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
>>>>>>>> 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...


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