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Improve generic fls64 for 64-bit machines

PATCH In review Alexander van Heukelum 5 participants 12 messages Mar 15, 2008 – Jul 18, 2008

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Alexander van Heukelum

author
#0 lore raw

cover Improve generic fls64 for 64-bit machines

This series of patches:

[1/3] adds __fls.h to asm-generic
[2/3] modifies asm-*/bitops.h for 64-bit archs to implement __fls [3/3] modifies asm-generic/fls64.h to make use of __fls

I have compiled i386 and x86_64, and they generate the same code as before the change. The changes to the other archs are a best effort. Please comment.

If this patch series is accepted, it will make one tiny bit of the x86-unification a tiny bit cleaner. The patches are against Linus' current tree.

Andrew, if no concensus can be reached that this is a bad patch series, would you be willing to add this to your tree?

Greetings,
Alexander
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author
#1 lore raw

PATCH 1/3 Introduce a generic __fls implementation

Add a generic __fls implementation in the same spirit as the generic __ffs one. It finds the last (most significant) set bit in the given long value.

Signed-off-by Alexander van Heukelum <[email protected]>
---
 include/asm-generic/bitops/__fls.h |   43 ++++++++++++++++++++++++++++++++++++
 1 files changed, 43 insertions(+), 0 deletions(-)
 create mode 100644 include/asm-generic/bitops/__fls.h
include/asm-generic/bitops/__fls.h new file +43 -0
new file mode 100644
index 0000000..be24465
@@ -0,0 +1,43 @@
+#ifndef _ASM_GENERIC_BITOPS___FLS_H_
+#define _ASM_GENERIC_BITOPS___FLS_H_
+
+#include <asm/types.h>
+
+/**
+ * __fls - find last (most-significant) set bit in a long word
+ * @word: the word to search
+ *
+ * Undefined if no set bit exists, so code should check against 0 first.
+ */
+static inline unsigned long __fls(unsigned long word)
+{
+	int num = BITS_PER_LONG - 1;
+
+#if BITS_PER_LONG == 64
+	if (!(word & (~0ul << 32))) {
+		num -= 32;
+		word <<= 32;
+	}
+#endif
+	if (!(word & (~0ul << (BITS_PER_LONG-16)))) {
+		num -= 16;
+		word <<= 16;
+	}
+	if (!(word & (~0ul << (BITS_PER_LONG-8)))) {
+		num -= 8;
+		word <<= 8;
+	}
+	if (!(word & (~0ul << (BITS_PER_LONG-4)))) {
+		num -= 4;
+		word <<= 4;
+	}
+	if (!(word & (~0ul << (BITS_PER_LONG-2)))) {
+		num -= 2;
+		word <<= 2;
+	}
+	if (!(word & (~0ul << (BITS_PER_LONG-1))))
+		num -= 1;
+	return num;
+}
+
+#endif /* _ASM_GENERIC_BITOPS___FLS_H_ */

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author
#2 lore raw

PATCH 2/3 Implement __fls on all 64-bit archs

In reply to Alexander van Heukelum

Implement __fls on all 64-bit archs:

alpha has an implementation of fls64.

	Added __fls(x) = fls64(x) - 1.

ia64 has fls, but not __fls.
Added __fls based on code of fls.

mips and powerpc have __ilog2, which is the same as __fls.

	Added __fls = __ilog2.

parisc, s390, sh and sparc64:
Include generic __fls.

x86_64 already has __fls.

Signed-off-by Alexander van Heukelum <[email protected]>
---
 include/asm-alpha/bitops.h   |    5 +++++
 include/asm-ia64/bitops.h    |   16 ++++++++++++++++
 include/asm-mips/bitops.h    |    5 +++++
 include/asm-parisc/bitops.h  |    1 +
 include/asm-powerpc/bitops.h |    5 +++++
 include/asm-s390/bitops.h    |    1 +
 include/asm-sh/bitops.h      |    1 +
 include/asm-sparc64/bitops.h |    1 +
 8 files changed, 35 insertions(+), 0 deletions(-)
include/asm-alpha/bitops.h +5 -0
index 9e19a70..15f3ae2 100644
@@ -388,6 +388,11 @@ static inline int fls64(unsigned long x)
 }
 #endif
 
+static inline unsigned long __fls(unsigned long x)
+{
+	return fls64(x) - 1;
+}
+
 static inline int fls(int x)
 {
 	return fls64((unsigned int) x);
include/asm-ia64/bitops.h +16 -0
index 953d3df..e2ca800 100644
@@ -407,6 +407,22 @@ fls (int t)
 	return ia64_popcnt(x);
 }
 
+/*
+ * Find the last (most significant) bit set.  Undefined for x==0.
+ * Bits are numbered from 0..63 (e.g., __fls(9) == 3).
+ */
+static inline unsigned long
+__fls (unsigned long x)
+{
+	x |= x >> 1;
+	x |= x >> 2;
+	x |= x >> 4;
+	x |= x >> 8;
+	x |= x >> 16;
+	x |= x >> 32;
+	return ia64_popcnt(x) - 1;
+}
+
 #include <asm-generic/bitops/fls64.h>
 
 /*
include/asm-mips/bitops.h +5 -0
index ec75ce4..c2bd126 100644
@@ -591,6 +591,11 @@ static inline int __ilog2(unsigned long x)
 	return 63 - lz;
 }
 
+static inline unsigned long __fls(unsigned long x)
+{
+	return __ilog2(x);
+}
+
 #if defined(CONFIG_CPU_MIPS32) || defined(CONFIG_CPU_MIPS64)
 
 /*
include/asm-parisc/bitops.h +1 -0
index f8eebcb..7a6ea10 100644
@@ -210,6 +210,7 @@ static __inline__ int fls(int x)
 	return ret;
 }
 
+#include <asm-generic/bitops/__fls.h>
 #include <asm-generic/bitops/fls64.h>
 #include <asm-generic/bitops/hweight.h>
 #include <asm-generic/bitops/lock.h>
include/asm-powerpc/bitops.h +5 -0
index 220d9a7..2fc0c45 100644
@@ -312,6 +312,11 @@ static __inline__ int fls(unsigned int x)
 	asm ("cntlzw %0,%1" : "=r" (lz) : "r" (x));
 	return 32 - lz;
 }
+
+static __inline__ unsigned long __fls(unsigned long x)
+{
+	return __ilog2(x);
+}
 #include <asm-generic/bitops/fls64.h>
 
 #include <asm-generic/bitops/hweight.h>
include/asm-s390/bitops.h +1 -0
index 965394e..b4eb24a 100644
@@ -769,6 +769,7 @@ static inline int sched_find_first_bit(unsigned long *b)
 }
 
 #include <asm-generic/bitops/fls.h>
+#include <asm-generic/bitops/__fls.h>
 #include <asm-generic/bitops/fls64.h>
 
 #include <asm-generic/bitops/hweight.h>
include/asm-sh/bitops.h +1 -0
index b6ba5a6..d7d382f 100644
@@ -95,6 +95,7 @@ static inline unsigned long ffz(unsigned long word)
 #include <asm-generic/bitops/ext2-atomic.h>
 #include <asm-generic/bitops/minix.h>
 #include <asm-generic/bitops/fls.h>
+#include <asm-generic/bitops/__fls.h>
 #include <asm-generic/bitops/fls64.h>
 
 #endif /* __KERNEL__ */
include/asm-sparc64/bitops.h +1 -0
index 982ce89..11f9d81 100644
@@ -34,6 +34,7 @@ extern void change_bit(unsigned long nr, volatile unsigned long *addr);
 #include <asm-generic/bitops/ffz.h>
 #include <asm-generic/bitops/__ffs.h>
 #include <asm-generic/bitops/fls.h>
+#include <asm-generic/bitops/__fls.h>
 #include <asm-generic/bitops/fls64.h>
 
 #ifdef __KERNEL__

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author
#3 lore raw

PATCH 3/3 Use __fls for fls64 on 64-bit archs

In reply to Alexander van Heukelum

Use __fls for fls64 on 64-bit archs. The implementation for 64-bit archs is moved from x86_64 to asm-generic.

Signed-off-by Alexander van Heukelum <[email protected]>
---
 include/asm-generic/bitops/fls64.h |   22 ++++++++++++++++++++++
 include/asm-x86/bitops_64.h        |   15 ++-------------
 2 files changed, 24 insertions(+), 13 deletions(-)
include/asm-generic/bitops/fls64.h +22 -0
index 1b6b17c..86d403f 100644
@@ -3,6 +3,18 @@
 
 #include <asm/types.h>
 
+/**
+ * fls64 - find last set bit in a 64-bit word
+ * @x: the word to search
+ *
+ * This is defined in a similar way as the libc and compiler builtin
+ * ffsll, but returns the position of the most significant set bit.
+ *
+ * fls64(value) returns 0 if value is 0 or the position of the last
+ * set bit if value is nonzero. The last (most significant) bit is
+ * at position 64.
+ */
+#if BITS_PER_LONG == 32
 static inline int fls64(__u64 x)
 {
 	__u32 h = x >> 32;

@@ -10,5 +22,15 @@ static inline int fls64(__u64 x)
 		return fls(h) + 32;
 	return fls(x);
 }
+#elif BITS_PER_LONG == 64
+static inline int fls64(__u64 x)
+{
+	if (x == 0)
+		return 0;
+	return __fls(x) + 1;
+}
+#else
+#error BITS_PER_LONG not 32 or 64
+#endif
 
 #endif /* _ASM_GENERIC_BITOPS_FLS64_H_ */
include/asm-x86/bitops_64.h +2 -13
index aaf1519..1f1f796 100644
@@ -112,19 +112,6 @@ static inline int ffs(int x)
 }
 
 /**
- * fls64 - find last bit set in 64 bit word
- * @x: the word to search
- *
- * This is defined the same way as fls.
- */
-static inline int fls64(__u64 x)
-{
-	if (x == 0)
-		return 0;
-	return __fls(x) + 1;
-}
-
-/**
  * fls - find last bit set
  * @x: the word to search
  *

@@ -146,6 +133,8 @@ static inline int fls(int x)
 
 #endif /* __KERNEL__ */
 
+#include <asm-generic/bitops/fls64.h>
+
 #ifdef __KERNEL__
 
 #include <asm-generic/bitops/ext2-non-atomic.h>

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#4 lore raw

Re: [3/3] Use __fls for fls64 on 64-bit archs

(Sorry, sending this again as I screwed up the previous mail).

