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

[Richard Biener <richard.guenther@gmail.com>]

On August 9, 2017 9:53:05 AM GMT+02:00, Richard Sandiford <richard.sandiford@linaro.org> wrote:
>Richard Biener <richard.guenther@gmail.com> writes:
>> 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?
>
>I guess it would have been splitting hairs in the days when they
>amounted to the same thing, i.e. when there was no behaviour that
>would match "-fomit-frame" and when the prologue and epilogue were
>glued to the start and end of the function.  But that was quite a
>long time ago.  Shrink-wrapping at least means that omitting the frame
>and omitting the frame pointer are different things, and it seems
>fair that -fomit-frame-pointer has followed the natural meaning.
>
>> You could also interpret -fno-omit-frame-pointer as obviously forcing
>a
>> frame as otherwise there's nothing to omit...
>
>But applying that kind of interpretation to something like
>-maccumulate-outgoing-args would make inlining all calls within a
>function invalid, since there'd no longer be arguments to accumulate.
>
>I think this kind of disagreement just emphasises that if we really
>need a "always emit a prologue at the very start, an epilogue at the
>very end, and always use a frame pointer" option, we should add it
>and document exactly what the guarantees are.  I don't think
>-fno-omit-frame-pointer should be it, since as the replies earlier in
>the thread said, the natural meaning of that option has its uses too.

OK, but then both -f[no-]omit-frame-pointer do not have clearly defined semantics and thus shouldn't be exposed to the user?

Richard.

>Thanks,
>Richard
>
>>
>>>> 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.