[PATCH, rs6000] Fixing PR 67145

[Segher Boessenkool]

On Thu, Feb 25, 2016 at 09:08:32PM -0800, Richard Henderson wrote:
> It's the simplify-rtx.c portion of the patch that fixes the i686 regression.
> 
> In the PR, Alan raises some good points, but I don't believe that we can 
> address those for gcc6.  A new rtl reassoc optimization that takes loop 
> invariance into account will have to wait.
> 
> But we do need to take care of the rs6000 ice that results, and that's the 
> bulk of the patch -- allowing CA to be sorted to any register of the plus 
> chain.

The rs6000 change is much too risky for stage 4 as well in my opinion.
It also seems it will regress codegen (for which we have no testcases
yet, sigh).

What is the rs6000 ICE?

> Some notes:
> 
> ca_operand doesn't work as written, since CA_REGNO is not an available 
> register, and thus doesn't satisfy register_operand.

Yes, curious.  But when it was written it *did* match.  Huh.

> Is there any particular reason that subf<>3_carry_in_m1 was written with 
> minus rather than plus like all of the other patterns?

It's what RTL simplification comes up with, so it is the only thing
combine tries.  The same reason why there are separate patterns for 0
and -1 at all.


Segher