On Mon, 2011-12-05 at 09:50 +0100, Joakim Tjernlund wrote:
> Benjamin Herrenschmidt <b...@kernel.crashing.org> 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)"
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