[RS6000] cost SLOW_UNALIGNED_ACCESS

Alan Modra amodra@gmail.com
Wed Aug 3 05:07:00 GMT 2016


On Tue, Aug 02, 2016 at 10:31:33AM -0500, Segher Boessenkool wrote:
> On Wed, Aug 03, 2016 at 12:05:07AM +0930, Alan Modra wrote:
> > As noted in the last patch, rs6000_rtx_costs ought to cost slow
> > unaligned mems.  This stops combine merging loads/stores with a
> > mode-changing SET subreg, if the load/store in the subreg mode would
> > be slow.  Costing slow mems at 100 insns is just an order of magnitude
> > estimate.  (The alignment interrupt does cost quite a lot.
> > Experiments on power8 with a misaligned lwarx showed taking the
> > alignment interrupt cost roughly 300 insns.)
> 
> Okay for trunk.  Do you have a testcase, too?

All of yesterday's patches were from investigating
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71680#c1
but that testcase isn't particularly useful past the fix for power8
SLOW_UNALIGNED_ACCESS.

-- 
Alan Modra
Australia Development Lab, IBM



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