[PATCH v3 2/2] powerpc: add support for csum_add()
segher at kernel.crashing.org
Sat May 23 07:39:56 AEST 2015
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.
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