Re: [PATCH 3.12 065/122] lib/checksum.c: fix carry in csum_tcpudp_nofold

[Karl Beldan]

On Tue, Feb 17, 2015 at 12:04:22PM +0000, David Laight wrote:
> > +static inline u32 from64to32(u64 x)
> > +{
> > +   /* add up 32-bit and 32-bit for 32+c bit */
> > +   x = (x & 0xffffffff) + (x >> 32);
> > +   /* add up carry.. */
> > +   x = (x & 0xffffffff) + (x >> 32);
> > +   return (u32)x;
> > +}
> 
> As a matter of interest, does the compiler optimise away the
> second (x & 0xffffffff) ?
> The code could just be:
>       x = (x & 0xffffffff) + (x >> 32);
>       return x + (x >> 32);
> 
On my side, from what I've seen so far, your version results in better
assembly, esp. with clang, but my first version
http://article.gmane.org/gmane.linux.kernel/1875407:
        x += (x << 32) + (x >> 32);
        return (__force __wsum)(x >> 32);
resulted in even better assembly, I just verified with gcc/clang,
x86_64/ARM and -O1,2,3.
 
Karl
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