On January 30, 2015 9:12:12 PM CET, Mike Stump <mikest...@comcast.net> wrote: >On Jan 30, 2015, at 9:52 AM, Jakub Jelinek <ja...@redhat.com> wrote: >> On Fri, Jan 30, 2015 at 09:45:26AM -0800, Mike Stump wrote: >>> I have a port that has: >>> >>> (insn 47 46 48 18 (parallel [ >>> (unspec_volatile:DI [ >>> (const_int 128 [0x80]) >>> (const_int 6 [0x6]) >>> ] UNSPECV_SPECIAL_OP) >>> (set (mem/v:BLK (scratch:DI) [0 A8]) >>> (unspec:BLK [ >>> (mem/v:BLK (scratch:DI) [0 A8]) >>> ] UNSPEC_MEMORY_BARRIER)) >> >> Why don't you just (clobber (mem/v:BLK (scratch:DI))) instead >> of the second set in the parallel? The instruction is already >> UNSPEC_VOLATILE, and to make the barrier effect clear a clobber >should be >> sufficient. > >So, f087d65d84655bc2d5fdd4bcc6bf0fe337a39893 seems to have introduced >the current way that all the ports do this currently. So, I’d leave >this to Richard to answer. > >If (clobber (mem/v:BLK (scratch:DI))) works, certainly that seems >simpler than what people do now, but, then I’m left wondering, why >didn’t we do that from that start?
There would be the subtle difference that the set keeps all previous memory live while the clobber is only a barrier for memory instruction movement. Richard.
