[FFmpeg-devel] [PATCH] intmath: remove av_ctz.

[Ronald S. Bultje]

Hi,

On Mon, Oct 12, 2015 at 7:12 AM, Ganesh Ajjanagadde <gajjanag at mit.edu>
wrote:

> On Mon, Oct 12, 2015 at 6:30 AM, Michael Niedermayer
> <michael at 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 <gajjanag at mit.edu>
> >> wrote:
> >>
> >> > On Sun, Oct 11, 2015 at 9:12 PM, Ganesh Ajjanagadde <gajjanag at mit.edu
> >
> >> > wrote:
> >> > > On Sun, Oct 11, 2015 at 6:04 PM, Ronald S. Bultje <
> rsbultje at gmail.com>
> >> > wrote:
> >> > >> Hi,
> >> > >>
> >> > >> On Sun, Oct 11, 2015 at 5:52 PM, Andreas Cadhalpun <
> >> > >> andreas.cadhalpun at 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.


I don't think we need ff_ctzll, since it would have no users. We typically
add functions when they are used.

Renaming ff_ctz to ff_ctz32 is OK with me, although I don't care much
either way. Let's continue that discussion in the patch thread.

Ronald