Re: [PATCH 8/9] powerpc: simplify csum_add(a, b) in case a or b is constant 0

[Christophe Leroy]

Le 23/10/2015 05:33, Scott Wood a Ãcrit :
> On Tue, 2015-09-22 at 16:34 +0200, Christophe Leroy wrote:
> > Simplify csum_add(a, b) in case a or b is constant 0
> >
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
> > ---
> >   arch/powerpc/include/asm/checksum.h | 6 ++++++
> >   1 file changed, 6 insertions(+)
> >
> > diff --git a/arch/powerpc/include/asm/checksum.h
> > b/arch/powerpc/include/asm/checksum.h
> > index 56deea8..f8a9704 100644
> > --- a/arch/powerpc/include/asm/checksum.h
> > +++ b/arch/powerpc/include/asm/checksum.h
> > @@ -119,7 +119,13 @@ static inline __wsum csum_add(__wsum csum, __wsum
> > addend)
> >   {
> >   #ifdef __powerpc64__
> >        u64 res = (__force u64)csum;
> > +#endif
> > +     if (__builtin_constant_p(csum) && csum == 0)
> > +             return addend;
> > +     if (__builtin_constant_p(addend) && addend == 0)
> > +             return csum;
> >
> > +#ifdef __powerpc64__
> >        res += (__force u64)addend;
> >        return (__force __wsum)((u32)res + (res >> 32));
> >   #else
> How often does this happen?
In the following patch (9/9), csum_add() is used to implement 
csum_partial() for small blocks.
In several places in the networking code, csum_partial() is called with 
0 as initial sum.

Christophe