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On ARMv5 reduce the value to the LSB before using CLZ. Otherwise we'll

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calculate the position of the MSB not the LSB.
This commit is contained in:
rearnsha 2005-05-06 09:50:25 +00:00
parent e8c879147d
commit dd7174bb3e
1 changed files with 7 additions and 4 deletions
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11
lib/libc/arch/arm/string/ffs.S
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@ -1,4 +1,4 @@
/* $NetBSD: ffs.S,v 1.7 2004/08/21 13:08:29 rearnsha Exp $ */
/* $NetBSD: ffs.S,v 1.8 2005/05/06 09:50:25 rearnsha Exp $ */
/*
* Copyright (c) 2001 Christopher Gilbert
* All rights reserved.
@ -30,7 +30,7 @@
#include <machine/asm.h>
RCSID("$NetBSD: ffs.S,v 1.7 2004/08/21 13:08:29 rearnsha Exp $")
RCSID("$NetBSD: ffs.S,v 1.8 2005/05/06 09:50:25 rearnsha Exp $")
/*
* ffs - find first set bit, this algorithm isolates the first set
@ -38,8 +38,8 @@ RCSID("$NetBSD: ffs.S,v 1.7 2004/08/21 13:08:29 rearnsha Exp $")
* 6 bits as an index into the table. This algorithm should be a win
* over the checking each bit in turn as per the C compiled version.
*
* under ARMv5 there's an instruction called CLZ (count leading Zero's) that
* could be used
* On ARMv5 we use CLZ (count leading Zero's) and then subtract the result
* from 32.
*
* This is the ffs algorithm devised by d.seal and posted to comp.sys.arm on
* 16 Feb 1994.
@ -47,6 +47,9 @@ RCSID("$NetBSD: ffs.S,v 1.7 2004/08/21 13:08:29 rearnsha Exp $")
ENTRY(ffs)
#ifdef _ARM_ARCH_5
/* (X & -X) gives LSB or zero. */
rsb r1, r0, #0
and r0, r0, r1
clz r0, r0
rsb r0, r0, #32
RET