for which versions of the function in question are available.
>>>>>
>>>>
>>>> Wouldn't it be better to put libmvec in GCC instead?
>>>
>>> That's certainly a discussion we can have.
>>>
>>> What do you see as the pros and cons?
>>>
>>
>> It depends on who are the main target users of this library.
>> If it is mainly for programmers to use them directly in their
>> applications, mostly independent of compilers, it should be
>> in glibc. But if it is mainly used by GCC, it should be in
>> GCC, just like other run-time libraries.
>
> The former. I want users to be able to call these functions
> directly regardless of the compiler. The same goes for the
> ppc-related API that has been in use for a long time by
> developers there.
>
> The compiler can certainly make use of these functions, and
> any more standard cross-machine GNU API we design, but it
> should always be possible to call them directly.
>
> Does this mean libmvec should be in glibc?
>
If the target users are programmers, we should make it easier
to use for programmers. We can provide a generic API with a
generic implementation. Each target can provide an optimized
version which is transparent to users. We can use IFUNC to
select the best version at run-time.
--
H.J.