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

[Jeff Law <jeffreyalaw@gmail.com>]

On 6/19/23 22:52, Tamar Christina wrote:

>> It's a bit hackish, but could we reject the stack pointer for operand1 in the
>> stack-tie?  And if we do so, does it help?
> 
> Yeah this one I had to defer until later this week to look at closer because what I'm
> wondering about is whether the optimization should apply to frame related
> RTX as well.
> 
> Looking at the description of RTX_FRAME_RELATED_P that this optimization may
> end up de-optimizing RISC targets by creating an offset that is larger than offset
> which can be used from a SP making reload having to spill.  i.e. sometimes the
> move was explicitly done. So perhaps it should not apply it to
> RTX_FRAME_RELATED_P in find_oldest_value_reg and copyprop_hardreg_forward_1?
> 
> Other parts of this pass already seems to bail out in similar situations.   So I needed to
> write some testcases to check what would happen in these cases hence the deferral.
> to later in the week.
Rejecting for RTX_FRAME_RELATED_P would seem reasonable and probably 
better in general to me.  The cases where we're looking to clean things 
up aren't really in the prologue/epilogue, but instead in the main body 
after register elimination has turned fp into sp + offset, thus making 
all kinds of things no longer valid.

jeff