Hi,

I have a question about fls64() which I hope you or someone else could clarify, please see below.

include/asm-generic/bitops/fls64.h static inline int fls64(__u64 x) · line 31

Quoted 7 lines · Alexander van Heukelum
include/asm-generic/bitops/fls64.h +7 -0
+#elif BITS_PER_LONG == 64
+static inline int fls64(__u64 x)
+{
+	if (x == 0)
+		return 0;
+	return __fls(x) + 1;
+}

It seems fls64() is implemented on top of __fls(), however the __fls() implementation on the x86-64 architecture states that the result is undefined if the argument does not have any zero bits.
So if I understand correctly, the statement "fls64(~0ULL)" would return an undefined result on x64-64 instead of 64 as one would expect.

Wouldn't it make sense to check for ~0ULL in fls64()?

Thanks,
Ricardo

author
#5 lore raw

PATCH 1/1 x86: fix description of __fls(): __fls(0) is undefined

Ricardo M. Correia spotted that the use of __fls() in fls64() did not seem to make sense. In fact fls64()'s implementation is fine, but the description of __fls() was wrong. Fix that.

Reported-by "Ricardo M. Correia" <[email protected]>
Signed-off-by Alexander van Heukelum <[email protected]>

---

Quoted 6 lines · Ricardo M. Correia

Hi,

I have a question about fls64() which I hope you or someone else could clarify, please see below.

On Sáb, 2008-03-15 at 18:32 +0100, Alexander van Heukelum wrote:

Quoted 11 lines

Review

Quoted 7 lines
(quoted diff) +7 -0
+#elif BITS_PER_LONG == 64
+static inline int fls64(__u64 x)
+{
+	if (x == 0)
+		return 0;
+	return __fls(x) + 1;
+}

It seems fls64() is implemented on top of __fls(), however the __fls() implementation on the x86-64 architecture states that the result is undefined if the argument does not have any zero bits.

You have found a bug. It's not in fls64, though, but a copy/paste one in the comment preceding __fls(). __fls() gives an undefined result if there are no _set_ bits: only __fls(0) gives an undefined result.

The inconsistency is well-spotted, though, thanks.

Patch is against current -tip.

Greetings,
Alexander

Quoted 7 lines

So if I understand correctly, the statement "fls64(~0ULL)" would return an undefined result on x64-64 instead of 64 as one would expect.

Wouldn't it make sense to check for ~0ULL in fls64()?

Thanks,
Ricardo

---

include/asm-x86/bitops.h +1 -1
index 96b1829..cfb2b64 100644
@@ -356,7 +356,7 @@ static inline unsigned long ffz(unsigned long word)
  * __fls: find last set bit in word
  * @word: The word to search
  *
- * Undefined if no zero exists, so code should check against ~0UL first.
+ * Undefined if no set bit exists, so code should check against 0 first.
  */
 static inline unsigned long __fls(unsigned long word)
 {
#7 lore raw
In reply to Alexander van Heukelum
Quoted 9 lines · Alexander van Heukelum

This series of patches:

[1/3] adds __fls.h to asm-generic
[2/3] modifies asm-*/bitops.h for 64-bit archs to implement __fls [3/3] modifies asm-generic/fls64.h to make use of __fls

I have compiled i386 and x86_64, and they generate the same code as before the change. The changes to the other archs are a best effort. Please comment.

i've applied #1 and #3 to x86.git/testing, to see how this works out on x86. But it looks good to me in general.

Ingo
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#8 lore raw
In reply to Alexander van Heukelum
Quoted 5 lines · Alexander van Heukelum

This series of patches:

[1/3] adds __fls.h to asm-generic
[2/3] modifies asm-*/bitops.h for 64-bit archs to implement __fls [3/3] modifies asm-generic/fls64.h to make use of __fls

I strongly support this.

I wish we'd also have a consistent naming convention for all the bitops functions so it will be clearer what data type the function is working on and is the result 0 or 1 based.

It seems like what we currently have is:

name type first bit#
---- ---- ----------
ffs int 1
fls int 1
__ffs ulong 0
__fls ulong 0 # in your proposal
ffz ulong 0
fls64 __u64 1

so it seems like
- ffz is misnamed and is rather confusing.
Apprently is should be renamed to __ffz.

- (new) ffz(x) can be defined to ffs(~(x))

- It'd be nice to have ffs64, and maybe ffz64.

Benny

Quoted 19 lines

I have compiled i386 and x86_64, and they generate the same code as before the change. The changes to the other archs are a best effort. Please comment.

If this patch series is accepted, it will make one tiny bit of the x86-unification a tiny bit cleaner. The patches are against Linus' current tree.

Andrew, if no concensus can be reached that this is a bad patch series, would you be willing to add this to your tree?

Greetings,
Alexander
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Please read the FAQ at http://www.tux.org/lkml/

author
#9 lore raw
Quoted 28 lines · Benny Halevy
Quoted 5 lines · Alexander van Heukelum

This series of patches:

[1/3] adds __fls.h to asm-generic
[2/3] modifies asm-*/bitops.h for 64-bit archs to implement __fls [3/3] modifies asm-generic/fls64.h to make use of __fls

I strongly support this.

I wish we'd also have a consistent naming convention for all the bitops functions so it will be clearer what data type the function is working on and is the result 0 or 1 based.

It seems like what we currently have is:

name type first bit#
---- ---- ----------
ffs int 1
fls int 1
__ffs ulong 0
__fls ulong 0 # in your proposal
ffz ulong 0
fls64 __u64 1

so it seems like
- ffz is misnamed and is rather confusing.
Apprently is should be renamed to __ffz.

Quoted 5 lines

- (new) ffz(x) can be defined to ffs(~(x))

- It'd be nice to have ffs64, and maybe ffz64.

Benny

I think every programmer who thinks in terms of bits realises that ffz(x) == __ffs(~x) and ffz(~x) == __ffs(x) etc... so I would rather get rid of ffz entirely by converting all uses to __ffs. Patch (against current linus) below. After that all implementations of ffz could be removed.

ffs64 would be a good addition to complete the set of functions, but that would be the same as glibc's (and gcc-builtin) ffsll.

Looking into that... the relevant gcc builtins are __builtin_ffs (find first set bit), __builtin_clz (count leading zeroes), __builtin_ctz (count trailing zeroes), __builtin_popcount, maybe __builtin_parity and their -l and -ll variants. Maybe the kernel should be changed to use those names instead of the current ones? ffs would stay as it is. __ffs would become ctz, __fls would become something like 31-clz, and hweight would become popcount.

Greetings,
Alexander

[RFC] how about getting rid of ffz?

The patch is not tested, but the conversion should be completely trivial.

Signed-off-by Alexander van Heukelum <[email protected]>

---

 arch/alpha/kernel/irq_i8259.c     |    2 +-
 arch/alpha/kernel/irq_pyxis.c     |    2 +-
 arch/alpha/kernel/sys_alcor.c     |    2 +-
 arch/alpha/kernel/sys_cabriolet.c |    2 +-
 arch/alpha/kernel/sys_dp264.c     |    2 +-
 arch/alpha/kernel/sys_eb64p.c     |    2 +-
 arch/alpha/kernel/sys_mikasa.c    |    2 +-
 arch/alpha/kernel/sys_noritake.c  |    2 +-
 arch/alpha/kernel/sys_rx164.c     |    2 +-
 arch/arm/kernel/smp.c             |    2 +-
 arch/ia64/hp/common/sba_iommu.c   |    2 +-
 arch/ia64/kernel/perfmon.c        |    2 +-
 arch/ia64/kernel/smp.c            |    2 +-
 arch/ia64/mm/init.c               |    2 +-
 arch/mips/kernel/irixelf.c        |    2 +-
 arch/parisc/kernel/smp.c          |    2 +-
 arch/sh/kernel/cpu/irq/imask.c    |    2 +-
 block/blk-barrier.c               |    2 +-
 crypto/lrw.c                      |    2 +-
 drivers/ieee1394/pcilynx.c        |    2 +-
 drivers/input/keyboard/hilkbd.c   |    2 +-
 drivers/md/bitmap.c               |    4 ++--
 drivers/md/md.c                   |    2 +-
 drivers/md/raid0.c                |    2 +-
 drivers/md/raid10.c               |    2 +-
 drivers/net/wan/cycx_x25.c        |    2 +-
 drivers/scsi/NCR_Q720.c           |    2 +-
 fs/adfs/map.c                     |    4 ++--
 fs/binfmt_elf.c                   |    2 +-
 fs/binfmt_elf_fdpic.c             |    2 +-
 fs/ntfs/mft.c                     |    2 +-
 fs/udf/balloc.c                   |    2 +-
 fs/xfs/xfs_bit.c                  |    2 +-
 include/asm-m68knommu/bitops.h    |    4 ++--
 include/linux/inetdevice.h        |    2 +-
 include/linux/signal.h            |    2 +-
 kernel/signal.c                   |    6 +++---
 lib/find_next_bit.c               |    6 +++---
 mm/page_alloc.c                   |    2 +-
 net/sched/sch_cbq.c               |    4 ++--
 net/sched/sch_htb.c               |   10 +++++-----
 sound/core/oss/mixer_oss.c        |    2 +-
 42 files changed, 54 insertions(+), 54 deletions(-)
arch/alpha/kernel/irq_i8259.c +1 -1
index 9405bee..f54afc8 100644
@@ -174,7 +174,7 @@ isa_no_iack_sc_device_interrupt(unsigned long vector)
 	pic &= 0xFFFB;				/* mask out cascade & hibits */
 
