Martin Storsjö: > Passing an uninitialized variable as argument to a function is > undefined behaviour (UB). The compiler can assume that UB does not > happen. > > Hence, the compiler can assume that the variables are never > uninitialized when passed as argument, which means that the codepaths > that initializes them must be taken. > > In ff_seek_frame_binary, this means that the compiler can assume > that the codepaths that initialize pos_min and pos_max are taken, > which means that the conditions "if (sti->index_entries)" and > "if (index >= 0)" can be optimized out. > > Current Clang git versions (upcoming Clang 14) enabled an optimization > that does this, which broke the current version of this function > (which intentionally left the variables uninitialized, but silencing > warnings about being uninitialized). See [1] for discussion on > the matter. > > [1] https://reviews.llvm.org/D105169#3069555 > > Signed-off-by: Martin Storsjö <mar...@martin.st> > --- > libavformat/seek.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/libavformat/seek.c b/libavformat/seek.c > index 40169736df..405ca316b3 100644 > --- a/libavformat/seek.c > +++ b/libavformat/seek.c > @@ -283,7 +283,7 @@ int ff_seek_frame_binary(AVFormatContext *s, int > stream_index, > int64_t target_ts, int flags) > { > const AVInputFormat *const avif = s->iformat; > - int64_t av_uninit(pos_min), av_uninit(pos_max), pos, pos_limit; > + int64_t pos_min = 0, pos_max = 0, pos, pos_limit; > int64_t ts_min, ts_max, ts; > int index; > int64_t ret; >
I already wanted to write that the compiler is wrong, but it seems it is not, as C11 differs from C99 in this respect (C11 6.3.2.1 2): "If the lvalue designates an object of automatic storage duration that could have been declared with the register storage class (never had its address taken), and that object is uninitialized (not declared with an initializer and no assignment to it has been performed prior to use), the behavior is undefined." For GCC and Clang av_uninit(x) is defined as x=x. And that is a problem: In case this macro is used to declare an automatic variable that is could be declared with the register storage class the pseudo-initialization is UB according to the above. So I think we will have to modify the macro to make it safe. Or just stop using it. (How could such a hack ever end up in a public header?) - Andreas _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
