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Re: [PATCH] i386: Don't use frame pointer without stack access
On August 8, 2017 7:36:35 PM GMT+02:00, Richard Sandiford <richard.sandiford@linaro.org> wrote:
>Richard Sandiford <richard.sandiford@linaro.org> writes:
>> Richard Biener <richard.guenther@gmail.com> writes:
>>> On August 8, 2017 6:38:30 PM GMT+02:00, "H.J. Lu"
><hjl.tools@gmail.com> wrote:
>>>>On Mon, Aug 7, 2017 at 1:05 PM, Richard Sandiford
>>>><richard.sandiford@linaro.org> wrote:
>>>>> Arjan van de Ven <arjan@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?
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...
Richard.