Re: [PATCH 1/3] powerpc, Makefile: Make it possible to safely select CONFIG_FRAME_POINTER

[Segher Boessenkool <segher@kernel.crashing.org>]

>>> When we add "-pg" to gcc, it automatically causes frame pointers  
>>> to be
>>> used.
>>
>> Nope, it does no such thing.
>
> Well, mcount is expected to be able to get to not just who called
> mcount, but also the parent of that function. The way mcount is
> implemented does not let you do that. If mcount was the first thing to
> be called in a function, then it would have been perfect. We could get
> to the caller, its parent, and even the parameters. But unfortunately,
> mcount is called after the stack is set up. Thus, without frame  
> pointers
> (the way to find a previous frame) there's no way (on some archs) to
> find the parent. Nor can we figure out the parameters, which really
> sucks.

Yes, and this is (supposedly) why GCC does not like seeing -pg and
-fomit-frame-pointer at the same time -- because that cannot work
*on some architectures*.  These are the same architectures that
do not enable -fomit-frame-pointer automatically at -O.

>> NO_NO_OMIT_FRAME_POINTER ?  Or better, just never use -fno-o-f-p,
>> I don't see why you would ever need it.
>
> Because on x86_64 it gives better back traces. x86_64 has no way to  
> get
> to the previous frames without it. There's code to use other debug
> metadata to get back tracing, but for uses of things like the stack
> tracer, we need to be able to use the actual stack frames.
>
> As you said above, -fomit-frame-pointer is default when we  
> optimize, and
> that is how the kernel is built. If we optimize on x86_64 and do  
> not use
> -fno-omit-frame-pointer, the stack tracer is useless.

No.  -fomit-frame-pointer is only the default when optimising on
archs/ABIs where it doesn't hinder debugging and -pg and all that
goodness; specifically, you do not get it by default on x86, not
at any optimisation level.


Segher