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.le...@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
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