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Re: [PATCH] i386: Don't use frame pointer without stack access

On Tue, Aug 8, 2017 at 11:00 AM, Richard Biener
<> wrote:
> On August 8, 2017 7:36:35 PM GMT+02:00, Richard Sandiford <> wrote:
>>Richard Sandiford <> writes:
>>> Richard Biener <> writes:
>>>> On August 8, 2017 6:38:30 PM GMT+02:00, "H.J. Lu"
>><> wrote:
>>>>>On Mon, Aug 7, 2017 at 1:05 PM, Richard Sandiford
>>>>><> wrote:
>>>>>> Arjan van de Ven <> 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
>>>>>>>>> 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
>>>>>>>> actually get what they are asking for?  This doesn't really make
>>>>>>>> It is perfectly fine to omit frame pointer by default, when it
>>>>>>>> required for something, but if the user asks for it, we
>>>>>ignore his
>>>>>>>> request.
>>>>>>> wanting a framepointer is very nice and desired...  ... but if
>>>>>>> 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),
>>>>>> kicks in when the following is compiled with -O3
>>>>>>     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
>>>>>and also handle:
>>>>>typedef int v8si __attribute__ ((vector_size (32)));
>>>>>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
>>>> 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
>>> rather than -fno-omit-frame.
> Isn't that a bit splitting hairs if you look at (past) history?
> You could also interpret -fno-omit-frame-pointer as obviously forcing a frame as otherwise there's nothing to omit...
>>> 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...

Should we also disable LTO and function inlining with -fno-omit-frame-pointer?
Both of them may eliminate frame pointer.


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