 	while (pic) {
-		int j = ffz(~pic);
+		int j = __ffs(pic);
 		pic &= pic - 1;
 		handle_irq(j);
 	}
arch/alpha/kernel/irq_pyxis.c +1 -1
index d53edbc..272339d 100644
@@ -95,7 +95,7 @@ pyxis_device_interrupt(unsigned long vector)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1; /* clear least bit set */
 		if (i == 7)
 			isa_device_interrupt(vector);
arch/alpha/kernel/sys_alcor.c +1 -1
index d187d01..4747c89 100644
@@ -113,7 +113,7 @@ alcor_device_interrupt(unsigned long vector)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1; /* clear least bit set */
 		if (i == 31) {
 			isa_device_interrupt(vector);
arch/alpha/kernel/sys_cabriolet.c +1 -1
index ace475c..e0f73b0 100644
@@ -95,7 +95,7 @@ cabriolet_device_interrupt(unsigned long v)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1;	/* clear least bit set */
 		if (i == 4) {
 			isa_device_interrupt(v);
arch/alpha/kernel/sys_dp264.c +1 -1
index c71b0fd..d7efd02 100644
@@ -233,7 +233,7 @@ dp264_device_interrupt(unsigned long vector)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1; /* clear least bit set */
 		if (i == 55)
 			isa_device_interrupt(vector);
arch/alpha/kernel/sys_eb64p.c +1 -1
index 9c5a306..4713aab 100644
@@ -93,7 +93,7 @@ eb64p_device_interrupt(unsigned long vector)
 	 * them and call the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1;	/* clear least bit set */
 
 		if (i == 5) {
arch/alpha/kernel/sys_mikasa.c +1 -1
index 8d3e942..26e1ad8 100644
@@ -94,7 +94,7 @@ mikasa_device_interrupt(unsigned long vector)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1; /* clear least bit set */
 		if (i < 16) {
 			isa_device_interrupt(vector);
arch/alpha/kernel/sys_noritake.c +1 -1
index eb2a1d6..a7fcc10 100644
@@ -100,7 +100,7 @@ noritake_device_interrupt(unsigned long vector)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1; /* clear least bit set */
 		if (i < 16) {
 			isa_device_interrupt(vector);
arch/alpha/kernel/sys_rx164.c +1 -1
index ce1faa6..c91ad0b 100644
@@ -99,7 +99,7 @@ rx164_device_interrupt(unsigned long vector)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1; /* clear least bit set */
 		if (i == 20) {
 			isa_no_iack_sc_device_interrupt(vector);
arch/arm/kernel/smp.c +1 -1
index eefae1d..dd372e0 100644
@@ -596,7 +596,7 @@ asmlinkage void __exception do_IPI(struct pt_regs *regs)
 
 			nextmsg = msgs & -msgs;
 			msgs &= ~nextmsg;
-			nextmsg = ffz(~nextmsg);
+			nextmsg = __ffs(nextmsg);
 
 			switch (nextmsg) {
 			case IPI_TIMER:
arch/ia64/hp/common/sba_iommu.c +1 -1
index 523eae6..4d21931 100644
@@ -502,7 +502,7 @@ sba_search_bitmap(struct ioc *ioc, unsigned long bits_wanted, int use_hint)
 		unsigned int bitshiftcnt;
 		for(; res_ptr < res_end ; res_ptr++) {
 			if (likely(*res_ptr != ~0UL)) {
-				bitshiftcnt = ffz(*res_ptr);
+				bitshiftcnt = __ffs(~(*res_ptr));
 				*res_ptr |= (1UL << bitshiftcnt);
 				pide = ((unsigned long)res_ptr - (unsigned long)ioc->res_map);
 				pide <<= 3;	/* convert to bit address */
arch/ia64/kernel/perfmon.c +1 -1
index a2aabfd..abebeba 100644
@@ -6683,7 +6683,7 @@ pfm_init(void)
 	       pmu_conf->num_pmcs,
 	       pmu_conf->num_pmds,
 	       pmu_conf->num_counters,
-	       ffz(pmu_conf->ovfl_val));
+	       __ffs(~(pmu_conf->ovfl_val)));
 
 	/* sanity check */
 	if (pmu_conf->num_pmds >= PFM_NUM_PMD_REGS || pmu_conf->num_pmcs >= PFM_NUM_PMC_REGS) {
arch/ia64/kernel/smp.c +1 -1
index 4e446aa..6f303c0 100644
@@ -134,7 +134,7 @@ handle_IPI (int irq, void *dev_id)
 		do {
 			unsigned long which;
 
-			which = ffz(~ops);
+			which = __ffs(ops);
 			ops &= ~(1 << which);
 
 			switch (which) {
arch/ia64/mm/init.c +1 -1
index a4ca657..d9c910a 100644
@@ -333,7 +333,7 @@ ia64_mmu_init (void *my_cpu_data)
 #	define vmlpt_bits		(impl_va_bits - PAGE_SHIFT + pte_bits)
 #	define POW2(n)			(1ULL << (n))
 
-	impl_va_bits = ffz(~(local_cpu_data->unimpl_va_mask | (7UL << 61)));
+	impl_va_bits = __ffs(local_cpu_data->unimpl_va_mask | (7UL << 61));
 
 	if (impl_va_bits < 51 || impl_va_bits > 61)
 		panic("CPU has bogus IMPL_VA_MSB value of %lu!\n", impl_va_bits - 1);
arch/mips/kernel/irixelf.c +1 -1
index 290d8e3..7a82960 100644
@@ -1207,7 +1207,7 @@ static int irix_core_dump(long signr, struct pt_regs *regs, struct file *file, u
 	notes[1].type = NT_PRPSINFO;
 	notes[1].datasz = sizeof(psinfo);
 	notes[1].data = &psinfo;
-	i = current->state ? ffz(~current->state) + 1 : 0;
+	i = current->state ? __ffs(current->state) + 1 : 0;
 	psinfo.pr_state = i;
 	psinfo.pr_sname = (i < 0 || i > 5) ? '.' : "RSDZTD"[i];
 	psinfo.pr_zomb = psinfo.pr_sname == 'Z';
arch/parisc/kernel/smp.c +1 -1
index 85fc775..992d3c6 100644
@@ -168,7 +168,7 @@ ipi_interrupt(int irq, void *dev_id)
 		    break;
 
 		while (ops) {
-			unsigned long which = ffz(~ops);
+			unsigned long which = __ffs(ops);
 
 			ops &= ~(1 << which);
arch/sh/kernel/cpu/irq/imask.c +1 -1
index 301b505..240fc63 100644
@@ -84,7 +84,7 @@ static void disable_imask_irq(unsigned int irq)
 static void enable_imask_irq(unsigned int irq)
 {
 	set_bit(irq, &imask_mask);
-	interrupt_priority = IMASK_PRIORITY - ffz(imask_mask);
+	interrupt_priority = IMASK_PRIORITY - __ffs(~imask_mask);
 
 	set_interrupt_registers(interrupt_priority);
 }
block/blk-barrier.c +1 -1
index 55c5f1f..45937d9 100644
@@ -57,7 +57,7 @@ inline unsigned blk_ordered_cur_seq(struct request_queue *q)
 {
 	if (!q->ordseq)
 		return 0;
-	return 1 << ffz(q->ordseq);
+	return 1 << __ffs(~(q->ordseq));
 }
 
 unsigned blk_ordered_req_seq(struct request *rq)
crypto/lrw.c +1 -1
index 9d52e58..ddf6303 100644
@@ -115,7 +115,7 @@ static inline int get_index128(be128 *block)
 		if (!~val)
 			continue;
 
