Segher Boessenkool wrote: > On Mon, Jan 08, 2018 at 01:27:24PM +0000, Wilco Dijkstra wrote: > >> Peepholing is very conservative about instructions using SP and won't touch >> anything frame related. If this was working better then the backend could >> just >> emit single loads/stores and let peepholing generate LDP/STP. > > How unfortunate; that should definitely be improved then.
Improving that helps indeed but won't fix the issue. The epilog may not always be duplicated and merged like in my example. Any subset of saves and restores may not be pairable, so the worst case still has twice as many memory accesses. > Always pairing two registers together *also* degrades code quality. No, while it's not optimal, it means smaller code and fewer memory accesses. >> Another issue is that after pro_and_epilogue pass I see multiple restores >> of the same registers and then a branch to the same block. We should try >> to avoid the unnecessary duplication. > > It already does that if *all* predecessors of that block do that. If you > want to do it in other cases, you end up with more jumps. That may be > beneficial in some cases, of course, but it is not an obvious win (and in > the general case it is, hrm let's use nice words, "terrible"). That may well be the problem. So if there are N predecessors, of which N-1 need to restore the same set of callee saves, but one was shrinkwrapped, N-1 copies of the same restores might be emitted. N could be the number of blocks in a function - I really hope it doesn't work out like that... Wilco
