RE: [PATCH v2 1/5] riscv: Checksum header

[David Laight]

...
> > > +/*
> > > + *	Fold a partial checksum without adding pseudo headers
> > > + */
> > > +static inline __sum16 csum_fold(__wsum sum)
> > > +{
> > > +	sum += (sum >> 16) | (sum << 16);
> > > +	return (__force __sum16)(~(sum >> 16));
> > > +}

I'm intrigued, gcc normally compiler that quite well.
The very similar (from arch/arc):
	return (~sum - rol32(sum, 16)) >> 16;
is slightly better on most architectures.
(Especially if the ~sum and rol() can be executed together.)

The only odd archs I saw were sparc32 (carry flag bug no rotate)
and arm (barrel shifter on all instructions).

It is better than the current asm for a lot of archs including x64.

	David

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