-		return x + ffz(val);
+		return x + __ffs(~val);
 	}
 
 	return x;
drivers/ieee1394/pcilynx.c +1 -1
index 8af01ab..0b9e3d1 100644
@@ -141,7 +141,7 @@ static pcl_t alloc_pcl(struct ti_lynx *lynx)
         int i, j;
 
         spin_lock(&lynx->lock);
-        /* FIXME - use ffz() to make this readable */
+        /* FIXME - use __ffs() to make this readable */
         for (i = 0; i < (LOCALRAM_SIZE / 1024); i++) {
                 m = lynx->pcl_bmap[i];
                 for (j = 0; j < 8; j++) {
drivers/input/keyboard/hilkbd.c +1 -1
index 50d80ec..94b6bfb 100644
@@ -254,7 +254,7 @@ hil_keyb_init(void)
 		kbid = -1;
 		printk(KERN_WARNING "HIL: no keyboard present\n");
 	} else {
-		kbid = ffz(~c);
+		kbid = __ffs(c);
 		printk(KERN_INFO "HIL: keyboard found at id %d\n", kbid);
 	}
drivers/md/bitmap.c +2 -2
index c14dacd..7b0d5f7 100644
@@ -538,7 +538,7 @@ static int bitmap_read_sb(struct bitmap *bitmap)
 		reason = "unrecognized superblock version";
 	else if (chunksize < PAGE_SIZE)
 		reason = "bitmap chunksize too small";
-	else if ((1 << ffz(~chunksize)) != chunksize)
+	else if ((1 << __ffs(chunksize)) != chunksize)
 		reason = "bitmap chunksize not a power of 2";
 	else if (daemon_sleep < 1 || daemon_sleep > MAX_SCHEDULE_TIMEOUT / HZ)
 		reason = "daemon sleep period out of range";

@@ -1540,7 +1540,7 @@ int bitmap_create(mddev_t *mddev)
 	if (err)
 		goto error;
 
-	bitmap->chunkshift = ffz(~bitmap->chunksize);
+	bitmap->chunkshift = __ffs(bitmap->chunksize);
 
 	/* now that chunksize and chunkshift are set, we can use these macros */
  	chunks = (blocks + CHUNK_BLOCK_RATIO(bitmap) - 1) /
drivers/md/md.c +1 -1
index 61ccbd2..a963235 100644
@@ -3316,7 +3316,7 @@ static int do_md_run(mddev_t * mddev)
 		/*
 		 * chunk-size has to be a power of 2 and multiples of PAGE_SIZE
 		 */
-		if ( (1 << ffz(~chunk_size)) != chunk_size) {
+		if ( (1 << __ffs(chunk_size)) != chunk_size) {
 			printk(KERN_ERR "chunk_size of %d not valid\n", chunk_size);
 			return -EINVAL;
 		}
drivers/md/raid0.c +1 -1
index 818b482..e680472 100644
@@ -407,7 +407,7 @@ static int raid0_make_request (struct request_queue *q, struct bio *bio)
 
 	chunk_size = mddev->chunk_size >> 10;
 	chunk_sects = mddev->chunk_size >> 9;
-	chunksize_bits = ffz(~chunk_size);
+	chunksize_bits = __ffs(chunk_size);
 	block = bio->bi_sector >> 1;
drivers/md/raid10.c +1 -1
index 32389d2..0f876b3 100644
@@ -2029,7 +2029,7 @@ static int run(mddev_t *mddev)
 	conf->copies = nc*fc;
 	conf->far_offset = fo;
 	conf->chunk_mask = (sector_t)(mddev->chunk_size>>9)-1;
-	conf->chunk_shift = ffz(~mddev->chunk_size) - 9;
+	conf->chunk_shift = __ffs(mddev->chunk_size) - 9;
 	size = mddev->size >> (conf->chunk_shift-1);
 	sector_div(size, fc);
 	size = size * conf->raid_disks;
drivers/net/wan/cycx_x25.c +1 -1
index d3b28b0..8d66031 100644
@@ -1207,7 +1207,7 @@ static int x25_place_call(struct cycx_device *card,
 		return -EAGAIN;
 	}
 
-	key = ffz(card->u.x.connection_keys);
+	key = __ffs(~(card->u.x.connection_keys));
 	set_bit(key, (void*)&card->u.x.connection_keys);
 	++key;
 	dprintk(1, KERN_INFO "%s:x25_place_call:key=%d\n", card->devname, key);
drivers/scsi/NCR_Q720.c +1 -1
index a8bbdc2..768d2c6 100644
@@ -66,7 +66,7 @@ NCR_Q720_intr(int irq, void *data)
 		return IRQ_NONE;
 
 
-	while((siop = ffz(sir)) < p->siops) {
+	while((siop = __ffs(~sir)) < p->siops) {
 		sir |= 1<<siop;
 		ncr53c8xx_intr(irq, p->hosts[siop]);
 	}
fs/adfs/map.c +2 -2
index 92ab4fb..74002c7 100644
@@ -101,7 +101,7 @@ lookup_zone(const struct adfs_discmap *dm, const unsigned int idlen,
 				v = le32_to_cpu(_map[mapptr >> 5]);
 			}
 
-			mapptr += 1 + ffz(~v);
+			mapptr += 1 + __ffs(v);
 		}
 
 		if (frag == frag_id)

@@ -179,7 +179,7 @@ scan_free_map(struct adfs_sb_info *asb, struct adfs_discmap *dm)
 				v = le32_to_cpu(_map[mapptr >> 5]);
 			}
 
-			mapptr += 1 + ffz(~v);
+			mapptr += 1 + __ffs(v);
 		}
 
 		total += mapptr - start;
fs/binfmt_elf.c +1 -1
index 5e1a4fb..c97c169 100644
@@ -1383,7 +1383,7 @@ static int fill_psinfo(struct elf_prpsinfo *psinfo, struct task_struct *p,
 	psinfo->pr_pgrp = task_pgrp_vnr(p);
 	psinfo->pr_sid = task_session_vnr(p);
 
-	i = p->state ? ffz(~p->state) + 1 : 0;
+	i = p->state ? __ffs(p->state) + 1 : 0;
 	psinfo->pr_state = i;
 	psinfo->pr_sname = (i > 5) ? '.' : "RSDTZW"[i];
 	psinfo->pr_zomb = psinfo->pr_sname == 'Z';
fs/binfmt_elf_fdpic.c +1 -1
index 32649f2..5fd6707 100644
@@ -1396,7 +1396,7 @@ static int fill_psinfo(struct elf_prpsinfo *psinfo, struct task_struct *p,
 	psinfo->pr_pgrp = task_pgrp_vnr(p);
 	psinfo->pr_sid = task_session_vnr(p);
 
-	i = p->state ? ffz(~p->state) + 1 : 0;
+	i = p->state ? __ffs(p->state) + 1 : 0;
 	psinfo->pr_state = i;
 	psinfo->pr_sname = (i > 5) ? '.' : "RSDTZW"[i];
 	psinfo->pr_zomb = psinfo->pr_sname == 'Z';
fs/ntfs/mft.c +1 -1
index 2ad5c8b..df7a352 100644
@@ -1207,7 +1207,7 @@ static int ntfs_mft_bitmap_find_and_alloc_free_rec_nolock(ntfs_volume *vol,
 				byte = buf + (bit >> 3);
 				if (*byte == 0xff)
 					continue;
-				b = ffz((unsigned long)*byte);
+				b = __ffs(~((unsigned long)*byte));
 				if (b < 8 && b >= (bit & 7)) {
 					ll = data_pos + (bit & ~7ull) + b;
 					if (unlikely(ll > (1ll << 32))) {
fs/udf/balloc.c +1 -1
index f855dcb..f021023 100644
@@ -75,7 +75,7 @@ static inline int find_next_one_bit(void *addr, int size, int offset)
 found_first:
 	tmp &= ~0UL >> (BITS_PER_LONG - size);
 found_middle:
-	return result + ffz(~tmp);
+	return result + __ffs(tmp);
 }
 
 #define find_first_one_bit(addr, size)\
fs/xfs/xfs_bit.c +1 -1
index fab0b6d..d09991d 100644
@@ -179,7 +179,7 @@ xfs_contig_bits(uint *map, uint	size, uint start_bit)
 	}
 	return result - start_bit;
 found:
-	return result + ffz(tmp) - start_bit;
+	return result + __ffs(~tmp) - start_bit;
 }
 
 /*
include/asm-m68knommu/bitops.h +2 -2
index c142fbf..476ad9f 100644
@@ -289,9 +289,9 @@ found_first:
 	 * see above. But then we have to swab tmp below for ffz, so
 	 * we might as well do this here.
 	 */
-	return result + ffz(__swab32(tmp) | (~0UL << size));
+	return result + __ffs(~(__swab32(tmp) | (~0UL << size)));
 found_middle:
-	return result + ffz(__swab32(tmp));
+	return result + __ffs(~(__swab32(tmp)));
 }
 
 #define ext2_find_next_bit(addr, size, off) \
include/linux/inetdevice.h +1 -1
index fc4e3db..306d62f 100644
@@ -217,7 +217,7 @@ static __inline__ int inet_mask_len(__be32 mask)
 	__u32 hmask = ntohl(mask);
 	if (!hmask)
 		return 0;
-	return 32 - ffz(~hmask);
+	return 32 - __ffs(hmask);
 }
include/linux/signal.h +1 -1
index 42d2e0a..5cce0a4 100644
@@ -64,7 +64,7 @@ static inline int sigismember(sigset_t *set, int _sig)
 
 static inline int sigfindinword(unsigned long word)
 {
-	return ffz(~word);
+	return __ffs(word);
 }
 
