On Mon, Oct 12, 2015 at 6:30 AM, Michael Niedermayer <mich...@niedermayer.cc> wrote: > On Sun, Oct 11, 2015 at 10:02:29PM -0400, Ronald S. Bultje wrote: >> Hi, >> >> On Sun, Oct 11, 2015 at 9:17 PM, Ganesh Ajjanagadde <gajja...@mit.edu> >> wrote: >> >> > On Sun, Oct 11, 2015 at 9:12 PM, Ganesh Ajjanagadde <gajja...@mit.edu> >> > wrote: >> > > On Sun, Oct 11, 2015 at 6:04 PM, Ronald S. Bultje <rsbul...@gmail.com> >> > wrote: >> > >> Hi, >> > >> >> > >> On Sun, Oct 11, 2015 at 5:52 PM, Andreas Cadhalpun < >> > >> andreas.cadhal...@googlemail.com> wrote: >> > >> >> > >>> On 11.10.2015 23:44, Ronald S. Bultje wrote: >> > >>> > It's a non-installed header and only used in one place (flacenc). >> > >>> > Since ff_ctz is static inline, it's fine to use that instead. >> > >>> > --- >> > >>> > doc/APIchanges | 3 --- >> > >>> > libavcodec/flacenc.c | 2 +- >> > >>> > libavutil/intmath.c | 5 ----- >> > >>> > libavutil/intmath.h | 14 ++++++-------- >> > >>> > 4 files changed, 7 insertions(+), 17 deletions(-) >> > >>> >> > >>> Should be fine. >> > >> >> > >> >> > >> Thanks, pushed. >> > > >> > > Since there is still time, and I did not think of this before, I would >> > > like to replace ff_ctz with ff_ctz32. There are a couple of reasons: >> > > 1. It is used with an int32 argument in flacenc. >> > > 2. I can do a deBruijn optimization for this as well. With an int also >> > > I could do it, but it would need some ifdefry depending on whether int >> > > is 32 bit or 64 bits. >> > > >> > > Let me see if I understand API/ABI with respect to this proposed >> > > change correctly now: >> > > API is not broken, as this is not public. >> > > ABI is broken, since the width of operands to ff_ctz has could change >> > > from 64 to 32 bits. >> > >> > That actually raises the question of whether int = 32 bits is assumed >> > everywhere. For instance, if int = 64 bits, at least on the Intel >> > compiler, ff_ctz is broken: AFAIK, _bit_scan_forward operates on 32 >> > bit operands. >> >> >> I think you can safely assume int is 32bit, we do so in numerous places. > > we assume int >= 32 bit all over (posix gurantees it) > int == 32bit is assumed in some arch specific code like simd asm > > generic code should not assume int to be 32bit > or rather IMHO we should not conciously add any such code as it just > violates C and will break on some (currently) uncommon platforms/configs
ff_ctz is currently not "generic", it is very much bit width specific. For instance, ff_ctz_c is broken if int = 64 bit, _bit_scan_forward is also broken if int = 64 bit. Thus, at least on ICC or non (MSC || GCC) it is broken (even on MSC it is, there is a _BitScanForward64; but at least Microsoft can dictate "int = 32"). By doing my proposed change, at the very least we are explicit in what we do - we have ff_ctz32 for 32 bits, ff_ctzll (I can rename this to ff_ctz64) force 64 bits. Moreover, ff_ctz's only use in flacenc is with a int_32. Thus, my proposal is a strict improvement over the current code. If we want then to move to a fully "generic" ff_ctz for ints, then in a subsequent patch I will add the needed ifdefry. > > [...] > -- > Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB > > If a bugfix only changes things apparently unrelated to the bug with no > further explanation, that is a good sign that the bugfix is wrong. > > _______________________________________________ > ffmpeg-devel mailing list > ffmpeg-devel@ffmpeg.org > http://ffmpeg.org/mailman/listinfo/ffmpeg-devel > _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
