Re: [PATCH 2/2] cprop_hardreg: Enable propagation of the stack pointer if possible.

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

On Thu, May 25, 2023 at 4:38 PM Jeff Law <jeffreyalaw@gmail.com> wrote:
>
>
>
> On 5/25/23 06:35, Manolis Tsamis wrote:
> > Propagation of the stack pointer in cprop_hardreg is currenty forbidden
> > in all cases, due to maybe_mode_change returning NULL. Relax this
> > restriction and allow propagation when no mode change is requested.
> >
> > gcc/ChangeLog:
> >
> >          * regcprop.cc (maybe_mode_change): Enable stack pointer propagation.
> I can't see how this can be correct given the stack pointer equality
> tests elsewhere in the compiler, particularly the various targets.
>
> The problem is if you change the mode then you end up with multiple REG
> expressions that reference the stack pointer.
>
> See rev: d1446456c3fcaa7be628726c9de4a877729490ca and the thread around
> the change which introduced this code.
>

Hi Jeff,

Isn't this fine for this case since:

  1) stack_pointer_rtx is used which won't cause issues with pointer
equalities (If I understand correctly).
  2) Propagation is guarded with `if (orig_mode == new_mode)` so only
when there is no mode change.

Thanks,
Manolis

>
> Jeff