* [PATCH] powerpc: Discard ffs() function and use builtin_ffs instead
@ 2016-05-12 15:32 Christophe Leroy
2016-05-13 6:16 ` Michael Ellerman
0 siblings, 1 reply; 5+ messages in thread
From: Christophe Leroy @ 2016-05-12 15:32 UTC (permalink / raw)
To: Benjamin Herrenschmidt, Paul Mackerras, Michael Ellerman, Scott Wood
Cc: linux-kernel, linuxppc-dev
With the ffs() function as defined in arch/powerpc/include/asm/bitops.h
GCC will not optimise the code in case of constant parameter, as shown
by the small exemple below.
int ffs_test(void)
{
return 4 << ffs(31);
}
c0012334 <ffs_test>:
c0012334: 39 20 00 01 li r9,1
c0012338: 38 60 00 04 li r3,4
c001233c: 7d 29 00 34 cntlzw r9,r9
c0012340: 21 29 00 20 subfic r9,r9,32
c0012344: 7c 63 48 30 slw r3,r3,r9
c0012348: 4e 80 00 20 blr
With this patch, the same function will compile as follows:
c0012334 <ffs_test>:
c0012334: 38 60 00 08 li r3,8
c0012338: 4e 80 00 20 blr
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
arch/powerpc/include/asm/bitops.h | 11 +----------
1 file changed, 1 insertion(+), 10 deletions(-)
diff --git a/arch/powerpc/include/asm/bitops.h b/arch/powerpc/include/asm/bitops.h
index 59abc62..75e3ebb 100644
--- a/arch/powerpc/include/asm/bitops.h
+++ b/arch/powerpc/include/asm/bitops.h
@@ -218,16 +218,7 @@ static __inline__ unsigned long __ffs(unsigned long x)
return __ilog2(x & -x);
}
-/*
- * ffs: find first bit set. This is defined the same way as
- * the libc and compiler builtin ffs routines, therefore
- * differs in spirit from the above ffz (man ffs).
- */
-static __inline__ int ffs(int x)
-{
- unsigned long i = (unsigned long)x;
- return __ilog2(i & -i) + 1;
-}
+#include <asm-generic/bitops/builtin-ffs.h>
/*
* fls: find last (most-significant) bit set.
--
2.1.0
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: powerpc: Discard ffs() function and use builtin_ffs instead
2016-05-12 15:32 [PATCH] powerpc: Discard ffs() function and use builtin_ffs instead Christophe Leroy
@ 2016-05-13 6:16 ` Michael Ellerman
2016-05-13 6:53 ` Christophe Leroy
2016-05-13 8:29 ` Gabriel Paubert
0 siblings, 2 replies; 5+ messages in thread
From: Michael Ellerman @ 2016-05-13 6:16 UTC (permalink / raw)
To: Christophe Leroy, Benjamin Herrenschmidt, Paul Mackerras, Scott Wood
Cc: linuxppc-dev, linux-kernel
On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote:
> With the ffs() function as defined in arch/powerpc/include/asm/bitops.h
> GCC will not optimise the code in case of constant parameter, as shown
> by the small exemple below.
>
> int ffs_test(void)
> {
> return 4 << ffs(31);
> }
>
> c0012334 <ffs_test>:
> c0012334: 39 20 00 01 li r9,1
> c0012338: 38 60 00 04 li r3,4
> c001233c: 7d 29 00 34 cntlzw r9,r9
> c0012340: 21 29 00 20 subfic r9,r9,32
> c0012344: 7c 63 48 30 slw r3,r3,r9
> c0012348: 4e 80 00 20 blr
>
> With this patch, the same function will compile as follows:
>
> c0012334 <ffs_test>:
> c0012334: 38 60 00 08 li r3,8
> c0012338: 4e 80 00 20 blr
But what code does it generate when it's not a constant?
And which gcc version first added the builtin version?
cheers
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: powerpc: Discard ffs() function and use builtin_ffs instead
2016-05-13 6:16 ` Michael Ellerman
@ 2016-05-13 6:53 ` Christophe Leroy
2016-09-16 10:01 ` Christophe Leroy
2016-05-13 8:29 ` Gabriel Paubert
1 sibling, 1 reply; 5+ messages in thread
From: Christophe Leroy @ 2016-05-13 6:53 UTC (permalink / raw)
To: Michael Ellerman, Benjamin Herrenschmidt, Paul Mackerras, Scott Wood
Cc: linuxppc-dev, linux-kernel
Le 13/05/2016 à 08:16, Michael Ellerman a écrit :
> On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote:
>> With the ffs() function as defined in arch/powerpc/include/asm/bitops.h
>> GCC will not optimise the code in case of constant parameter, as shown
>> by the small exemple below.
>>
>> int ffs_test(void)
>> {
>> return 4 << ffs(31);
>> }
>>
>> c0012334 <ffs_test>:
>> c0012334: 39 20 00 01 li r9,1
>> c0012338: 38 60 00 04 li r3,4
>> c001233c: 7d 29 00 34 cntlzw r9,r9
>> c0012340: 21 29 00 20 subfic r9,r9,32
>> c0012344: 7c 63 48 30 slw r3,r3,r9
>> c0012348: 4e 80 00 20 blr
>>
>> With this patch, the same function will compile as follows:
>>
>> c0012334 <ffs_test>:
>> c0012334: 38 60 00 08 li r3,8
>> c0012338: 4e 80 00 20 blr
>
> But what code does it generate when it's not a constant?
