[PATCH 02/10] ARM: ftrace: document mcount formats

[Uwe Kleine-König]

Hello,

On Sun, Feb 14, 2010 at 01:18:30AM +0530, Rabin Vincent wrote:
> Add a comment describing the mcount variants and how the callsites look
> like.
> 
> Signed-off-by: Rabin Vincent <rabin at rab.in>
> ---
>  arch/arm/kernel/entry-common.S |   32 ++++++++++++++++++++++++++++++++
>  1 files changed, 32 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> index 0b042bd..d412d7c 100644
> --- a/arch/arm/kernel/entry-common.S
> +++ b/arch/arm/kernel/entry-common.S
> @@ -92,6 +92,38 @@ ENDPROC(ret_from_fork)
>  #define CALL(x) .long x
>  
>  #ifdef CONFIG_FUNCTION_TRACER
> +/*
> + * When compiling with -pg, gcc inserts a call to the mcount routine at the
> + * start of every function.  In mcount, apart from the function's address (in
> + * lr), we need to get hold of the function's caller's address.
> + *
> + * Older GCCs (pre-4.4) inserted a call to a routine called mcount like this:
> + *
> + *	bl	mcount
> + *
> + * These versions have the limitation that in order for the mcount routine to
> + * be able to determine the function's caller's address, an APCS-style frame
> + * pointer (which is set up with something like the code below) is required.
> + *
> + *	mov     ip, sp
> + *	push    {fp, ip, lr, pc}
> + *	sub     fp, ip, #4
> + *
> + * With EABI, these frame pointers are not available unless -mapcs-frame is
> + * specified, and if building as Thumb-2, not even then.
> + *
> + * Newer GCCs (4.4+) solve this problem by introducing a new version of mcount,
> + * with call sites like:
> + *
> + *	push	{lr}
> + *	bl	__gnu_mcount_nc
> + *
> + * With these compilers, frame pointers are not necessary.
> + *
> + * With both the mcount types, we need to restore the original lr before
> + * returning.  In the __gnu_mcount_nc, version we're allowed to clobber ip.
> + * No other registers should be clobbered.
> + */
Very nice.

Maybe make the last two sentences:

In the __gnu_mcount_nc case the ip register is clobbered which is OK as
the calling convention for ARM allow clobbering this value for
subroutines and it doesn't contain parameters.

Best regards
Uwe

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