[FFmpeg-devel] [PATCH 4/4] avcodec/apedec: use ff_clz() instead of while loop

[Paul B Mahol]

On Thu, Oct 08, 2020 at 09:33:35PM +0200, Michael Niedermayer wrote:
> On Wed, Oct 07, 2020 at 05:12:47PM +0200, Paul B Mahol wrote:
> > On Wed, Oct 07, 2020 at 04:45:56PM +0200, Michael Niedermayer wrote:
> > > On Tue, Oct 06, 2020 at 12:08:27PM +0200, Anton Khirnov wrote:
> > > > Quoting Paul B Mahol (2020-10-06 10:19:13)
> > > > > On Tue, Oct 06, 2020 at 10:08:58AM +0200, Anton Khirnov wrote:
> > > > > > Quoting Paul B Mahol (2020-10-06 02:17:14)
> > > > > > > Signed-off-by: Paul B Mahol <onemda at gmail.com>
> > > > > > > ---
> > > > > > >  libavcodec/apedec.c | 10 +++-------
> > > > > > >  1 file changed, 3 insertions(+), 7 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/libavcodec/apedec.c b/libavcodec/apedec.c
> > > > > > > index 8fe7b5ee86..ea36247eb9 100644
> > > > > > > --- a/libavcodec/apedec.c
> > > > > > > +++ b/libavcodec/apedec.c
> > > > > > > @@ -575,14 +575,10 @@ static inline int ape_decode_value_3990(APEContext *ctx, APERice *rice)
> > > > > > >          base = range_decode_culfreq(ctx, pivot);
> > > > > > >          range_decode_update(ctx, 1, base);
> > > > > > >      } else {
> > > > > > > -        int base_hi = pivot, base_lo;
> > > > > > > -        int bbits = 0;
> > > > > > > +        int base_hi, base_lo;
> > > > > > > +        int bbits = 16 - ff_clz(pivot);
> > > > > > 
> > > > > > This assumes unsigned is always 32bit, no?
> > > > > 
> > > > > ksum is 32 bit, from which pivot is derived.
> > > > > 
> > > > > Should I use explicitly uint32_t type instead?
> > > > > Where is unsigned not 32bit?
> > > > 
> > > > I don't think uint32_t would help, as ff_clz() can expand to a compiler
> > > > builtin.
> > > > 
> > > > What I'm concerned about is the unstated assumption about
> > > > sizeof(int/unsigned) == 4 spreading through the codebase. It's already
> > > > present in plenty of places, so I certainly don't intend to block your
> > > > patch over this. But we should consider explitly documenting this, and
> > > > maybe testing in configure.
> > > 
> > > At least in code i wrote and write i consider it a bug if it would
> > > assume sizeof(int/unsigned) == 4
> > > 
> > > We also could add a ILP64 fate target, that way we would better understand
> > > how much really assumes 32bit int
> > 
> > So you basically saying we should not use ff_clz() at all because of this.
> > Then we should remove it from our code.
> 
> i guess thats the conclusion yes or
> the hardcoded 32 that is used by code calling it would need to be changed

Will apply this ape patchset soon, without this last controversial patch.

> 
> thx
> 
> 
> [...]
> -- 
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
> 
> Rewriting code that is poorly written but fully understood is good.
> Rewriting code that one doesnt understand is a sign that one is less smart
> then the original author, trying to rewrite it will not make it better.



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