On Thu, Mar 12, 2015 at 04:17:30PM +0100, Michael Niedermayer wrote: > On Thu, Mar 12, 2015 at 03:39:51PM +0100, Christophe Gisquet wrote: > > Hi, > > > > 2015-03-12 14:37 GMT+01:00 Michael Niedermayer <michae...@gmx.at>: > > >> > const int mx = h->mv_cache[list][scan8[n]][0] + src_x_offset > > >> > * 8; > > >> > int my = h->mv_cache[list][scan8[n]][1] + src_y_offset > > >> > * 8; > > >> > const int luma_xy = (mx & 3) + ((my & 3) << 2); > > >> > - ptrdiff_t offset = ((mx >> 2) << pixel_shift) + (my >> 2) * > > >> > h->mb_linesize; > > >> > + ptrdiff_t offset = (mx >> 2) * (1 << pixel_shift) + (my >> 2) * > > >> > h->mb_linesize; > > >> > > >> > > >> Why is this undefined? > > > > > > Because C sucks > > > > I actually have an equivalent question to Ronald's. Is there a valid > > input that causes the undefined behaviour? For an invalid input (e.g. > > DoS tentative), is the behaviour worsened? > > i belive any motion vector that points left outside the picture will > trigger this one, its also happening with multiple fate samples > This issue is about undefined behavor according to the C specification > not about current gcc generating actually bad code from it AFAIK
also a compiler could very easily generate bad code from this, that is it would be both allowed in C and make sense for it to do so consider ((mx >> 2) << pixel_shift) ... const int full_mx = mx >> 2; .... if (full_mx < 0 - extra_width || from "(mx >> 2) << pixel_shift)" a compiler could imply that (mx >> 2) is >= 0 as the alterantive is undefined that makes full_mx >= 0 at this point the only way for full_mx < - extra_width is a non zero extra_width and that is impossible on the else path to "if (mx & 7)" above so the compiler could optimize this check out in that case luckily gcc is not smart enough to do that [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB The real ebay dictionary, page 2 "100% positive feedback" - "All either got their money back or didnt complain" "Best seller ever, very honest" - "Seller refunded buyer after failed scam"
signature.asc
Description: Digital signature
_______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
