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

James Almer jamrial at gmail.com
Sat May 7 02:02:48 CEST 2016

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.

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