[FFmpeg-devel] [PATCH 2/2] configure: Enable GCC vectorization on ≥4.9

[Hendrik Leppkes]

On Tue, Feb 9, 2016 at 1:28 AM, Timothy Gu <timothygu99 at gmail.com> wrote:
> On Sun, Jan 31, 2016 at 3:38 PM Timothy Gu <timothygu99 at gmail.com> wrote:
>>
>> On Sat, Jan 30, 2016 at 07:27:22PM +0000, Derek Buitenhuis wrote:
>> > On 1/30/2016 7:15 PM, Timothy Gu wrote:
>> > > FATE passes here on a x86-64 machine with both GCC 4.9.2 and 5.3.1.
>> >
>> > Perhaps this should be restricted to x86?
>>
>> Fair enough.
>
>
> Pushed.
>

Just to document it, this has caused build breakage in various
scenarios, even in GCC 5.3 (6.1 not tested).

The latest reported on IRC just today here:
libavcodec/sbrdsp.c: In function 'sbr_neg_odd_64_c':
libavcodec/sbrdsp.c:47:13: internal compiler error: in
vect_analyze_data_ref_accesses, at tree-vect-data-refs.c:2596
 static void sbr_neg_odd_64_c(float *x)

There are various other cases which usually involve inline asm when
building with SIMD (ie. --cpu=host) and the optimizer running out of
registers, for example:
libavcodec/x86/cabac.h:192:5: error: 'asm' operand has impossible constraints

IMHO this feature is not quite ready to be enabled unconditionally in
our code base, and we should re-evaluate this change.

- Hendrik