Re: [PATCH] Always enable LRA

["Koning, Paul" <Paul.Koning@dell.com>]

> On Oct 14, 2022, at 12:18 PM, Segher Boessenkool <segher@kernel.crashing.org> wrote:
> 
> On Fri, Oct 14, 2022 at 12:36:47AM +0000, Koning, Paul wrote:
>> I guess I'll have to look harder to see if it's possible to make LRA handle CISC addressing modes like memory indirect, autoincrement, autodecrement, and others that the old reload handles at least somewhat.  Ideally LRA should do a better job; right now I believe it doesn't really do these things at all.  Targets like pdp11 and vax would like these.
> 
> So what does it do now?  Break every more complex addressing mode apart
> again?  Or ICE?  Or something in between?

The former.  LRA does handle some cases but not all that the target permits and not as many as the old reload.

Example:

unsigned int cksum (unsigned int *buf, unsigned int len)
{
    unsigned int ret = 0;
    do {
        ret += *buf++;
    } while (--len != 0);
    return ret;
}

The loop looks like this:

L_2:
	add	(r2)+,r0
	sob	r1,L_2

which is what I would expect.  Now throw in an indirection:

Old reload produces this loop:

L_2:
	add	@(r2)+,r0
	sob	r1,L_2

while LRA doesn't understand it can use the autoincrement indirect mode:

L_2:
	mov	(r2)+,r3
	add	(r3),r0
	sob	r1,L_2

This is from a GCC 13.0 test build of last June, with -O2 -m45, with and without -mlra.

    paul