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.
>
> 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.
