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);
}
c000b8f4 <ffs_test>:
c000b8f4: 38 00 00 01 li r0,1
c000b8f8: 38 60 00 04 li r3,4
c000b8fc: 7c 00 00 34 cntlzw r0,r0
c000b900: 20 00 00 20 subfic r0,r0,32
c000b904: 7c 63 00 30 slw r3,r3,r0
c000b908: 4e 80 00 20 blr
With this patch, the same function will compile as follows:
c000b8f4 <ffs_test>:
c000b8f4: 38 60 00 08 li r3,8
c000b8f8: 4e 80 00 20 blr
With non constant parameters, the generated code is identical on PPC32
without and with the patch:
int ffs_test2(long x)
{
return ffs(x);
}
Before:
c000b90c <ffs_test2>:
c000b90c: 7c 03 00 d0 neg r0,r3
c000b910: 7c 03 18 38 and r3,r0,r3
c000b914: 7c 63 00 34 cntlzw r3,r3
c000b918: 20 63 00 20 subfic r3,r3,32
c000b91c: 4e 80 00 20 blr
After:
c000b8fc <ffs_test2>:
c000b8fc: 7c 03 00 d0 neg r0,r3
c000b900: 7c 03 18 38 and r3,r0,r3
c000b904: 7c 63 00 34 cntlzw r3,r3
c000b908: 20 63 00 20 subfic r3,r3,32
c000b90c: 4e 80 00 20 blr
__builtin_ffs() is already used in arch/powerpc/include/asm/page_32.h
__builtin_ffs() has been in GCC since at least gcc-2.95
On PPC64, the generated code is different, so we do this change
only on PPC32:
Before the change:
c000000000027c00 <.ffs_test2>:
c000000000027c00: 7c 03 00 d0 neg r0,r3
c000000000027c04: 7c 03 18 38 and r3,r0,r3
c000000000027c08: 7c 63 00 74 cntlzd r3,r3
c000000000027c0c: 20 63 00 40 subfic r3,r3,64
c000000000027c10: 7c 63 07 b4 extsw r3,r3
c000000000027c14: 4e 80 00 20 blr
After the change:
c000000000027bf0 <.ffs_test2>:
c000000000027bf0: 7c 03 00 d0 neg r0,r3
c000000000027bf4: 7c 03 18 38 and r3,r0,r3
c000000000027bf8: 7c 63 00 34 cntlzw r3,r3
c000000000027bfc: 20 63 00 20 subfic r3,r3,32
c000000000027c00: 7c 63 07 b4 extsw r3,r3
c000000000027c04: 4e 80 00 20 blr
Signed-off-by: Christophe Leroy <christophe.le...@c-s.fr>
---
v2: limit the change to PPC32
arch/powerpc/include/asm/bitops.h | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/arch/powerpc/include/asm/bitops.h
b/arch/powerpc/include/asm/bitops.h
index 59abc62..0e84f0c 100644
--- a/arch/powerpc/include/asm/bitops.h
+++ b/arch/powerpc/include/asm/bitops.h
@@ -223,11 +223,15 @@ static __inline__ unsigned long __ffs(unsigned long x)
* the libc and compiler builtin ffs routines, therefore
* differs in spirit from the above ffz (man ffs).
*/
+#ifdef CONFIG_PPC64
static __inline__ int ffs(int x)
{
unsigned long i = (unsigned long)x;
return __ilog2(i & -i) + 1;
}
+#else
+#include <asm-generic/bitops/builtin-ffs.h>
+#endif
/*
* fls: find last (most-significant) bit set.
--
2.10.1
