Re: [PATCH] Fix PR46932: Block auto increment on frame pointer

[Jeff Law <law@redhat.com>]

On 07/25/2017 03:25 PM, Wilco Dijkstra wrote:
> Jeff Law wrote:
> 
>> My only concern here would be cases where we don't end up eliminating FP
>> to SP.  But I'd think it's unlikely that we'd have enough auto-inc
>> opportunities on the frame pointer for it to matter much anyway.
> 
> What kind of case are you thinking of? Whether it is SP or FP doesn't matter,
> we cannot copy propagate the pointer:
None in particular.  My point was that we're unlikely to even see that
many cases where an autoinc opportunity exists using the frame pointer
to begin with.  So I don't see that restricting auto-incs involving FP
should ever cause significant problems.




> 
>         add     x1, fp, 32
>         strb    w0, [x1, -1]!
>     
> Not even if the elimination offset is zero:
> 
>         mov     x1, fp
>         strb    w0, [x1, 31]!
> 
> Basically an independent add is better than the above:
> 
>         strb    w0, [fp, 31]
>         add     x1, fp, 31
> 
> I think the phase is still overly aggressive and there are more cases where it 
> doesn't make sense to use auto increment. For example you will extra moves
> just like the 2nd case above when it's not the last use of the source.
Certainly possible in the general sense, though we have some targets (SH
IIRC) that try really hard to exploit autoinc addressing modes.    On
modern processors I would think the independent add is usually a better
choice.

Instructions with multiple outputs are problematic for out-of-order
processors.  You often end up holding resources within the CPU until the
instruction can fully retire and often no dependent insns are allowed to
move forward either, even if they just depended on the embedded
arithmetic which may be ready to retire had it been issued as a separate
instruction.

Jeff