[FFmpeg-devel] [PATCH] swresample/resample: Properly empty MMX state

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

There is a x86-32 MMXEXT implementation for resampling
planar 16bit data. multiple_resample() therefore calls
emms_c() if it thinks that this needed. And this is bad:

1. It is a maintenance nightmare because changes to the
x86 resample DSP code would necessitate changes to the check
whether to call emms_c().
2. The return value of av_get_cpu_flags() does not tell
whether the MMX DSP functions are in use, as they could
have been overridden by av_force_cpu_flags().
3. The MMX DSP functions will never be overridden in case of
an x86-32 build with --disable-sse2. In this scenario lots of
resampling tests (like swr-resample_exact_lin_async-s16p-8000-48000)
fail because the cpuflags indicate that SSE2 is available
(presuming that the test is run on a CPU with SSE2).
4. The check includes a call to av_get_cpu_flags(). This is not
optimized away for arches other than x86-32.
5. The check takes about as much time as emms_c() itself,
making it pointless.

This commit therefore removes the check and calls emms_c()
unconditionally (it is a no-op for non-x86).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
---
The reason I don't add an ARCH_X86_32 check is that I intend
to remove this emms_c() again shortly. I have just updated
my branch [1] that removes obsolete MMX(EXT) by a commit that
removes the MMXEXT resampling functions that are the cause
of this issue. A follow-up commit then removes the emms_c()
completely.

[1]: https://github.com/mkver/FFmpeg/commits/mmx2

 libswresample/resample.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/libswresample/resample.c b/libswresample/resample.c
index f1ec77f54b..9c5b7fee72 100644
--- a/libswresample/resample.c
+++ b/libswresample/resample.c
@@ -452,9 +452,6 @@ static int set_compensation(ResampleContext *c, int sample_delta, int compensati
 
 static int multiple_resample(ResampleContext *c, AudioData *dst, int dst_size, AudioData *src, int src_size, int *consumed){
     int i;
-    int av_unused mm_flags = av_get_cpu_flags();
-    int need_emms = c->format == AV_SAMPLE_FMT_S16P && ARCH_X86_32 &&
-                    (mm_flags & (AV_CPU_FLAG_MMX2 | AV_CPU_FLAG_SSE2)) == AV_CPU_FLAG_MMX2;
     int64_t max_src_size = (INT64_MAX/2 / c->phase_count) / c->src_incr;
 
     if (c->compensation_distance)
@@ -500,8 +497,7 @@ static int multiple_resample(ResampleContext *c, AudioData *dst, int dst_size, A
         }
     }
 
-    if(need_emms)
-        emms_c();
+    emms_c();
 
     if (c->compensation_distance) {
         c->compensation_distance -= dst_size;
-- 
2.34.1

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