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Re: [PATCH 1/N] x86_64 vectorization support: vectorized math functions addition to Glibc

On Fri, Sep 12, 2014 at 8:10 AM, Carlos O'Donell <> wrote:
> On 09/12/2014 11:01 AM, H.J. Lu wrote:
>> On Thu, Sep 11, 2014 at 5:10 PM, Carlos O'Donell <> 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.


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