On Sat, May 03, 2025 at 07:42:22AM -0600, Jeff Law wrote: > On 5/3/25 4:52 AM, Richard Biener wrote: > >On Fri, 2 May 2025, Paul Koning wrote: > >>>On May 2, 2025, at 12:27 PM, Maciej W. Rozycki <ma...@orcam.me.uk> > >>>wrote: > >>Indeed, I have noticed that LRA doesn't take advantage of PDP-11 (and I > >>would guess VAX) addressing modes not found in RISC type machines. A > >>notable example are the pre-dec and post-inc modes, and I think memory > >>indirect (i.e., MEM(MEM(xyz)) modes). What isn't clear to me is whether > >>there is interest in LRA doing those things, or if the answer is that > >>they only are part of reload and therefore now unsupported. I would > >>like to be able to take advantage of those features, but have not dug > >>into it so far -- modern register alllocators are rather intimidating > >>beasts. > > > >I think the auto-inc/dec addressing modes are used by many targets so > >there's definitely the chance to adapt LRA (and/or the auto-inc/dec pass) > >to handle these better.
Yup. If there is a benefit to it, it will be done. It isn't especially difficult to do, unless I am missing something of course. > There's also the chance this is easier to pull > >off when we do not have to care for the IRA + reload combo not regressing. > Yup. There's no inherent reason why an auto-inc/auto-dec target won't > work with LRA (we have some in the tree), but there's probably places > that support can be improved. Absolutely. And a lot can already be done by the instruction combiner. > >As for MEM(MEM(xyz)) addressing modes I'm less sure - I suppose those > >are usually formed at RTL expansion time (rather than, say, by > >RTL combine)? If PDP-11 is the only target with those then it might > >be easier to recover those post-LRA during late-combine or peephole > >or alternatively in a target specific pass? But of course I know > >nothing about the constraints of said addressing mode or the challenges > >those present to LRA. > Double-indirect is *very* uncommon. I wouldn't want to pollute LRA with > support that for those addressing modes if we could avoid it. Pushing > it to the target seems better from an overall project maintenance > standpoint, though it may not be the best solution for the targets which > support double-indirect addressing modes. It is pretty hard to work with double-indirect things, often have to make sure the two memory accesses are not to the same address, etc. Segher
