Check whether compiler supports xmm registers in asm clobber list

Originally committed as revision 25362 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1074,6 +1074,7 @@ HAVE_LIST="
     VirtualAlloc
     winsock2_h
     xform_asm
+    xmm_clobbers
     yasm
 "
 
@@ -2585,6 +2586,9 @@ EOF
     check_asm ebx_available '""::"b"(0)' &&
         check_asm ebx_available '"":::"%ebx"'
 
+    # check whether xmm clobbers are supported
+    check_asm xmm_clobbers '"":::"%xmm0"'
+
     # check whether more than 10 operands are supported
     check_cc <<EOF && enable ten_operands
 int main(void) {

libavutil/x86_cpu.h

@@ -75,4 +75,24 @@ typedef int x86_reg;
 #    define BROKEN_RELOCATIONS 1
 #endif
 
+/*
+ * If gcc is not set to support sse (-msse) it will not accept xmm registers
+ * in the clobber list for inline asm. XMM_CLOBBERS takes a list of xmm
+ * registers to be marked as clobbered and evaluates to nothing if they are
+ * not supported, or to the list itself if they are supported. Since a clobber
+ * list may not be empty, XMM_CLOBBERS_ONLY should be used if the xmm
+ * registers are the only in the clobber list.
+ * For example a list with "eax" and "xmm0" as clobbers should become:
+ * : XMM_CLOBBERS("xmm0",) "eax"
+ * and a list with only "xmm0" should become:
+ * XMM_CLOBBERS_ONLY("xmm0")
+ */
+#if HAVE_XMM_CLOBBERS
+#    define XMM_CLOBBERS(...)        __VA_ARGS__
+#    define XMM_CLOBBERS_ONLY(...) : __VA_ARGS__
+#else
+#    define XMM_CLOBBERS(...)
+#    define XMM_CLOBBERS_ONLY(...)
+#endif
+
 #endif /* AVUTIL_X86_CPU_H */