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

[Ganesh Ajjanagadde]

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.

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.
>
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