Clang dies on dsputil_mmx.c with -fsanitize=undefined unless it is an
optimizing compilation. Adding -O1 to the CFLAGS of --toolchain=clang-usan
works around the compilation error that otherwise occurs when Libav
is configured with --toolchain=clang-usan --disable-optimization
(the latter disables only some optimizations, but allows optimizations
which are necessary for compilation to succeed).
This workaround has been discussed with lu_zero.

http://bugzilla.libav.org/show_bug.cgi?id=683
http://llvm.org/bugs/show_bug.cgi?id=19763
---
 configure | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/configure b/configure
index da77f69..caa3759 100755
--- a/configure
+++ b/configure
@@ -2433,7 +2433,7 @@ case "$toolchain" in
     ;;
     clang-usan)
         cc_default="clang"
-        add_cflags  -fsanitize=undefined
+        add_cflags  -fsanitize=undefined -O1
         add_ldflags -fsanitize=undefined
     ;;
     gcc-asan)
-- 
1.9.1

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