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frob the mask so that only the low set bit is set, to avoid lossage.

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cgd 1996-07-15 05:20:19 +00:00
parent 3f38b4d7ad
commit a906c164d2
1 changed files with 23 additions and 17 deletions
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40
sys/lib/libkern/arch/alpha/ffs.S
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@ -1,4 +1,4 @@
/* $NetBSD: ffs.S,v 1.1 1996/07/15 03:13:42 cgd Exp $ */
/* $NetBSD: ffs.S,v 1.2 1996/07/15 05:20:19 cgd Exp $ */
/*
* Copyright (c) 1995 Christopher G. Demetriou
@ -34,46 +34,52 @@
#include "DEFS.h"
LEAF(ffs, 1)
addl a0, 0, a0
beq a0, Lallzero
addl a0, 0, t0
beq t0, Lallzero
/*
* Initialize return value (v0), and set up t1 so that it
* contains the mask with only the lowest bit set.
*/
subl zero, t0, t1
ldil v0, 1
and t0, t1, t1
and a0, 0xff, t0
bne t0, Ldo8
and t1, 0xff, t2
bne t2, Ldo8
/*
* If lower 16 bits empty, add 16 to result and use upper 16.
*/
zapnot a0, 0x03, t1
bne t1, Ldo16
sra a0, 16, a0
zapnot t1, 0x03, t3
bne t3, Ldo16
sra t1, 16, t1
addl v0, 16, v0
Ldo16:
/*
* If lower 8 bits empty, add 8 to result and use upper 8.
*/
and a0, 0xff, t2
bne t2, Ldo8
sra a0, 8, a0
and t1, 0xff, t4
bne t4, Ldo8
sra t1, 8, t1
addl v0, 8, v0
Ldo8:
and a0, 0x0f, t3 /* lower 4 of 8 empty? */
and a0, 0x33, t4 /* lower 2 of each 4 empty? */
and a0, 0x55, t5 /* lower 1 of each 2 empty? */
and t1, 0x0f, t5 /* lower 4 of 8 empty? */
and t1, 0x33, t6 /* lower 2 of each 4 empty? */
and t1, 0x55, t7 /* lower 1 of each 2 empty? */
/* If lower 4 bits empty, add 4 to result. */
bne t3, Ldo4
bne t5, Ldo4
addl v0, 4, v0
Ldo4: /* If lower 2 bits of each 4 empty, add 2 to result. */
bne t4, Ldo2
bne t6, Ldo2
addl v0, 2, v0
Ldo2: /* If lower bit of each 2 empty, add 1 to result. */
bne t5, Ldone
bne t7, Ldone
addl v0, 1, v0
Ldone: