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

[Richard Sandiford <richard dot sandiford at linaro dot org>]

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.

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.

Thanks,
Richard