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

[James Almer]

On 5/6/2016 9:56 PM, Hendrik Leppkes wrote:
> On Sat, May 7, 2016 at 2:02 AM, James Almer <jamrial at gmail.com> wrote:
>> On 5/6/2016 8:48 PM, Timothy Gu wrote:
>>> On Fri, May 06, 2016 at 12:08:14PM +0200, Hendrik Leppkes wrote:
>>>>
>>>> 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.
>>>
>>> I don't have a problem with reverting this commit, but as James mentioned I do
>>> prefer the bug to be reported to GCC if possible.
>>>
>>> Have you also considered the possibility to enable this feature only if inline
>>> assembly is not enabled?
>>
>> Nobody disables inline asm when using GCC, so it'd be the same as removing tree
>> vectorization altogether to begin with.
>>
>> This feature gives some nice speed boost on parts of the code that don't have
>> hand written asm, so I'd very much rather keep it and try to get GCC to fix bugs
>> on their compilers.
> 
> Fixing would be nice of course, but it should then only be enabled in
> versions we know do not have problems, which is none right now.
> 
> - Hendrik

Again, do you know the configure options or compile flags that triggered these 
compiler errors? None of the many GCC 5.3 FATE clients reproduce them, so it must
be a specific combination of compile flags.