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


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...

Thanks,
Richard


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