 #endif /* __HAVE_ARCH_SIG_BITOPS */
kernel/signal.c +3 -3
index 6af1210..8814bce 100644
@@ -142,7 +142,7 @@ int next_signal(struct sigpending *pending, sigset_t *mask)
 	default:
 		for (i = 0; i < _NSIG_WORDS; ++i, ++s, ++m)
 			if ((x = *s &~ *m) != 0) {
-				sig = ffz(~x) + i*_NSIG_BPW + 1;
+				sig = __ffs(x) + i*_NSIG_BPW + 1;
 				break;
 			}
 		break;

@@ -153,11 +153,11 @@ int next_signal(struct sigpending *pending, sigset_t *mask)
 			sig = _NSIG_BPW + 1;
 		else
 			break;
-		sig += ffz(~x);
+		sig += __ffs(x);
 		break;
 
 	case 1: if ((x = *s &~ *m) != 0)
-			sig = ffz(~x) + 1;
+			sig = __ffs(x) + 1;
 		break;
 	}
lib/find_next_bit.c +3 -3
index 78ccd73..173359c 100644
@@ -103,7 +103,7 @@ found_first:
 	if (tmp == ~0UL)	/* Are any bits zero? */
 		return result + size;	/* Nope. */
 found_middle:
-	return result + ffz(tmp);
+	return result + __ffs(~tmp);
 }
 
 EXPORT_SYMBOL(find_next_zero_bit);

@@ -170,10 +170,10 @@ found_first:
 	if (tmp == ~0UL)	/* Are any bits zero? */
 		return result + size; /* Nope. Skip ffz */
 found_middle:
-	return result + ffz(tmp);
+	return result + __ffs(~tmp);
 
 found_middle_swap:
-	return result + ffz(ext2_swab(tmp));
+	return result + __ffs(~(ext2_swab(tmp)));
 }
 
 EXPORT_SYMBOL(generic_find_next_zero_le_bit);
mm/page_alloc.c +1 -1
index 402a504..a98e344 100644
@@ -2447,7 +2447,7 @@ static inline unsigned long wait_table_hash_nr_entries(unsigned long pages)
  */
 static inline unsigned long wait_table_bits(unsigned long size)
 {
-	return ffz(~size);
+	return __ffs(size);
 }
 
 #define LONG_ALIGN(x) (((x)+(sizeof(long))-1)&~((sizeof(long))-1))
net/sched/sch_cbq.c +2 -2
index 09969c1..df8b935 100644
@@ -637,7 +637,7 @@ static enum hrtimer_restart cbq_undelay(struct hrtimer *timer)
 	q->pmask = 0;
 
 	while (pmask) {
-		int prio = ffz(~pmask);
+		int prio = __ffs(pmask);
 		psched_tdiff_t tmp;
 
 		pmask &= ~(1<<prio);

@@ -961,7 +961,7 @@ cbq_dequeue_1(struct Qdisc *sch)
 
 	activemask = q->activemask&0xFF;
 	while (activemask) {
-		int prio = ffz(~activemask);
+		int prio = __ffs(activemask);
 		activemask &= ~(1<<prio);
 		skb = cbq_dequeue_prio(sch, prio);
 		if (skb)
net/sched/sch_htb.c +5 -5
index 66148cc..848053a 100644
@@ -343,7 +343,7 @@ static inline void htb_add_class_to_row(struct htb_sched *q,
 {
 	q->row_mask[cl->level] |= mask;
 	while (mask) {
-		int prio = ffz(~mask);
+		int prio = __ffs(mask);
 		mask &= ~(1 << prio);
 		htb_add_to_id_tree(q->row[cl->level] + prio, cl, prio);
 	}

@@ -373,7 +373,7 @@ static inline void htb_remove_class_from_row(struct htb_sched *q,
 	int m = 0;
 
 	while (mask) {
-		int prio = ffz(~mask);
+		int prio = __ffs(mask);
 
 		mask &= ~(1 << prio);
 		if (q->ptr[cl->level][prio] == cl->node + prio)

@@ -401,7 +401,7 @@ static void htb_activate_prios(struct htb_sched *q, struct htb_class *cl)
 	while (cl->cmode == HTB_MAY_BORROW && p && mask) {
 		m = mask;
 		while (m) {
-			int prio = ffz(~m);
+			int prio = __ffs(m);
 			m &= ~(1 << prio);
 
 			if (p->un.inner.feed[prio].rb_node)

@@ -436,7 +436,7 @@ static void htb_deactivate_prios(struct htb_sched *q, struct htb_class *cl)
 		m = mask;
 		mask = 0;
 		while (m) {
-			int prio = ffz(~m);
+			int prio = __ffs(m);
 			m &= ~(1 << prio);
 
 			if (p->un.inner.ptr[prio] == cl->node + prio) {

@@ -925,7 +925,7 @@ static struct sk_buff *htb_dequeue(struct Qdisc *sch)
 
 		m = ~q->row_mask[level];
 		while (m != (int)(-1)) {
-			int prio = ffz(m);
+			int prio = __ffs(~m);
 			m |= 1 << prio;
 			skb = htb_dequeue_tree(q, prio, level);
 			if (likely(skb != NULL)) {
sound/core/oss/mixer_oss.c +1 -1
index 75daed2..70535d8 100644
@@ -217,7 +217,7 @@ static int snd_mixer_oss_set_recsrc(struct snd_mixer_oss_file *fmixer, int recsr
 	if (mixer->get_recsrc && mixer->put_recsrc) {	/* exclusive input */
 		if (recsrc & ~mixer->oss_recsrc)
 			recsrc &= ~mixer->oss_recsrc;
-		mixer->put_recsrc(fmixer, ffz(~recsrc));
+		mixer->put_recsrc(fmixer, __ffs(recsrc));
 		mixer->get_recsrc(fmixer, &result);
 		result = 1 << result;
 	}
#10 lore raw
Quoted 32 lines · Alexander van Heukelum
Quoted 31 lines · Benny Halevy
Quoted 5 lines · Alexander van Heukelum

This series of patches:

[1/3] adds __fls.h to asm-generic
[2/3] modifies asm-*/bitops.h for 64-bit archs to implement __fls [3/3] modifies asm-generic/fls64.h to make use of __fls

I strongly support this.

I wish we'd also have a consistent naming convention for all the bitops functions so it will be clearer what data type the function is working on and is the result 0 or 1 based.

It seems like what we currently have is:

name type first bit#
---- ---- ----------
ffs int 1
fls int 1
__ffs ulong 0
__fls ulong 0 # in your proposal
ffz ulong 0
fls64 __u64 1

so it seems like
- ffz is misnamed and is rather confusing.
Apprently is should be renamed to __ffz.

- (new) ffz(x) can be defined to ffs(~(x))

- It'd be nice to have ffs64, and maybe ffz64.

Quoted 7 lines
Quoted 1 line

Benny

I think every programmer who thinks in terms of bits realises that ffz(x) == __ffs(~x) and ffz(~x) == __ffs(x) etc... so I would rather get rid of ffz entirely by converting all uses to __ffs. Patch (against current linus) below. After that all implementations of ffz could be removed.

Yeah, very few architectures have an optimized version of ffz that will perform noticeably better than __ffs(~x). (e.g. h8300, sh)

Quoted 4 lines

ffs64 would be a good addition to complete the set of functions, but that would be the same as glibc's (and gcc-builtin) ffsll.

Quoted 8 lines

Looking into that... the relevant gcc builtins are __builtin_ffs (find first set bit), __builtin_clz (count leading zeroes), __builtin_ctz (count trailing zeroes), __builtin_popcount, maybe __builtin_parity and their -l and -ll variants. Maybe the kernel should be changed to use those names instead of the current ones? ffs would stay as it is. __ffs would become ctz, __fls would become something like 31-clz, and hweight would become popcount.

Interesting idea. ctz much better than __ffs with regards to the return value's first bit number, but unless you expose clz and convert the code how do you get rid of the __fls vs. fls confusion?
(BTW for __fls, I'd use BITS_PER_LONG - 1, not 31 :)

I think that adopting libc's convention might make more sense, i.e., define ffs, ffsl, ffsll, and fls, flsl, flsll, and have *all* be 1-based.

Benny

Quoted 703 lines

Greetings,
Alexander

[RFC] how about getting rid of ffz?

The patch is not tested, but the conversion should be completely trivial.

