Subject: Re: oprofile callgraph support missing for common cpus

From: Benjamin Herrenschmidt <[email-masked]>
Date: Mon, 05 Dec 2011 20:01:26 +1100

On Mon, 2011-12-05 at 09:50 +0100, Joakim Tjernlund wrote:
> Benjamin Herrenschmidt <[email-masked]> wrote on 2011/11/25 06:24:32:
> >
> > On Fri, 2011-11-18 at 09:22 +0100, Joakim Tjernlund wrote:
> >
> > > I forgot to ask, oprofile mentions setting -no-omit-framepointer to get
> > > correct backtrace but I cannot turn on frame pointers for the ppc kernel.
> > > Isn't frame pointers needed for pcc? what about user space?
> >
> > PowerPC always has frame pointers, ignore that :-)
> 
> A bit late but consider this:

 .../...

Right I wasn't clear. We do have frame pointers for non-leaf functions,
and we can trace from LR when we are on a leaf function, we can use
__builtin_return_address as well.

We also explicitely prevent -fno-omit-frame-pointer, iirc, due to a bug
with older versions of gcc which could cause miscompiles under some
circumstances (though I don't remember the details).

Cheers,
Ben.

> int leaf(int x)
> {
> 	return x+3;
> }
> 
> which yields(with gcc -O2 -S):
> 	.file	"leaf.c"
> 	.section	".text"
> 	.align 2
> 	.globl leaf
> 	.type	leaf, @function
> leaf:
> 	addi 3,3,3
> 	blr
> 	.size	leaf, .-leaf
> 	.section	.note.GNU-stack,"",@progbits
> 	.ident	"GCC: (GNU) 3.4.6 (Gentoo 3.4.6-r2, ssp-3.4.6-1.0, pie-8.7.9)"
> 
> 
> Here there is with frame pointer(I guess that the messing around with r11 and r31 is a defect?):
> (With gcc -O2 -S -fno-omit-frame-pointer)
> 
> 	.file	"leaf.c"
> 	.section	".text"
> 	.align 2
> 	.globl leaf
> 	.type	leaf, @function
> leaf:
> 	stwu 1,-16(1)
> 	addi 3,3,3
> 	lwz 11,0(1)
> 	stw 31,12(1)
> 	mr 31,1
> 	lwz 31,-4(11)
> 	mr 1,11
> 	blr
> 	.size	leaf, .-leaf
> 	.section	.note.GNU-stack,"",@progbits
> 	.ident	"GCC: (GNU) 3.4.6 (Gentoo 3.4.6-r2, ssp-3.4.6-1.0, pie-8.7.9)"