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Subject[PATCH 3/4] powerpc: Replace ffz() by equivalent generic function
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With the ffz() function as defined in arch/powerpc/include/asm/bitops.h
GCC will not optimise the code in case of constant parameter.

This patch replaces ffz() by the generic function.

The generic ffz(x) expects to never be called with ~x == 0
as written in the comment in include/asm-generic/bitops/ffz.h
The only user of ffz() within arch/powerpc/ is
platforms/512x/mpc5121_ads_cpld.c, which checks if x is not 0xff

For non constant calls, the generated code is doing the same:

unsigned long testffz(unsigned long x)
{
return ffz(x);
}

On PPC32, before the patch:
00000018 <testffz>:
18: 7c 63 18 f9 not. r3,r3
1c: 40 82 00 0c bne 28 <testffz+0x10>
20: 38 60 00 20 li r3,32
24: 4e 80 00 20 blr
28: 7d 23 00 d0 neg r9,r3
2c: 7d 23 18 38 and r3,r9,r3
30: 7c 63 00 34 cntlzw r3,r3
34: 20 63 00 1f subfic r3,r3,31
38: 4e 80 00 20 blr

On PPC32, after the patch:
00000018 <testffz>:
18: 39 23 00 01 addi r9,r3,1
1c: 7d 23 18 78 andc r3,r9,r3
20: 7c 63 00 34 cntlzw r3,r3
24: 20 63 00 1f subfic r3,r3,31
28: 4e 80 00 20 blr

On PPC64, before the patch:
0000000000000030 <.testffz>:
30: 7c 60 18 f9 not. r0,r3
34: 38 60 00 40 li r3,64
38: 4d 82 00 20 beqlr
3c: 7c 60 00 d0 neg r3,r0
40: 7c 63 00 38 and r3,r3,r0
44: 7c 63 00 74 cntlzd r3,r3
48: 20 63 00 3f subfic r3,r3,63
4c: 7c 63 07 b4 extsw r3,r3
50: 4e 80 00 20 blr

On PPC64, after the patch:
0000000000000030 <.testffz>:
30: 38 03 00 01 addi r0,r3,1
34: 7c 03 18 78 andc r3,r0,r3
38: 7c 63 00 74 cntlzd r3,r3
3c: 20 63 00 3f subfic r3,r3,63
40: 4e 80 00 20 blr

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
arch/powerpc/include/asm/bitops.h | 20 +-------------------
1 file changed, 1 insertion(+), 19 deletions(-)

diff --git a/arch/powerpc/include/asm/bitops.h b/arch/powerpc/include/asm/bitops.h
index af36b404dbe8..d835cd697d6b 100644
--- a/arch/powerpc/include/asm/bitops.h
+++ b/arch/powerpc/include/asm/bitops.h
@@ -233,25 +233,7 @@ int __ilog2_u64(u64 n)
}
#endif

-/*
- * Determines the bit position of the least significant 0 bit in the
- * specified double word. The returned bit position will be
- * zero-based, starting from the right side (63/31 - 0).
- */
-static __inline__ unsigned long ffz(unsigned long x)
-{
- /* no zero exists anywhere in the 8 byte area. */
- if ((x = ~x) == 0)
- return BITS_PER_LONG;
-
- /*
- * Calculate the bit position of the least significant '1' bit in x
- * (since x has been changed this will actually be the least significant
- * '0' bit in * the original x). Note: (x & -x) gives us a mask that
- * is the least significant * (RIGHT-most) 1-bit of the value in x.
- */
- return __ilog2(x & -x);
-}
+#include <asm-generic/bitops/ffz.h>

#include <asm-generic/bitops/builtin-__ffs.h>

--
2.12.0
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