Signed-off-by Alexander van Heukelum <[email protected]>

---

 arch/alpha/kernel/irq_i8259.c     |    2 +-
 arch/alpha/kernel/irq_pyxis.c     |    2 +-
 arch/alpha/kernel/sys_alcor.c     |    2 +-
 arch/alpha/kernel/sys_cabriolet.c |    2 +-
 arch/alpha/kernel/sys_dp264.c     |    2 +-
 arch/alpha/kernel/sys_eb64p.c     |    2 +-
 arch/alpha/kernel/sys_mikasa.c    |    2 +-
 arch/alpha/kernel/sys_noritake.c  |    2 +-
 arch/alpha/kernel/sys_rx164.c     |    2 +-
 arch/arm/kernel/smp.c             |    2 +-
 arch/ia64/hp/common/sba_iommu.c   |    2 +-
 arch/ia64/kernel/perfmon.c        |    2 +-
 arch/ia64/kernel/smp.c            |    2 +-
 arch/ia64/mm/init.c               |    2 +-
 arch/mips/kernel/irixelf.c        |    2 +-
 arch/parisc/kernel/smp.c          |    2 +-
 arch/sh/kernel/cpu/irq/imask.c    |    2 +-
 block/blk-barrier.c               |    2 +-
 crypto/lrw.c                      |    2 +-
 drivers/ieee1394/pcilynx.c        |    2 +-
 drivers/input/keyboard/hilkbd.c   |    2 +-
 drivers/md/bitmap.c               |    4 ++--
 drivers/md/md.c                   |    2 +-
 drivers/md/raid0.c                |    2 +-
 drivers/md/raid10.c               |    2 +-
 drivers/net/wan/cycx_x25.c        |    2 +-
 drivers/scsi/NCR_Q720.c           |    2 +-
 fs/adfs/map.c                     |    4 ++--
 fs/binfmt_elf.c                   |    2 +-
 fs/binfmt_elf_fdpic.c             |    2 +-
 fs/ntfs/mft.c                     |    2 +-
 fs/udf/balloc.c                   |    2 +-
 fs/xfs/xfs_bit.c                  |    2 +-
 include/asm-m68knommu/bitops.h    |    4 ++--
 include/linux/inetdevice.h        |    2 +-
 include/linux/signal.h            |    2 +-
 kernel/signal.c                   |    6 +++---
 lib/find_next_bit.c               |    6 +++---
 mm/page_alloc.c                   |    2 +-
 net/sched/sch_cbq.c               |    4 ++--
 net/sched/sch_htb.c               |   10 +++++-----
 sound/core/oss/mixer_oss.c        |    2 +-
 42 files changed, 54 insertions(+), 54 deletions(-)
arch/alpha/kernel/irq_i8259.c +1 -1
index 9405bee..f54afc8 100644
@@ -174,7 +174,7 @@ isa_no_iack_sc_device_interrupt(unsigned long vector)
 	pic &= 0xFFFB;				/* mask out cascade & hibits */
 
 	while (pic) {
-		int j = ffz(~pic);
+		int j = __ffs(pic);
 		pic &= pic - 1;
 		handle_irq(j);
 	}
arch/alpha/kernel/irq_pyxis.c +1 -1
index d53edbc..272339d 100644
@@ -95,7 +95,7 @@ pyxis_device_interrupt(unsigned long vector)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1; /* clear least bit set */
 		if (i == 7)
 			isa_device_interrupt(vector);
arch/alpha/kernel/sys_alcor.c +1 -1
index d187d01..4747c89 100644
@@ -113,7 +113,7 @@ alcor_device_interrupt(unsigned long vector)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1; /* clear least bit set */
 		if (i == 31) {
 			isa_device_interrupt(vector);
arch/alpha/kernel/sys_cabriolet.c +1 -1
index ace475c..e0f73b0 100644
@@ -95,7 +95,7 @@ cabriolet_device_interrupt(unsigned long v)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1;	/* clear least bit set */
 		if (i == 4) {
 			isa_device_interrupt(v);
arch/alpha/kernel/sys_dp264.c +1 -1
index c71b0fd..d7efd02 100644
@@ -233,7 +233,7 @@ dp264_device_interrupt(unsigned long vector)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1; /* clear least bit set */
 		if (i == 55)
 			isa_device_interrupt(vector);
arch/alpha/kernel/sys_eb64p.c +1 -1
index 9c5a306..4713aab 100644
@@ -93,7 +93,7 @@ eb64p_device_interrupt(unsigned long vector)
 	 * them and call the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1;	/* clear least bit set */
 
 		if (i == 5) {
arch/alpha/kernel/sys_mikasa.c +1 -1
index 8d3e942..26e1ad8 100644
@@ -94,7 +94,7 @@ mikasa_device_interrupt(unsigned long vector)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1; /* clear least bit set */
 		if (i < 16) {
 			isa_device_interrupt(vector);
arch/alpha/kernel/sys_noritake.c +1 -1
index eb2a1d6..a7fcc10 100644
@@ -100,7 +100,7 @@ noritake_device_interrupt(unsigned long vector)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1; /* clear least bit set */
 		if (i < 16) {
 			isa_device_interrupt(vector);
arch/alpha/kernel/sys_rx164.c +1 -1
index ce1faa6..c91ad0b 100644
@@ -99,7 +99,7 @@ rx164_device_interrupt(unsigned long vector)
 	 * the appropriate interrupt handler.
 	 */
 	while (pld) {
-		i = ffz(~pld);
+		i = __ffs(pld);
 		pld &= pld - 1; /* clear least bit set */
 		if (i == 20) {
 			isa_no_iack_sc_device_interrupt(vector);
arch/arm/kernel/smp.c +1 -1
index eefae1d..dd372e0 100644
@@ -596,7 +596,7 @@ asmlinkage void __exception do_IPI(struct pt_regs *regs)
 
 			nextmsg = msgs & -msgs;
 			msgs &= ~nextmsg;
-			nextmsg = ffz(~nextmsg);
+			nextmsg = __ffs(nextmsg);
 
 			switch (nextmsg) {
 			case IPI_TIMER:
arch/ia64/hp/common/sba_iommu.c +1 -1
index 523eae6..4d21931 100644
@@ -502,7 +502,7 @@ sba_search_bitmap(struct ioc *ioc, unsigned long bits_wanted, int use_hint)
 		unsigned int bitshiftcnt;
 		for(; res_ptr < res_end ; res_ptr++) {
 			if (likely(*res_ptr != ~0UL)) {
-				bitshiftcnt = ffz(*res_ptr);
+				bitshiftcnt = __ffs(~(*res_ptr));
 				*res_ptr |= (1UL << bitshiftcnt);
 				pide = ((unsigned long)res_ptr - (unsigned long)ioc->res_map);
 				pide <<= 3;	/* convert to bit address */
arch/ia64/kernel/perfmon.c +1 -1
index a2aabfd..abebeba 100644
@@ -6683,7 +6683,7 @@ pfm_init(void)
 	       pmu_conf->num_pmcs,
 	       pmu_conf->num_pmds,
 	       pmu_conf->num_counters,
-	       ffz(pmu_conf->ovfl_val));
+	       __ffs(~(pmu_conf->ovfl_val)));
 
 	/* sanity check */
 	if (pmu_conf->num_pmds >= PFM_NUM_PMD_REGS || pmu_conf->num_pmcs >= PFM_NUM_PMC_REGS) {
arch/ia64/kernel/smp.c +1 -1
index 4e446aa..6f303c0 100644
@@ -134,7 +134,7 @@ handle_IPI (int irq, void *dev_id)
 		do {
 			unsigned long which;
 
-			which = ffz(~ops);
+			which = __ffs(ops);
 			ops &= ~(1 << which);
 
 			switch (which) {
arch/ia64/mm/init.c +1 -1
index a4ca657..d9c910a 100644
@@ -333,7 +333,7 @@ ia64_mmu_init (void *my_cpu_data)
 #	define vmlpt_bits		(impl_va_bits - PAGE_SHIFT + pte_bits)
 #	define POW2(n)			(1ULL << (n))
 
-	impl_va_bits = ffz(~(local_cpu_data->unimpl_va_mask | (7UL << 61)));
+	impl_va_bits = __ffs(local_cpu_data->unimpl_va_mask | (7UL << 61));
 
 	if (impl_va_bits < 51 || impl_va_bits > 61)
 		panic("CPU has bogus IMPL_VA_MSB value of %lu!\n", impl_va_bits - 1);
arch/mips/kernel/irixelf.c +1 -1
index 290d8e3..7a82960 100644
@@ -1207,7 +1207,7 @@ static int irix_core_dump(long signr, struct pt_regs *regs, struct file *file, u
 	notes[1].type = NT_PRPSINFO;
 	notes[1].datasz = sizeof(psinfo);
 	notes[1].data = &psinfo;
-	i = current->state ? ffz(~current->state) + 1 : 0;
+	i = current->state ? __ffs(current->state) + 1 : 0;
 	psinfo.pr_state = i;
 	psinfo.pr_sname = (i < 0 || i > 5) ? '.' : "RSDZTD"[i];
 	psinfo.pr_zomb = psinfo.pr_sname == 'Z';
arch/parisc/kernel/smp.c +1 -1
index 85fc775..992d3c6 100644
@@ -168,7 +168,7 @@ ipi_interrupt(int irq, void *dev_id)
 		    break;
 
 		while (ops) {
-			unsigned long which = ffz(~ops);
+			unsigned long which = __ffs(ops);
 
 			ops &= ~(1 << which);
arch/sh/kernel/cpu/irq/imask.c +1 -1
index 301b505..240fc63 100644
@@ -84,7 +84,7 @@ static void disable_imask_irq(unsigned int irq)
 static void enable_imask_irq(unsigned int irq)
 {
 	set_bit(irq, &imask_mask);
-	interrupt_priority = IMASK_PRIORITY - ffz(imask_mask);
+	interrupt_priority = IMASK_PRIORITY - __ffs(~imask_mask);
 
 	set_interrupt_registers(interrupt_priority);
 }
block/blk-barrier.c +1 -1
index 55c5f1f..45937d9 100644
@@ -57,7 +57,7 @@ inline unsigned blk_ordered_cur_seq(struct request_queue *q)
 {
 	if (!q->ordseq)
 		return 0;
-	return 1 << ffz(q->ordseq);
+	return 1 << __ffs(~(q->ordseq));
 }
 
 unsigned blk_ordered_req_seq(struct request *rq)
crypto/lrw.c +1 -1
index 9d52e58..ddf6303 100644
@@ -115,7 +115,7 @@ static inline int get_index128(be128 *block)
 		if (!~val)
 			continue;
 
