[Pixman] Patch to fix test for GCC vector extensions

[Rob Tsuk]

Thanks! It had been so long since I’d submitted the patch that I’d forgotten on which machine I’d done the work and what I needed to do to get set up to modify it. I’m glad you found it easy to do for me.

> On Sep 18, 2017, at 9:54 AM, Siarhei Siamashka <siarhei.siamashka at gmail.com> wrote:
> 
> On Mon, 18 Sep 2017 07:24:49 -0700
> Rob Tsuk <rob at tsuk.com> wrote:
> 
>>> On Sep 17, 2017, at 6:40 AM, Siarhei Siamashka <siarhei.siamashka at gmail.com> wrote:
>>> 
>>> On Mon, 7 Aug 2017 07:52:29 -0700
>>> Rob Tsuk <rob at tsuk.com <mailto:rob at tsuk.com>> wrote:
>>> 
>>>> From 662b17d0d691b567a3343af3439f5fbefe989584 Mon Sep 17 00:00:00 2001
>>>> From: Rob Tsuk <robtsuk at google.com>
>>>> Date: Mon, 7 Aug 2017 07:11:10 -0700
>>>> Subject: [PATCH 1/1] Fix test for GCC vector extensions
>>>> 
>>>> The previous test was getting a false positive for
>>>> clang version 4.
>>>> ---
>>>> configure.ac | 2 +-
>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>> 
>>>> diff --git a/configure.ac b/configure.ac
>>>> index e833e45..cbebc82 100644
>>>> --- a/configure.ac
>>>> +++ b/configure.ac
>>>> @@ -1101,7 +1101,7 @@ support_for_gcc_vector_extensions=no
>>>> AC_MSG_CHECKING(for GCC vector extensions)
>>>> AC_LINK_IFELSE([AC_LANG_SOURCE([[
>>>> unsigned int __attribute__ ((vector_size(16))) e, a, b;
>>>> -int main (void) { e = a - ((b << 27) + (b >> (32 - 27))) + 1; return e[0]; }
>>>> +int main (void) { __builtin_shuffle(a,b);e = a - ((b << 27) + (b >> (32 - 27))) + 1; return e[0]; }
>>>> ]])], support_for_gcc_vector_extensions=yes)
>>>> 
>>>> if test x$support_for_gcc_vector_extensions = xyes; then  
>>> 
>>> Thanks, the patch looks mostly good. We indeed have to add
>>> the use of __builtin_shuffle() to the configure check code
>>> snippet because it is used by pixman and Clang implements
>>> this functionality in a different and incompatible way.
>>> 
>>> We did not see this problem with older versions of Clang
>>> because it did not support the shift vector by scalar operation
>>> in its vector extensions.
>>> 
>>> Too bad that GCC and Clang parted their ways and we can't have
>>> a single unified implementation. The whole reason to use GCC
>>> vector extension was that we could avoid platform-dependent
>>> intrinsics in this code.
>>> 
>>> About the patch. It would be great to use the result of
>>> __builtin_shuffle() in the intermediate calculations just in
>>> case. See
>>>  https://cgit.freedesktop.org/pixman/commit/?id=a566f627dbd6ea8f2cba70a446e62caaa2ecbd26 <https://cgit.freedesktop.org/pixman/commit/?id=a566f627dbd6ea8f2cba70a446e62caaa2ecbd26>
>> 
>> Your proposed modification to my patch makes sense. I’m not certain when I”ll have time to perform it.
> 
> It does not really require much time. I just did that for you:
> https://lists.freedesktop.org/archives/pixman/2017-September/004680.html
> 
> -- 
> Best regards,
> Siarhei Siamashka