[Pixman] Patch to fix test for GCC vector extensions

[Rob Tsuk]

Your proposed modification to my patch makes sense. I’m not certain when I”ll have time to perform it.

> 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>
> 
> -- 
> Best regards,
> Siarhei Siamashka

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