| Message ID | 20160523084602.53F771A239A@localhost.localdomain (mailing list archive) |
|---|---|
| State | Rejected |
| Headers | show |
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@c-s.fr> > --- > 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@lists.ozlabs.org > https://lists.ozlabs.org/listinfo/linuxppc-dev
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: >> +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: > >>+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 07:17:38PM +0200, Gabriel Paubert wrote: > 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. 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
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);
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 <christophe.leroy@c-s.fr> --- 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(-)