Re: [PATCH v2 3/8] s390/string: Convert memmove() to C

[Heiko Carstens <hca@linux.ibm.com>]

> > +	if (!n)
> > +		return dest;
> > +	if ((d <= s || d >= s + n)) {
> 
> While this might work on current compilers, technically it is UB.
> Pointer comparisons are only defined if pointers "point to the same
> object".  To be on the safe side, you would have to do the unsigned
> long dance around this C standard definition.

There is a GNU C extension, which says the above code is valid:
https://www.gnu.org/software/c-intro-and-ref/manual/html_node/Pointer-Comparison.html

Since GNU C is relevant for the kernel, I'll leave it as is, unless
I'm completely wrong :)

> > +		while (n >= 256) {
> > +			asm volatile(
> > +				"	mvc	0(256,%[d]),0(%[s])\n"
> > +				:
> > +				: [d] "a" (d), [s] "a" (s)
> > +				: "memory");
> > +			d += 256;
> > +			s += 256;
> > +			n -= 256;
> > +		}
> 
> I am wondering here if it is worth also doing a 16 byte loop and,
> maybe, a single 8 byte move.

I'll give it a try at a later point in time. For now this is just
converting the existing code. Thanks for giving the hint, and looking
at the code!