[FFmpeg-devel] [PATCH 1/9] avcodec/x86/simple_idct: Empty MMX state in ff_simple_idct_mmx

[Andreas Rheinhardt]

We currently mostly do not empty the MMX state in our MMX
DSP functions; instead we only do so before code that might
be using x87 code. This is a violation of the System V i386 ABI
(and maybe of other ABIs, too):
"The CPU shall be in x87 mode upon entry to a function. Therefore,
every function that uses the MMX registers is required to issue an
emms or femms instruction after using MMX registers, before returning
or calling another function." (See 2.2.1 in [1])
This patch does not intend to change all these functions to abide
by the ABI; it only does so for ff_simple_idct_mmx(), as this
function can by called by external users, because it is exported
via AVDCT (i.e. via avcodec_dct_init()). Without this, the following
fragment will assert (in i386):
    av_force_cpu_flags(AV_CPU_FLAG_MMX | AV_CPU_FLAG_MMXEXT);
    int16_t *blk = av_malloc(64 * sizeof(*blk));
    AVDCT *avdct = avcodec_dct_alloc();
    avcodec_dct_init(avdct);
    avdct->idct(blk);
    av_assert0_fpu();

[1]: https://raw.githubusercontent.com/wiki/hjl-tools/x86-psABI/intel386-psABI-1.1.pdf

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
---
 libavcodec/x86/simple_idct.asm | 1 +
 1 file changed, 1 insertion(+)

diff --git a/libavcodec/x86/simple_idct.asm b/libavcodec/x86/simple_idct.asm
index dcf0da6df1..e3a29efc33 100644
--- a/libavcodec/x86/simple_idct.asm
+++ b/libavcodec/x86/simple_idct.asm
@@ -845,6 +845,7 @@ INIT_MMX mmx
 
 cglobal simple_idct, 1, 2, 8, 128, block, t0
     IDCT
+    emms
 RET
 
 INIT_XMM sse2
-- 
2.34.1