Subject: Re: [PATCH v3 2/2] powerpc: add support for csum_add()

From: Segher Boessenkool <[email-masked]>
Date: Fri, 22 May 2015 16:39:56 -0500

On Fri, May 22, 2015 at 02:32:42PM -0500, Scott Wood wrote:
> > I'd also have thought that the 64bit C version above would be generally 'good'.
> 
> It doesn't generate the addc/addze sequence.  At least with GCC 4.8.2,
> it does something like:
> 
> 	mr	tmp0, csum
> 	li	tmp1, 0
> 	li	tmp2, 0
> 	addc	tmp3, addend, tmp0
> 	adde	csum, tmp2, tmp1
> 	add	csum, csum, tmp3

Right.  Don't expect older compilers to do sane things here.

All this begs a question...  If it is worth spending so much time
micro-optimising this, why not pick the low-hanging fruit first?
Having a 32-bit accumulator for ones' complement sums, on a 64-bit
system, is not such a great idea.

Segher