Messages in this thread | | | Date | Fri, 22 May 2015 16:39:56 -0500 | From | Segher Boessenkool <> | Subject | Re: [PATCH v3 2/2] powerpc: add support for csum_add() |
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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
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