Re: [PATCH] powerpc: inline current_stack_pointer()

From: Gabriel Paubert
Date: Tue May 24 2016 - 02:44:35 EST


On Mon, May 23, 2016 at 10:46:02AM +0200, Christophe Leroy wrote:
> current_stack_pointeur() is a single instruction function. it
> It is not worth breaking the execution flow with a bl/blr for a
> single instruction

Are you sure that the result is always the same?

Calling an external function prevents the compiler from considering the
caller of of current_stack_pointer a leaf function, which certainly
does not help the compiler, but in a leaf function the compiler is free
not to establish a new frame.

If the compiler decides to establishes a new frame (typically with
"stwu r1,-frame_size(r1)"), *r1 is the previous stack pointer, or
the caller's stack pointer, or the current function frame pointer if
I remember correctly the ABI definitions.

However, if the compiler decides that it can get away without a frame
for the function, *r1 is the stack pointer of the caller's caller.

Depending on the application, this may or may not be important.

By the way, isn't there a GCC builtin which can perform this task,
for example builtin_frame_address()?

Gabriel
>
> Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
> ---
> arch/powerpc/include/asm/reg.h | 7 ++++++-
> arch/powerpc/kernel/misc.S | 4 ----
> arch/powerpc/kernel/ppc_ksyms.c | 2 --
> 3 files changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
> index c1e82e9..7ce6777 100644
> --- a/arch/powerpc/include/asm/reg.h
> +++ b/arch/powerpc/include/asm/reg.h
> @@ -1301,7 +1301,12 @@ static inline unsigned long mfvtb (void)
>
> #define proc_trap() asm volatile("trap")
>
> -extern unsigned long current_stack_pointer(void);
> +static inline unsigned long current_stack_pointer(void)
> +{
> + register unsigned long *ptr asm("r1");
> +
> + return *ptr;
> +}
>
> extern unsigned long scom970_read(unsigned int address);
> extern void scom970_write(unsigned int address, unsigned long value);
> diff --git a/arch/powerpc/kernel/misc.S b/arch/powerpc/kernel/misc.S
> index 0d43219..7ce26d4 100644
> --- a/arch/powerpc/kernel/misc.S
> +++ b/arch/powerpc/kernel/misc.S
> @@ -114,7 +114,3 @@ _GLOBAL(longjmp)
> mtlr r0
> mr r3,r4
> blr
> -
> -_GLOBAL(current_stack_pointer)
> - PPC_LL r3,0(r1)
> - blr
> diff --git a/arch/powerpc/kernel/ppc_ksyms.c b/arch/powerpc/kernel/ppc_ksyms.c
> index 9f01e28..eb5c5dc 100644
> --- a/arch/powerpc/kernel/ppc_ksyms.c
> +++ b/arch/powerpc/kernel/ppc_ksyms.c
> @@ -33,5 +33,3 @@ EXPORT_SYMBOL(store_vr_state);
> #ifdef CONFIG_EPAPR_PARAVIRT
> EXPORT_SYMBOL(epapr_hypercall_start);
> #endif
> -
> -EXPORT_SYMBOL(current_stack_pointer);
> --
> 2.1.0
> _______________________________________________
> Linuxppc-dev mailing list
> Linuxppc-dev@xxxxxxxxxxxxxxxx
> https://lists.ozlabs.org/listinfo/linuxppc-dev