Re: [PATCH 5/9] regrename: Don't run if function was separately shrink-wrapped

[Segher Boessenkool <segher@kernel.crashing.org>]

On Wed, Sep 14, 2016 at 12:11:50PM -0600, Jeff Law wrote:
> On 09/14/2016 07:04 AM, Segher Boessenkool wrote:
> >>I'd
> >>probably start by fixing the dataflow issues and see if that fixes the
> >>regrename thing as a side effect.
> >
> >Have you seen https://gcc.gnu.org/ml/gcc-patches/2016-07/msg00091.html ?
> I missed it.

Yeah thought so, too much stuff in flight here.

> My interpretation....
> 
> The uses at each "strange" exit fixing the first issue with 
> shrink-wrapping definitely sounds like we've got a dataflow problem of 
> some sort.
> 
> If you think about it, conceptually we want the return insn to make the 
> callee saved registers "used" so that DCE, regrename and friends don't 
> muck with them.  The fact that we don't is as much never having to care 
> all that much until recently.

(There is no return insn at those exits; these are exits *without*
successor block, not the exit block).

It is puzzling to me why adding USEs for just the registers that *are*
separately shrink-wrapped does not work; only all those that *could* be
shrink-wrapped does.  Do you have any idea about that?

> I continue to wonder if we could add something similar to 
> CALL_INSN_FUNCTION_USAGE where we attach uses for all the call-saved 
> registers onto a return insn.  We would attach them at the end of 
> prologue/epilogue generation and only attach those where were live 
> somewhere in the code.

Maybe adding the new insns to the {pro,epi}logue_insn_hash will help
something.  Or maybe it will blow up spectacularly.  Will know in a
bit.

> By deferring that step until after prologue/epilogue generation we 
> shouldn't cause unnecessary register saves/restores.

Hrm.  I'll see about that as well.

> I pondered just doing it for the separately wrapped components on that 
> particular path, but I've yet to convince myself that's actually correct.

If that is not correct, how is the status quo correct?  That is what
puzzles me above, too.

> Bernd knows the regrename code better than anyone.  Is there any way the 
> two of you could work together to try and track down what's going on in 
> the hash_map_rand case?  Even throwing in some more debug stuff might 
> help narrow things down since it's renaming something to a non-volatile, 
> non-separately shrink wrapped register that's causing problems.

Okay with me, I could certainly use his help.  I'll try the above things
first though, so not before friday.

> Can we agree that there's a set of targets that will improve and a set 
> that are harmed? And that to enable regrename by default we need to 
> either better describe the pipeline characteristics we're optimizing for 
> or a well defined way for targets to turn it off?

There is a well-defined way to turn it off, via common/config/*/*-common.c ,
TARGET_OPTION_OPTIMIZATION_TABLE.  We disagree on whether most targets will
want it enabled, i.e. whether it should (eventually) be enabled by default.


Segher