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      1 /*	$NetBSD: ffs.S,v 1.5 2020/05/05 06:20:55 skrll Exp $	*/
      2 
      3 /*	$OpenBSD: ffs.S,v 1.3 2001/06/04 23:14:02 mickey Exp $	*/
      4 
      5 /*
      6  * Copyright (c) 1990, 1991, 1992, 1994, The University of Utah and
      7  * the Computer Systems Laboratory at the University of Utah (CSL).
      8  * All rights reserved.
      9  *
     10  * Permission to use, copy, modify and distribute this software and its
     11  * documentation is hereby granted, provided that both the copyright
     12  * notice and this permission notice appear in all copies of the
     13  * software, derivative works or modified versions, and any portions
     14  * thereof, and that both notices appear in supporting documentation.
     15  *
     16  * THE UNIVERSITY OF UTAH AND CSL ALLOW FREE USE OF THIS SOFTWARE IN ITS "AS
     17  * IS" CONDITION.  THE UNIVERSITY OF UTAH AND CSL DISCLAIM ANY LIABILITY OF
     18  * ANY KIND FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
     19  *
     20  * CSL requests users of this software to return to csl-dist (at) cs.utah.edu any
     21  * improvements that they make and grant CSL redistribution rights.
     22  *
     23  */
     24 
     25 #include <machine/asm.h>
     26 
     27 #ifdef SYSLIBC_SCCS
     28 	.text
     29 	.asciz "$OpenBSD: ffs.S,v 1.3 2001/06/04 23:14:02 mickey Exp $"
     30 	.align	4
     31 #endif
     32 
     33 /*
     34  * ffs(bitmask)
     35  *
     36  * Return the position of the "most significant" bit in `bitmask'.
     37  * Since this is similar to the VAX ffs instruction, bits in a word
     38  * are numbered as "32, 31, ... 1", 0 is returned if no bits are set.
     39  */
     40 
     41 LEAF_ENTRY(ffs)
     42 	comb,=	%arg0,%r0,ffsdone	; If arg0 is 0
     43 	 or	%r0,%r0,%ret0		;   return 0
     44 	ldi	32,%ret0		; Set return to high bit
     45 	extru,=	%arg0,31,16,%r0		; If low 16 bits are non-zero
     46 	addi,tr	-16,%ret0,%ret0		;   subtract 16 from bitpos
     47 	shd	%r0,%arg0,16,%arg0	; else shift right 16 bits
     48 	extru,=	%arg0,31,8,%r0		; If low 8 bits are non-zero
     49 	addi,tr	-8,%ret0,%ret0		;   subtract 8 from bitpos
     50 	shd	%r0,%arg0,8,%arg0	; else shift right 8 bits
     51 	extru,=	%arg0,31,4,%r0		; If low 4 bits are non-zero
     52 	addi,tr	-4,%ret0,%ret0		;   subtract 4 from bitpos
     53 	shd	%r0,%arg0,4,%arg0	; else shift right 4 bits
     54 	extru,=	%arg0,31,2,%r0		; If low 2 bits are non-zero
     55 	addi,tr	-2,%ret0,%ret0		;   subtract 2 from bitpos
     56 	shd	%r0,%arg0,2,%arg0	; else shift right 2 bits
     57 	extru,=	%arg0,31,1,%r0		; If low bit is non-zero
     58 	addi	-1,%ret0,%ret0		;   subtract 1 from bitpos
     59 ffsdone:
     60 	bv,n	%r0(%rp)
     61 EXIT(ffs)
     62 
     63 	.end
     64