Re: [PATCH] df: Keep return address register undefined until epilogue_completed

[Jeff Law <law@redhat.com>]

On 09/10/2016 01:17 AM, Segher Boessenkool wrote:
> On Fri, Sep 09, 2016 at 04:40:12PM -0600, Jeff Law wrote:
>> Right now the dataflow is conservatively correct WRT the return register.
>
> And conservatively incorrect wrt all other callee-saved regs!
But prior to prologue/epilogue insertion it shouldn't matter.  In fact, 
explicit references to callee saved regs prior to register allocation 
has always been problematical.

I do think our handling of life information for callee-saved regs after 
insertion of the prologue/epilogue could be improved.

In a separate shrink wrapped world ISTM that we want to move to a model 
where the return insn itself is a use of the appropriate callee saved 
regs.  If we did that I suspect some of the hacks from the separate 
shrink wrapping kit would just "go away".

Attaching actual USEs to those insns is obviously problematical though 
from a recognition standpoint.  It'd probably have to be structured more 
like what we do with CALL_INSNs to mark registers used/set.


>
>> If we made the change you want to make than the dataflow becomes overly
>> optimistic about the range over which the return register is live prior
>> to inserting the prologue/epilogue into the insn chain.
>>
>> This seems unsafe to me.
>
> Yes, but so does the current situation.  And it all seems to work
> nevertheless :-)
But that doesn't make it right...

Jeff