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. 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.