diff options
Diffstat (limited to 'lib/libc/arch/arm/string/ffs.S')
| -rw-r--r-- | lib/libc/arch/arm/string/ffs.S | 49 |
1 files changed, 6 insertions, 43 deletions
diff --git a/lib/libc/arch/arm/string/ffs.S b/lib/libc/arch/arm/string/ffs.S index 9cb18ce92f7..2503f334635 100644 --- a/lib/libc/arch/arm/string/ffs.S +++ b/lib/libc/arch/arm/string/ffs.S @@ -1,4 +1,4 @@ -/* $OpenBSD: ffs.S,v 1.8 2018/01/19 11:10:43 jca Exp $ */ +/* $OpenBSD: ffs.S,v 1.9 2018/01/21 21:56:02 kettenis Exp $ */ /* $NetBSD: ffs.S,v 1.5 2003/04/05 23:08:52 bjh21 Exp $ */ /* * Copyright (c) 2001 Christopher Gilbert @@ -31,49 +31,12 @@ #include "DEFS.h" -/* - * ffs - find first set bit, this algorithm isolates the first set - * bit, then multiplies the number by 0x0450fbaf which leaves the top - * 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 - * - * This is the ffs algorithm devised by d.seal and posted to comp.sys.arm on - * 16 Feb 1994. - */ - - .syntax unified - ENTRY(ffs) /* Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry */ - rsb r1, r0, #0 - ands r0, r0, r1 - /* - * now r0 has at most one set bit, call this X - * if X = 0, all further instructions are skipped - */ - adrne r2, .L_ffs_table - orrne r0, r0, r0, lsl #4 /* r0 = X * 0x11 */ - orrne r0, r0, r0, lsl #6 /* r0 = X * 0x451 */ - rsbne r0, r0, r0, lsl #16 /* r0 = X * 0x0450fbaf */ - - /* now lookup in table indexed on top 6 bits of r0 */ - ldrbne r0, [ r2, r0, lsr #26 ] - - mov pc, lr + rsb r1, r0, #0 + ands r0, r0, r1 + clzne r0, r0 + rsbne r0, r0, #32 + bx lr END(ffs) .protected ffs - -.type .L_ffs_table, _ASM_TYPE_OBJECT; -.L_ffs_table: -/* 0 1 2 3 4 5 6 7 */ - .byte 0, 1, 2, 13, 3, 7, 0, 14 /* 0- 7 */ - .byte 4, 0, 8, 0, 0, 0, 0, 15 /* 8-15 */ - .byte 11, 5, 0, 0, 9, 0, 0, 26 /* 16-23 */ - .byte 0, 0, 0, 0, 0, 22, 28, 16 /* 24-31 */ - .byte 32, 12, 6, 0, 0, 0, 0, 0 /* 32-39 */ - .byte 10, 0, 0, 25, 0, 0, 21, 27 /* 40-47 */ - .byte 31, 0, 0, 0, 0, 24, 0, 20 /* 48-55 */ - .byte 30, 0, 23, 19, 29, 18, 17, 0 /* 56-63 */ |