-		return x + ffz(val);
+		return x + __ffs(~val);
 	}
 
 	return x;
drivers/ieee1394/pcilynx.c +1 -1
index 8af01ab..0b9e3d1 100644
@@ -141,7 +141,7 @@ static pcl_t alloc_pcl(struct ti_lynx *lynx)
         int i, j;
 
         spin_lock(&lynx->lock);
-        /* FIXME - use ffz() to make this readable */
+        /* FIXME - use __ffs() to make this readable */
         for (i = 0; i < (LOCALRAM_SIZE / 1024); i++) {
                 m = lynx->pcl_bmap[i];
                 for (j = 0; j < 8; j++) {
drivers/input/keyboard/hilkbd.c +1 -1
index 50d80ec..94b6bfb 100644
@@ -254,7 +254,7 @@ hil_keyb_init(void)
 		kbid = -1;
 		printk(KERN_WARNING "HIL: no keyboard present\n");
 	} else {
-		kbid = ffz(~c);
+		kbid = __ffs(c);
 		printk(KERN_INFO "HIL: keyboard found at id %d\n", kbid);
 	}
drivers/md/bitmap.c +2 -2
index c14dacd..7b0d5f7 100644
@@ -538,7 +538,7 @@ static int bitmap_read_sb(struct bitmap *bitmap)
 		reason = "unrecognized superblock version";
 	else if (chunksize < PAGE_SIZE)
 		reason = "bitmap chunksize too small";
-	else if ((1 << ffz(~chunksize)) != chunksize)
+	else if ((1 << __ffs(chunksize)) != chunksize)
 		reason = "bitmap chunksize not a power of 2";
 	else if (daemon_sleep < 1 || daemon_sleep > MAX_SCHEDULE_TIMEOUT / HZ)
 		reason = "daemon sleep period out of range";

@@ -1540,7 +1540,7 @@ int bitmap_create(mddev_t *mddev)
 	if (err)
 		goto error;
 
-	bitmap->chunkshift = ffz(~bitmap->chunksize);
+	bitmap->chunkshift = __ffs(bitmap->chunksize);
 
 	/* now that chunksize and chunkshift are set, we can use these macros */
  	chunks = (blocks + CHUNK_BLOCK_RATIO(bitmap) - 1) /
drivers/md/md.c +1 -1
index 61ccbd2..a963235 100644
@@ -3316,7 +3316,7 @@ static int do_md_run(mddev_t * mddev)
 		/*
 		 * chunk-size has to be a power of 2 and multiples of PAGE_SIZE
 		 */
-		if ( (1 << ffz(~chunk_size)) != chunk_size) {
+		if ( (1 << __ffs(chunk_size)) != chunk_size) {
 			printk(KERN_ERR "chunk_size of %d not valid\n", chunk_size);
 			return -EINVAL;
 		}
drivers/md/raid0.c +1 -1
index 818b482..e680472 100644
@@ -407,7 +407,7 @@ static int raid0_make_request (struct request_queue *q, struct bio *bio)
 
 	chunk_size = mddev->chunk_size >> 10;
 	chunk_sects = mddev->chunk_size >> 9;
-	chunksize_bits = ffz(~chunk_size);
+	chunksize_bits = __ffs(chunk_size);
 	block = bio->bi_sector >> 1;
drivers/md/raid10.c +1 -1
index 32389d2..0f876b3 100644
@@ -2029,7 +2029,7 @@ static int run(mddev_t *mddev)
 	conf->copies = nc*fc;
 	conf->far_offset = fo;
 	conf->chunk_mask = (sector_t)(mddev->chunk_size>>9)-1;
-	conf->chunk_shift = ffz(~mddev->chunk_size) - 9;
+	conf->chunk_shift = __ffs(mddev->chunk_size) - 9;
 	size = mddev->size >> (conf->chunk_shift-1);
 	sector_div(size, fc);
 	size = size * conf->raid_disks;
drivers/net/wan/cycx_x25.c +1 -1
index d3b28b0..8d66031 100644
@@ -1207,7 +1207,7 @@ static int x25_place_call(struct cycx_device *card,
 		return -EAGAIN;
 	}
 
-	key = ffz(card->u.x.connection_keys);
+	key = __ffs(~(card->u.x.connection_keys));
 	set_bit(key, (void*)&card->u.x.connection_keys);
 	++key;
 	dprintk(1, KERN_INFO "%s:x25_place_call:key=%d\n", card->devname, key);
drivers/scsi/NCR_Q720.c +1 -1
index a8bbdc2..768d2c6 100644
@@ -66,7 +66,7 @@ NCR_Q720_intr(int irq, void *data)
 		return IRQ_NONE;
 
 
-	while((siop = ffz(sir)) < p->siops) {
+	while((siop = __ffs(~sir)) < p->siops) {
 		sir |= 1<<siop;
 		ncr53c8xx_intr(irq, p->hosts[siop]);
 	}
fs/adfs/map.c +2 -2
index 92ab4fb..74002c7 100644
@@ -101,7 +101,7 @@ lookup_zone(const struct adfs_discmap *dm, const unsigned int idlen,
 				v = le32_to_cpu(_map[mapptr >> 5]);
 			}
 
-			mapptr += 1 + ffz(~v);
+			mapptr += 1 + __ffs(v);
 		}
 
 		if (frag == frag_id)

@@ -179,7 +179,7 @@ scan_free_map(struct adfs_sb_info *asb, struct adfs_discmap *dm)
 				v = le32_to_cpu(_map[mapptr >> 5]);
 			}
 
-			mapptr += 1 + ffz(~v);
+			mapptr += 1 + __ffs(v);
 		}
 
 		total += mapptr - start;
fs/binfmt_elf.c +1 -1
index 5e1a4fb..c97c169 100644
@@ -1383,7 +1383,7 @@ static int fill_psinfo(struct elf_prpsinfo *psinfo, struct task_struct *p,
 	psinfo->pr_pgrp = task_pgrp_vnr(p);
 	psinfo->pr_sid = task_session_vnr(p);
 
-	i = p->state ? ffz(~p->state) + 1 : 0;
+	i = p->state ? __ffs(p->state) + 1 : 0;
 	psinfo->pr_state = i;
 	psinfo->pr_sname = (i > 5) ? '.' : "RSDTZW"[i];
 	psinfo->pr_zomb = psinfo->pr_sname == 'Z';
fs/binfmt_elf_fdpic.c +1 -1
index 32649f2..5fd6707 100644
@@ -1396,7 +1396,7 @@ static int fill_psinfo(struct elf_prpsinfo *psinfo, struct task_struct *p,
 	psinfo->pr_pgrp = task_pgrp_vnr(p);
 	psinfo->pr_sid = task_session_vnr(p);
 
-	i = p->state ? ffz(~p->state) + 1 : 0;
+	i = p->state ? __ffs(p->state) + 1 : 0;
 	psinfo->pr_state = i;
 	psinfo->pr_sname = (i > 5) ? '.' : "RSDTZW"[i];
 	psinfo->pr_zomb = psinfo->pr_sname == 'Z';
fs/ntfs/mft.c +1 -1
index 2ad5c8b..df7a352 100644
@@ -1207,7 +1207,7 @@ static int ntfs_mft_bitmap_find_and_alloc_free_rec_nolock(ntfs_volume *vol,
 				byte = buf + (bit >> 3);
 				if (*byte == 0xff)
 					continue;
-				b = ffz((unsigned long)*byte);
+				b = __ffs(~((unsigned long)*byte));
 				if (b < 8 && b >= (bit & 7)) {
 					ll = data_pos + (bit & ~7ull) + b;
 					if (unlikely(ll > (1ll << 32))) {
fs/udf/balloc.c +1 -1
index f855dcb..f021023 100644
@@ -75,7 +75,7 @@ static inline int find_next_one_bit(void *addr, int size, int offset)
 found_first:
 	tmp &= ~0UL >> (BITS_PER_LONG - size);
 found_middle:
-	return result + ffz(~tmp);
+	return result + __ffs(tmp);
 }
 
 #define find_first_one_bit(addr, size)\
fs/xfs/xfs_bit.c +1 -1
index fab0b6d..d09991d 100644
@@ -179,7 +179,7 @@ xfs_contig_bits(uint *map, uint	size, uint start_bit)
 	}
 	return result - start_bit;
 found:
-	return result + ffz(tmp) - start_bit;
+	return result + __ffs(~tmp) - start_bit;
 }
 
 /*
include/asm-m68knommu/bitops.h +2 -2
index c142fbf..476ad9f 100644
@@ -289,9 +289,9 @@ found_first:
 	 * see above. But then we have to swab tmp below for ffz, so
 	 * we might as well do this here.
 	 */
-	return result + ffz(__swab32(tmp) | (~0UL << size));
+	return result + __ffs(~(__swab32(tmp) | (~0UL << size)));
 found_middle:
-	return result + ffz(__swab32(tmp));
+	return result + __ffs(~(__swab32(tmp)));
 }
 
