Re: [PATCH v2] Implement new RTL optimizations pass: fold-mem-offsets.

[Manolis Tsamis <manolis.tsamis@vrull.eu>]

On Mon, Jul 10, 2023 at 12:37 AM Hans-Peter Nilsson <hp@bitrange.com> wrote:
>
> On Thu, 15 Jun 2023, Manolis Tsamis wrote:
>
> > This is a new RTL pass that tries to optimize memory offset calculations
> > by moving them from add immediate instructions to the memory loads/stores.
> > For example it can transform this:
> >
> >   addi t4,sp,16
> >   add  t2,a6,t4
> >   shl  t3,t2,1
> >   ld   a2,0(t3)
> >   addi a2,1
> >   sd   a2,8(t2)
> >
> > into the following (one instruction less):
> >
> >   add  t2,a6,sp
> >   shl  t3,t2,1
> >   ld   a2,32(t3)
> >   addi a2,1
> >   sd   a2,24(t2)
> >
> > Although there are places where this is done already, this pass is more
> > powerful and can handle the more difficult cases that are currently not
> > optimized. Also, it runs late enough and can optimize away unnecessary
> > stack pointer calculations.
>
> It punts on all "use" insns that are not SET.
> Why not use single_set there too?
>

The issue was that single_set will potentially discard clobbers, but
if we have any clobbers it may be invalid to propagate through that
instruction.
Rejecting anything that is not a SET is enough to handle anything strange.
Although this can be improved (look through clobbers/use?) the
implementation will be more complicated without any obvious (large)
benefit.

Manolis

> brgds, H-P