> On May 3, 2025, at 6:52 AM, Richard Biener <rguent...@suse.de> 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:
>>>
>>> ...
>>> NB I understand your position and the need to cut the line sometime, and
>>> I knew what the situation is with the VAX backend and that it would be
>>> manageable. In principle it might be that it's only that single ICE that
>>> needs debugging before we can claim LRA usable, even if poor RISC-like
>>> code results (FWIW the PDP-11 backend suffers from the same fate). As
>>> long as user code builds and runs we can improve code gen gradually.
>>
>> 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. 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.
Thanks, that's helpful to know.
> 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.
VAX also has them, in fact VAX addressing modes are a superset of the PDP11
ones.
paul
