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