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

[Jeff Law <jeffreyalaw@gmail.com>]

On 5/31/23 06:15, Manolis Tsamis wrote:
> 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.
I must have missed #2 -- is that something that changed since the first 
iteration for Ventana many months ago?

Anyway, hoping to make meaningful progress on these two patches over the 
next couple days.

jeff