[PATCH] powerpc: inline current_stack_pointer()

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

Signed-off-by: Christophe Leroy <redacted>
---
 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

On Mon, May 23, 2016 at 10:46:02AM +0200, Christophe Leroy wrote:

+static inline unsigned long current_stack_pointer(void)
+{
+	register unsigned long *ptr asm("r1");
+
+	return *ptr;
+}

Register asm is only guaranteed to work as input to inline asm.  NAK.


Segher

Le 23/05/2016 à 22:22, Segher Boessenkool a écrit :

On Mon, May 23, 2016 at 10:46:02AM +0200, Christophe Leroy wrote:

quoted

+static inline unsigned long current_stack_pointer(void)
+{
+	register unsigned long *ptr asm("r1");
+
+	return *ptr;
+}

Register asm is only guaranteed to work as input to inline asm.  NAK.

Does it mean that the following declaration in 
arch/powerpc/include/asm/paca.h is wrong too ?

register struct paca_struct *local_paca asm("r13");

Christophe

On Tue, May 24, 2016 at 07:39:59AM +0200, Christophe Leroy wrote:

quoted

quoted

+static inline unsigned long current_stack_pointer(void)
+{
+	register unsigned long *ptr asm("r1");
+
+	return *ptr;
+}

Register asm is only guaranteed to work as input to inline asm.  NAK.

Does it mean that the following declaration in 
arch/powerpc/include/asm/paca.h is wrong too ?

register struct paca_struct *local_paca asm("r13");

That one is fine, because it is a global var.

https://gcc.gnu.org/onlinedocs/gcc/Explicit-Register-Variables.html


Segher

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

On Mon, May 23, 2016 at 07:17:38PM +0200, Gabriel Paubert wrote:

On Mon, May 23, 2016 at 10:46:02AM +0200, Christophe Leroy wrote:

quoted

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.

Right.  I think I wrote the original current_stack_pointer()
implementation, and that I deliberately didn't make it an inline
so that the caller would have to establish its own stack frame,
and thus its stack pointer value would be a well-defined thing.

Paul.

Hi,

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

Check out bfe9a2cfe91a ("powerpc: Reimplement __get_SP() as a function
not a define") to see why we made it a function.

Anton