On August 9, 2017 9:53:05 AM GMT+02:00, Richard Sandiford <richard.sandif...@linaro.org> wrote: >Richard Biener <richard.guent...@gmail.com> writes: >> On August 8, 2017 7:36:35 PM GMT+02:00, Richard Sandiford >> <richard.sandif...@linaro.org> wrote: >>>Richard Sandiford <richard.sandif...@linaro.org> writes: >>>> Richard Biener <richard.guent...@gmail.com> writes: >>>>> On August 8, 2017 6:38:30 PM GMT+02:00, "H.J. Lu" >>><hjl.to...@gmail.com> wrote: >>>>>>On Mon, Aug 7, 2017 at 1:05 PM, Richard Sandiford >>>>>><richard.sandif...@linaro.org> wrote: >>>>>>> Arjan van de Ven <ar...@linux.intel.com> writes: >>>>>>>> On 8/7/2017 8:43 AM, Jakub Jelinek wrote: >>>>>>>>> On Mon, Aug 07, 2017 at 08:39:24AM -0700, H.J. Lu wrote: >>>>>>>>>> When Linux/x86-64 kernel is compiled with >>>-fno-omit-frame-pointer. >>>>>>>>>> this optimization removes more than 730 >>>>>>>>>> >>>>>>>>>> pushq %rbp >>>>>>>>>> movq %rsp, %rbp >>>>>>>>>> popq %rbp >>>>>>>>> >>>>>>>>> If you don't want the frame pointer, why are you compiling >with >>>>>>>>> -fno-omit-frame-pointer? Are you going to add >>>>>>>>> -fforce-no-omit-frame-pointer or something similar so that >>>people >>>>>>can >>>>>>>>> actually get what they are asking for? This doesn't really >make >>>>>>sense. >>>>>>>>> It is perfectly fine to omit frame pointer by default, when it >>>>>>isn't >>>>>>>>> required for something, but if the user asks for it, we >>>shouldn't >>>>>>ignore his >>>>>>>>> request. >>>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> wanting a framepointer is very nice and desired... ... but if >>>the >>>>>>>> optimizer/ins scheduler moves instructions outside of the >frame'd >>>>>>>> portion, (it does it for cases like below as well), the value >is >>>>>>>> already negative for these functions that don't have stack use. >>>>>>>> >>>>>>>> <MPIDU_Sched_are_pending@@Base>: >>>>>>>> mov all_schedules@@Base-0x38460,%rax >>>>>>>> push %rbp >>>>>>>> mov %rsp,%rbp >>>>>>>> pop %rbp >>>>>>>> cmpq $0x0,(%rax) >>>>>>>> setne %al >>>>>>>> movzbl %al,%eax >>>>>>>> retq >>>>>>> >>>>>>> Yeah, and it could be even weirder for big single-block >functions. >>>>>>> I think GCC has been doing this kind of scheduling of prologue >and >>>>>>> epilogue instructions for a while, so there hasn*t really been a >>>>>>> guarantee which parts of the function will have a new FP and >which >>>>>>> will still have the old one. >>>>>>> >>>>>>> Also, with an arbitrarily-picked host compiler (GCC 6.3.1), >>>>>>shrink-wrapping >>>>>>> kicks in when the following is compiled with -O3 >>>>>>-fno-omit-frame-pointer: >>>>>>> >>>>>>> void f (int *); >>>>>>> void >>>>>>> g (int *x) >>>>>>> { >>>>>>> for (int i = 0; i < 1000; ++i) >>>>>>> x[i] += 1; >>>>>>> if (x[0]) >>>>>>> { >>>>>>> int temp; >>>>>>> f (&temp); >>>>>>> } >>>>>>> } >>>>>>> >>>>>>> so only the block with the call to f sets up FP. The relatively >>>>>>> long-running loop runs with the caller's FP. >>>>>>> >>>>>>> I hope we can go for a target-independent position that what >HJ*s >>>>>>> patch does is OK... >>>>>>> >>>>>> >>>>>>In light of this, I am resubmitting my patch. I added 3 more >>>>>>testcases >>>>>>and also handle: >>>>>> >>>>>>typedef int v8si __attribute__ ((vector_size (32))); >>>>>> >>>>>>void >>>>>>foo (v8si *out_start, v8si *out_end, v8si *regions) >>>>>>{ >>>>>> v8si base = regions[3]; >>>>>> *out_start = base; >>>>>> *out_end = base; >>>>>>} >>>>>> >>>>>>OK for trunk? >>>>> >>>>> The invoker specified -fno-omit-frame-pointer, why did you >eliminate >>>it? >>>>> I'd argue it's OK when neither -f nor -fno- is explicitly >specified >>>>> irrespective of the default in case we document the change but an >>>>> explicit -fno- is pretty clear. >>>> >>>> I don't buy that we're ignoring the user. -fomit-frame-pointer >says >>>> that, when you're creating a frame, it's OK not to set up the frame >>>> pointer. Forcing it off means that if you create a frame, you need >>>> to set up the frame pointer too. But it doesn't say anything about >>>> whether the frame itself is needed. I.e. it's >>>-fno-omit-frame*-pointer* >>>> rather than -fno-omit-frame. >> >> Isn't that a bit splitting hairs if you look at (past) history? > >I guess it would have been splitting hairs in the days when they >amounted to the same thing, i.e. when there was no behaviour that >would match "-fomit-frame" and when the prologue and epilogue were >glued to the start and end of the function. But that was quite a >long time ago. Shrink-wrapping at least means that omitting the frame >and omitting the frame pointer are different things, and it seems >fair that -fomit-frame-pointer has followed the natural meaning. > >> You could also interpret -fno-omit-frame-pointer as obviously forcing >a >> frame as otherwise there's nothing to omit... > >But applying that kind of interpretation to something like >-maccumulate-outgoing-args would make inlining all calls within a >function invalid, since there'd no longer be arguments to accumulate. > >I think this kind of disagreement just emphasises that if we really >need a "always emit a prologue at the very start, an epilogue at the >very end, and always use a frame pointer" option, we should add it >and document exactly what the guarantees are. I don't think >-fno-omit-frame-pointer should be it, since as the replies earlier in >the thread said, the natural meaning of that option has its uses too.
OK, but then both -f[no-]omit-frame-pointer do not have clearly defined semantics and thus shouldn't be exposed to the user? Richard. >Thanks, >Richard > >> >>>> It seems like the responses have been treating it more like >>>> a combination of: >>>> >>>> -fno-shrink-wrapping >>>> -fno-omit-frame-pointer >>>> the equivalent of the old textual prologues and epilogues >>>> >>>> but the positive option -fomit-frame-pointer doesn't have any >effect >>>> on the last two. >>> >>>er, you know what I mean :-) It doesn't have any effect on >>>-fshrink-wrapping or the textual-style prologues and epilogues. >> >> True. But I think people do not appreciate new options too much if >> existing ones worked in the past... >> >> Richard.