[FFmpeg-devel] [PATCH 1/2] Fix miscompilation for i586.

[Mikulas Patocka]

Hi

Here I'm sending two patches to fix portability for 586-class machines 
(Pentium, K6, etc.)


If the CPU is generic, 386, 486 or pentium, we must not use cmov in inline
assembler.

Note that some Linux distributions are compiled for i686, and for them it is
possible to use cmov in the assembler (because gcc uses it anyway). To test for
this case, we test for defined(__i686__) || defined(__athlon__) ||
defined(__SSE__) (these macros are driven by -march flag to gcc) and use cmov if
one of these conditions is true.

---
 configure                |    4 ++++
 libavcodec/x86/mathops.h |    2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

Index: ffmpeg/configure
===================================================================
--- ffmpeg.orig/configure	2014-09-12 21:46:25.710512294 +0200
+++ ffmpeg/configure	2014-09-12 21:46:32.099587711 +0200
@@ -3840,8 +3840,12 @@ elif enabled sparc; then
 elif enabled x86; then
 
     case $cpu in
+        generic)
+            disable i686
+        ;;
         i[345]86|pentium)
             cpuflags="-march=$cpu"
+            disable i686
             disable mmx
         ;;
         # targets that do NOT support nopl and conditional mov (cmov)
Index: ffmpeg/libavcodec/x86/mathops.h
===================================================================
--- ffmpeg.orig/libavcodec/x86/mathops.h	2014-09-12 21:46:05.823390224 +0200
+++ ffmpeg/libavcodec/x86/mathops.h	2014-09-12 21:46:32.116251966 +0200
@@ -69,7 +69,7 @@ static av_always_inline av_const int64_t
 
 #endif /* ARCH_X86_32 */
 
-#if HAVE_I686
+#if HAVE_I686 || defined(__i686__) || defined(__athlon__) || defined(__SSE__)
 /* median of 3 */
 #define mid_pred mid_pred
 static inline av_const int mid_pred(int a, int b, int c)