The generated code is the same with and without the patch when not a
constant:
int ffs_test2(int x)
{
return ffs(x);
}
c001233c <ffs_test2>:
c001233c: 7d 23 00 d0 neg r9,r3
c0012340: 7d 23 18 38 and r3,r9,r3
c0012344: 7c 63 00 34 cntlzw r3,r3
c0012348: 20 63 00 20 subfic r3,r3,32
c001234c: 4e 80 00 20 blr
>
> And which gcc version first added the builtin version?
Don't know, but __builtin_ffs() is already used in
arch/powerpc/include/asm/page_32.h
Christophe
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: powerpc: Discard ffs() function and use builtin_ffs instead
2016-05-13 6:16 ` Michael Ellerman
2016-05-13 6:53 ` Christophe Leroy
@ 2016-05-13 8:29 ` Gabriel Paubert
1 sibling, 0 replies; 5+ messages in thread
From: Gabriel Paubert @ 2016-05-13 8:29 UTC (permalink / raw)
To: Michael Ellerman
Cc: Christophe Leroy, Benjamin Herrenschmidt, Paul Mackerras,
Scott Wood, linuxppc-dev, linux-kernel
On Fri, May 13, 2016 at 04:16:57PM +1000, Michael Ellerman wrote:
> On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote:
> > With the ffs() function as defined in arch/powerpc/include/asm/bitops.h
> > GCC will not optimise the code in case of constant parameter, as shown
> > by the small exemple below.
> >
> > int ffs_test(void)
> > {
> > return 4 << ffs(31);
> > }
> >
> > c0012334 <ffs_test>:
> > c0012334: 39 20 00 01 li r9,1
> > c0012338: 38 60 00 04 li r3,4
> > c001233c: 7d 29 00 34 cntlzw r9,r9
> > c0012340: 21 29 00 20 subfic r9,r9,32
> > c0012344: 7c 63 48 30 slw r3,r3,r9
> > c0012348: 4e 80 00 20 blr
> >
> > With this patch, the same function will compile as follows:
> >
> > c0012334 <ffs_test>:
> > c0012334: 38 60 00 08 li r3,8
> > c0012338: 4e 80 00 20 blr
>
>
> But what code does it generate when it's not a constant?
>
> And which gcc version first added the builtin version?
It already existed in gcc-2.95, which you do not want to use to compile
anything today but I have in a corner of a chroot environment to maintain
~1997 vintage embedded stuff, running a 2.2.12 kernel!
Hopefully this clears up your concerns :-)
Cheers,
Gabriel
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: powerpc: Discard ffs() function and use builtin_ffs instead
2016-05-13 6:53 ` Christophe Leroy
@ 2016-09-16 10:01 ` Christophe Leroy
0 siblings, 0 replies; 5+ messages in thread
From: Christophe Leroy @ 2016-09-16 10:01 UTC (permalink / raw)
To: Michael Ellerman
Cc: Benjamin Herrenschmidt, Paul Mackerras, Scott Wood, linuxppc-dev,
linux-kernel
Le 13/05/2016 à 08:53, Christophe Leroy a écrit :
>
>
> Le 13/05/2016 à 08:16, Michael Ellerman a écrit :
>> On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote:
>>> With the ffs() function as defined in arch/powerpc/include/asm/bitops.h
>>> GCC will not optimise the code in case of constant parameter, as shown
>>> by the small exemple below.
>>>
>>> int ffs_test(void)
>>> {
>>> return 4 << ffs(31);
>>> }
>>>
>>> c0012334 <ffs_test>:
>>> c0012334: 39 20 00 01 li r9,1
>>> c0012338: 38 60 00 04 li r3,4
>>> c001233c: 7d 29 00 34 cntlzw r9,r9
>>> c0012340: 21 29 00 20 subfic r9,r9,32
>>> c0012344: 7c 63 48 30 slw r3,r3,r9
>>> c0012348: 4e 80 00 20 blr
>>>
>>> With this patch, the same function will compile as follows:
>>>
>>> c0012334 <ffs_test>:
>>> c0012334: 38 60 00 08 li r3,8
>>> c0012338: 4e 80 00 20 blr
>>
>> But what code does it generate when it's not a constant?
>
> The generated code is the same with and without the patch when not a
> constant:
>
> int ffs_test2(int x)
> {
> return ffs(x);
> }
>
> c001233c <ffs_test2>:
> c001233c: 7d 23 00 d0 neg r9,r3
> c0012340: 7d 23 18 38 and r3,r9,r3
> c0012344: 7c 63 00 34 cntlzw r3,r3
> c0012348: 20 63 00 20 subfic r3,r3,32
> c001234c: 4e 80 00 20 blr
>
>>
>> And which gcc version first added the builtin version?
> Don't know, but __builtin_ffs() is already used in
> arch/powerpc/include/asm/page_32.h
>
Hi Michael,
Any change to get it into 4.9 ?
Christophe
^ permalink raw reply [flat|nested] 5+ messages in thread
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2016-05-12 15:32 [PATCH] powerpc: Discard ffs() function and use builtin_ffs instead Christophe Leroy
2016-05-13 6:16 ` Michael Ellerman
2016-05-13 6:53 ` Christophe Leroy
2016-09-16 10:01 ` Christophe Leroy
2016-05-13 8:29 ` Gabriel Paubert
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