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

[Arjan van de Ven <arjan at linux dot intel dot com>]

On 8/7/2017 9:16 AM, Michael Matz wrote:
> Hi,
>
> On Mon, 7 Aug 2017, Arjan van de Ven wrote:
>
> > 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
>
> Then don't compile with -fno-omit-frame-pointer.  You explicitely
> requested one, so why are you surprised to see one?

I'm not surprised to see one.
I'm surprised to see a useless one.

The "perf" benefit is real, and that's why I asked for one... but the reorder made it an expensive
3 instruction nop for all intents and purposes.
If the pop was just before the ret, sure. It's not.

Maybe a different angle would be for a peephole phase to just eliminate the useless (even for those
who do want a frame pointer) push/mov/pop