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SubjectRe: CONFIG_FRAME_POINTER [was [PATCH] x86: BUILD_IRQ say .text]
On Sat, 26 Jul 2008, Benjamin Herrenschmidt wrote:
> On Fri, 2008-07-25 at 19:45 +0100, Hugh Dickins wrote:
> >
> > I've Cc'ed Ben and linuxppc-dev because I wonder if they're aware
> > that several options (I got it from LATENCYTOP, but I think LOCKDEP
> > and FTRACE and some others) are doing a "select FRAME_POINTER",
> > which forces CONFIG_FRAME_POINTER=y on PowerPC, even though
> > FRAME_POINTER is not an option offered on PowerPC. The
> > resulting kernels appear to run okay, but I was surprised.
>
> Because the option just does nothing for us ? :-) We always have frame
> pointers on powerpc except in some case for leaf functions. I don't know
> if the option has any actual effect on the later, but I don't think we
> have a case where doing either way would break things.

Thanks, that's reassuring.

I raised the question partly because I'd noticed CONFIG_FRAME_POINTER=y
does increase the size of powerpc kernels: part of that will have been
because of the -fno-optimize-sibling-calls bundled in, but when I edit
that out of the Makefile I'm left with

text data bss dec hex filename
4773061 856632 232052 5861745 597171 FPN/vmlinux
4943653 856632 232052 6032337 5c0bd1 FPY/vmlinux

Going to the first divergence between them,
the 2.6.26-git6 vmlinux built without CONFIG_FRAME_POINTER has

c000000000008024 <.run_init_process>:
c000000000008024: 7c 08 02 a6 mflr r0
c000000000008028: fb c1 ff f0 std r30,-16(r1)
c00000000000802c: eb c2 80 48 ld r30,-32696(r2)
c000000000008030: f8 01 00 10 std r0,16(r1)
c000000000008034: f8 21 ff 81 stdu r1,-128(r1)
c000000000008038: e9 3e 80 10 ld r9,-32752(r30)
c00000000000803c: f8 69 00 00 std r3,0(r9)
c000000000008040: 7d 24 4b 78 mr r4,r9
c000000000008044: 38 a9 01 10 addi r5,r9,272
c000000000008048: 48 01 70 4d bl c00000000001f094
<.kernel_execve>
c00000000000804c: 60 00 00 00 nop
c000000000008050: 38 21 00 80 addi r1,r1,128
c000000000008054: e8 01 00 10 ld r0,16(r1)
c000000000008058: eb c1 ff f0 ld r30,-16(r1)
c00000000000805c: 7c 08 03 a6 mtlr r0
c000000000008060: 4e 80 00 20 blr
Whereas the vmlinux built with -fno-omit-frame_pointer has

c000000000008024 <.run_init_process>:
c000000000008024: 7c 08 02 a6 mflr r0
c000000000008028: fb c1 ff f0 std r30,-16(r1)
c00000000000802c: eb c2 80 48 ld r30,-32696(r2)
c000000000008030: fb e1 ff f8 std r31,-8(r1)
c000000000008034: f8 01 00 10 std r0,16(r1)
c000000000008038: f8 21 ff 81 stdu r1,-128(r1)
c00000000000803c: e9 3e 80 10 ld r9,-32752(r30)
c000000000008040: f8 69 00 00 std r3,0(r9)
c000000000008044: 7d 24 4b 78 mr r4,r9
c000000000008048: 38 a9 01 10 addi r5,r9,272
c00000000000804c: 7c 3f 0b 78 mr r31,r1
c000000000008050: 48 01 8c 91 bl c000000000020ce0
<.kernel_execve>
c000000000008054: 60 00 00 00 nop
c000000000008058: e8 21 00 00 ld r1,0(r1)
c00000000000805c: e8 01 00 10 ld r0,16(r1)
c000000000008060: eb c1 ff f0 ld r30,-16(r1)
c000000000008064: eb e1 ff f8 ld r31,-8(r1)
c000000000008068: 7c 08 03 a6 mtlr r0
c00000000000806c: 4e 80 00 20 blr
That's for

static void run_init_process(char *init_filename)
{
argv_init[0] = init_filename;
kernel_execve(init_filename, argv_init, envp_init);
}
Hmm, perhaps it is doing sibling calls differently even without the
explicit -fno-optimize-sibling-calls (but when I add that option,
the vmlinux size does go up another 4400).

Sorry, I'm most probably fussing over nothing,
and wasting your time with my ignorance.

Hugh


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