 #define ext2_find_next_bit(addr, size, off) \
include/linux/inetdevice.h +1 -1
index fc4e3db..306d62f 100644
@@ -217,7 +217,7 @@ static __inline__ int inet_mask_len(__be32 mask)
 	__u32 hmask = ntohl(mask);
 	if (!hmask)
 		return 0;
-	return 32 - ffz(~hmask);
+	return 32 - __ffs(hmask);
 }
include/linux/signal.h +1 -1
index 42d2e0a..5cce0a4 100644
@@ -64,7 +64,7 @@ static inline int sigismember(sigset_t *set, int _sig)
 
 static inline int sigfindinword(unsigned long word)
 {
-	return ffz(~word);
+	return __ffs(word);
 }
 
 #endif /* __HAVE_ARCH_SIG_BITOPS */
kernel/signal.c +3 -3
index 6af1210..8814bce 100644
@@ -142,7 +142,7 @@ int next_signal(struct sigpending *pending, sigset_t *mask)
 	default:
 		for (i = 0; i < _NSIG_WORDS; ++i, ++s, ++m)
 			if ((x = *s &~ *m) != 0) {
-				sig = ffz(~x) + i*_NSIG_BPW + 1;
+				sig = __ffs(x) + i*_NSIG_BPW + 1;
 				break;
 			}
 		break;

@@ -153,11 +153,11 @@ int next_signal(struct sigpending *pending, sigset_t *mask)
 			sig = _NSIG_BPW + 1;
 		else
 			break;
-		sig += ffz(~x);
+		sig += __ffs(x);
 		break;
 
 	case 1: if ((x = *s &~ *m) != 0)
-			sig = ffz(~x) + 1;
+			sig = __ffs(x) + 1;
 		break;
 	}
lib/find_next_bit.c +3 -3
index 78ccd73..173359c 100644
@@ -103,7 +103,7 @@ found_first:
 	if (tmp == ~0UL)	/* Are any bits zero? */
 		return result + size;	/* Nope. */
 found_middle:
-	return result + ffz(tmp);
+	return result + __ffs(~tmp);
 }
 
 EXPORT_SYMBOL(find_next_zero_bit);

@@ -170,10 +170,10 @@ found_first:
 	if (tmp == ~0UL)	/* Are any bits zero? */
 		return result + size; /* Nope. Skip ffz */
 found_middle:
-	return result + ffz(tmp);
+	return result + __ffs(~tmp);
 
 found_middle_swap:
-	return result + ffz(ext2_swab(tmp));
+	return result + __ffs(~(ext2_swab(tmp)));
 }
 
 EXPORT_SYMBOL(generic_find_next_zero_le_bit);
mm/page_alloc.c +1 -1
index 402a504..a98e344 100644
@@ -2447,7 +2447,7 @@ static inline unsigned long wait_table_hash_nr_entries(unsigned long pages)
  */
 static inline unsigned long wait_table_bits(unsigned long size)
 {
-	return ffz(~size);
+	return __ffs(size);
 }
 
 #define LONG_ALIGN(x) (((x)+(sizeof(long))-1)&~((sizeof(long))-1))
net/sched/sch_cbq.c +2 -2
index 09969c1..df8b935 100644
@@ -637,7 +637,7 @@ static enum hrtimer_restart cbq_undelay(struct hrtimer *timer)
 	q->pmask = 0;
 
 	while (pmask) {
-		int prio = ffz(~pmask);
+		int prio = __ffs(pmask);
 		psched_tdiff_t tmp;
 
 		pmask &= ~(1<<prio);

@@ -961,7 +961,7 @@ cbq_dequeue_1(struct Qdisc *sch)
 
 	activemask = q->activemask&0xFF;
 	while (activemask) {
-		int prio = ffz(~activemask);
+		int prio = __ffs(activemask);
 		activemask &= ~(1<<prio);
 		skb = cbq_dequeue_prio(sch, prio);
 		if (skb)
net/sched/sch_htb.c +5 -5
index 66148cc..848053a 100644
@@ -343,7 +343,7 @@ static inline void htb_add_class_to_row(struct htb_sched *q,
 {
 	q->row_mask[cl->level] |= mask;
 	while (mask) {
-		int prio = ffz(~mask);
+		int prio = __ffs(mask);
 		mask &= ~(1 << prio);
 		htb_add_to_id_tree(q->row[cl->level] + prio, cl, prio);
 	}

@@ -373,7 +373,7 @@ static inline void htb_remove_class_from_row(struct htb_sched *q,
 	int m = 0;
 
 	while (mask) {
-		int prio = ffz(~mask);
+		int prio = __ffs(mask);
 
 		mask &= ~(1 << prio);
 		if (q->ptr[cl->level][prio] == cl->node + prio)

@@ -401,7 +401,7 @@ static void htb_activate_prios(struct htb_sched *q, struct htb_class *cl)
 	while (cl->cmode == HTB_MAY_BORROW && p && mask) {
 		m = mask;
 		while (m) {
-			int prio = ffz(~m);
+			int prio = __ffs(m);
 			m &= ~(1 << prio);
 
 			if (p->un.inner.feed[prio].rb_node)

@@ -436,7 +436,7 @@ static void htb_deactivate_prios(struct htb_sched *q, struct htb_class *cl)
 		m = mask;
 		mask = 0;
 		while (m) {
-			int prio = ffz(~m);
+			int prio = __ffs(m);
 			m &= ~(1 << prio);
 
 			if (p->un.inner.ptr[prio] == cl->node + prio) {

@@ -925,7 +925,7 @@ static struct sk_buff *htb_dequeue(struct Qdisc *sch)
 
 		m = ~q->row_mask[level];
 		while (m != (int)(-1)) {
-			int prio = ffz(m);
+			int prio = __ffs(~m);
 			m |= 1 << prio;
 			skb = htb_dequeue_tree(q, prio, level);
 			if (likely(skb != NULL)) {
sound/core/oss/mixer_oss.c +1 -2
index 75daed2..70535d8 100644
@@ -217,7 +217,7 @@ static int snd_mixer_oss_set_recsrc(struct snd_mixer_oss_file *fmixer, int recsr
 	if (mixer->get_recsrc && mixer->put_recsrc) {	/* exclusive input */
 		if (recsrc & ~mixer->oss_recsrc)
 			recsrc &= ~mixer->oss_recsrc;
-		mixer->put_recsrc(fmixer, ffz(~recsrc));
+		mixer->put_recsrc(fmixer, __ffs(recsrc));
 		mixer->get_recsrc(fmixer, &result);
 		result = 1 << result;
 	}
 
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#11 lore raw
Quoted 42 lines · Benny Halevy
Quoted 32 lines · Alexander van Heukelum
Quoted 31 lines · Benny Halevy
Quoted 5 lines · Alexander van Heukelum

This series of patches:

[1/3] adds __fls.h to asm-generic
[2/3] modifies asm-*/bitops.h for 64-bit archs to implement __fls [3/3] modifies asm-generic/fls64.h to make use of __fls

I strongly support this.

I wish we'd also have a consistent naming convention for all the bitops functions so it will be clearer what data type the function is working on and is the result 0 or 1 based.

It seems like what we currently have is:

name type first bit#
---- ---- ----------
ffs int 1
fls int 1
__ffs ulong 0
__fls ulong 0 # in your proposal
ffz ulong 0
fls64 __u64 1

so it seems like
- ffz is misnamed and is rather confusing.
Apprently is should be renamed to __ffz.

- (new) ffz(x) can be defined to ffs(~(x))

- It'd be nice to have ffs64, and maybe ffz64.

Quoted 7 lines
Quoted 1 line

Benny

I think every programmer who thinks in terms of bits realises that ffz(x) == __ffs(~x) and ffz(~x) == __ffs(x) etc... so I would rather get rid of ffz entirely by converting all uses to __ffs. Patch (against current linus) below. After that all implementations of ffz could be removed.

Quoted 3 lines

Yeah, very few architectures have an optimized version of ffz that will perform noticeably better than __ffs(~x). (e.g. h8300, sh)

Yeah, and these implementations seem to be based on a loop over all bits in the word. I don't think adding one extra not-operation to convert ffz to __ffs will hurt much ;).

Quoted 12 lines
Quoted 11 lines

ffs64 would be a good addition to complete the set of functions, but that would be the same as glibc's (and gcc-builtin) ffsll.

Looking into that... the relevant gcc builtins are __builtin_ffs (find first set bit), __builtin_clz (count leading zeroes), __builtin_ctz (count trailing zeroes), __builtin_popcount, maybe __builtin_parity and their -l and -ll variants. Maybe the kernel should be changed to use those names instead of the current ones? ffs would stay as it is. __ffs would become ctz, __fls would become something like 31-clz, and hweight would become popcount.

Quoted 4 lines

Interesting idea. ctz much better than __ffs with regards to the return value's first bit number, but unless you expose clz and convert the code how do you get rid of the __fls vs. fls confusion?

Exposing clz/ctz on all architectures will be the harder part. Changing all current uses of ffs/fls (and __fls) will take some time. Mostly because converting code using fls to use clz instead needs to be done a bit carefully, because fls(0) has defined behaviour, while clz(0) is undefined.

Quoted 1 line

(BTW for __fls, I'd use BITS_PER_LONG - 1, not 31 :)

:)

Quoted 3 lines

I think that adopting libc's convention might make more sense, i.e., define ffs, ffsl, ffsll, and fls, flsl, flsll, and have *all* be 1-based.

I agree that it makes sense for fls. For clz (and ctz) I would choose clz(unsigned long), clz32(u32), and clz64(u64).

Greetings,
Alexander

Quoted 1 line

Benny

Signature · 5 lines
  Alexander van Heukelum
  [email protected]

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