glibc/sysdeps/alpha/strlen.S

/* Copyright (C) 1996-2022 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library.  If not, see
   <https://www.gnu.org/licenses/>.  */

/* Finds length of a 0-terminated string.  Optimized for the Alpha
   architecture:

      - memory accessed as aligned quadwords only
      - uses cmpbge to compare 8 bytes in parallel
      - does binary search to find 0 byte in last quadword (HAKMEM
	needed 12 instructions to do this instead of the 8 instructions
	that the binary search needs).
*/

#include <sysdep.h>

	.set noreorder
	.set noat

ENTRY(strlen)
#ifdef PROF
	ldgp	gp, 0(pv)
	lda	AT, _mcount
	jsr	AT, (AT), _mcount
	.prologue 1
#else
	.prologue 0
#endif

	ldq_u   t0, 0(a0)	# load first quadword (a0 may be misaligned)
	lda     t1, -1(zero)
	insqh   t1, a0, t1
	andnot  a0, 7, v0
	or      t1, t0, t0
	nop			# dual issue the next two on ev5
	cmpbge  zero, t0, t1	# t1 <- bitmask: bit i == 1 <==> i-th byte == 0
	bne     t1, $found

$loop:	ldq     t0, 8(v0)
	addq    v0, 8, v0	# addr += 8
	cmpbge  zero, t0, t1
	beq     t1, $loop

$found:	negq    t1, t2		# clear all but least set bit
	and     t1, t2, t1

	and     t1, 0xf0, t2	# binary search for that set bit
	and	t1, 0xcc, t3
	and	t1, 0xaa, t4
	cmovne	t2, 4, t2
	cmovne	t3, 2, t3
	cmovne	t4, 1, t4
	addq	t2, t3, t2
	addq	v0, t4, v0
	addq	v0, t2, v0
	nop			# dual issue next two on ev4 and ev5

	subq    v0, a0, v0
	ret

	END(strlen)
libc_hidden_builtin_def (strlen)