summary refs log tree commit diff
path: root/sysdeps/powerpc/strlen.s
blob: fa1c670912366c663646750afe760e6a4e4edadb (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
 # Optimized strlen implementation for PowerPC.
 # Copyright (C) 1997 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 Library General Public License as
 # published by the Free Software Foundation; either version 2 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
 # Library General Public License for more details.
 #
 # You should have received a copy of the GNU Library General Public
 # License along with the GNU C Library; see the file COPYING.LIB.  If not,
 # write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 # Boston, MA 02111-1307, USA.

 # The algorithm here uses the following techniques:
 #
 # 1) Given a word 'x', we can test to see if it contains any 0 bytes
 #    by subtracting 0x01010101, and seeing if any of the high bits of each
 #    byte changed from 0 to 1. This works because the least significant
 #    0 byte must have had no incoming carry (otherwise it's not the least
 #    significant), so it is 0x00 - 0x01 == 0xff. For all other
 #    byte values, either they have the high bit set initially, or when
 #    1 is subtracted you get a value in the range 0x00-0x7f, none of which
 #    have their high bit set. The expression here is
 #    (x + 0xfefefeff) & ~(x | 0x7f7f7f7f), which gives 0x00000000 when
 #    there were no 0x00 bytes in the word.
 #
 # 2) Given a word 'x', we can test to see _which_ byte was zero by
 #    calculating ~(((x & 0x7f7f7f7f) + 0x7f7f7f7f) | x | 0x7f7f7f7f).
 #    This produces 0x80 in each byte that was zero, and 0x00 in all
 #    the other bytes. The '| 0x7f7f7f7f' clears the low 7 bits in each
 #    byte, and the '| x' part ensures that bytes with the high bit set
 #    produce 0x00. The addition will carry into the high bit of each byte
 #    iff that byte had one of its low 7 bits set. We can then just see
 #    which was the most significant bit set and divide by 8 to find how
 #    many to add to the index.
 #    This is from the book 'The PowerPC Compiler Writer's Guide',
 #    by Steve Hoxey, Faraydon Karim, Bill Hay and Hank Warren.
 #
 # We deal with strings not aligned to a word boundary by taking the
 # first word and ensuring that bytes not part of the string
 # are treated as nonzero. To allow for memory latency, we unroll the
 # loop a few times, being careful to ensure that we do not read ahead
 # across cache line boundaries.
 #
 # Questions to answer:
 # 1) How long are strings passed to strlen? If they're often really long,
 # we should probably use cache management instructions and/or unroll the
 # loop more. If they're often quite short, it might be better to use
 # fact (2) in the inner loop than have to recalculate it.
 # 2) How popular are bytes with the high bit set? If they are very rare,
 # on some processors it might be useful to use the simpler expression
 # ~((x - 0x01010101) | 0x7f7f7f7f) (that is, on processors with only one
 # ALU), but this fails when any character has its high bit set.

 # Some notes on register usage: Under the SVR4 ABI, we can use registers
 # 0 and 3 through 12 (so long as we don't call any procedures) without
 # saving them. We can also use registers 14 through 31 if we save them.
 # We can't use r1 (it's the stack pointer), nor r2 or r13 because the user
 # program may expect them to be hold their usual value if we get sent
 # a signal. Integer parameters are passed in r3 through r10.
 # We can use condition registers cr0, cr1, cr5, cr6, and cr7 without saving
 # them, the others we must save.

	.section ".text"
	.align 2
	.globl strlen
	.type strlen,@function
strlen:
 # On entry, r3 points to the string, and it's left that way.
 # We use r6 to store 0xfefefeff, and r7 to store 0x7f7f7f7f.
 # r4 is used to keep the current index into the string; r5 holds
 # the number of padding bits we prepend to the string to make it
 # start at a word boundary. r8 holds the 'current' word.
 # r9-12 are temporaries. r0 is used as a temporary and for discarded
 # results.
	clrrwi %r4,%r3,2
	lis   %r6,0xfeff
	lis   %r7,0x7f7f
	rlwinm %r10,%r3,0,29,29
	lwz   %r8,0(%r4)
	addi  %r7,%r7,0x7f7f
	rlwinm %r5,%r3,3,27,28
	cmpwi %cr1,%r10,0
	li    %r9,-1
 # That's the setup done, now do the first pair of words.
 # We make an exception and use method (2) on the first two words, to reduce
 # overhead.
	srw   %r9,%r9,%r5
	and   %r0,%r7,%r8
	or    %r10,%r7,%r8
	add   %r0,%r0,%r7
	nor   %r0,%r10,%r0
	and.  %r8,%r0,%r9
	bne   done0
 # Handle second word of pair. Put addi between branches to avoid hurting
 # branch prediction.
	addi  %r6,%r6,0xfffffeff
	
	bne   %cr1,loop
	lwzu  %r8,4(%r4)
	and   %r0,%r7,%r8
	or    %r10,%r7,%r8
	add   %r0,%r0,%r7
	nor.  %r8,%r10,%r0
	bne   done0

 # The loop.
	
loop:	lwz   %r8,4(%r4)
	lwzu  %r9,8(%r4)
	add   %r0,%r6,%r8
	nor   %r10,%r7,%r8
	and.  %r0,%r0,%r10
	add   %r11,%r6,%r9
	nor   %r12,%r7,%r9
	bne   done1
	and.  %r0,%r11,%r12
	beq   loop

	and   %r0,%r7,%r9
	add   %r0,%r0,%r7
	andc  %r8,%r12,%r0
	b     done0
	
done1:	and   %r0,%r7,%r8
	subi  %r4,%r4,4
	add   %r0,%r0,%r7
	andc  %r8,%r10,%r0
	
 # When we get to here, r4 points to the first word in the string that
 # contains a zero byte, and the most significant set bit in r8 is in that
 # byte.
done0:	cntlzw %r11,%r8
	subf  %r0,%r3,%r4
	srwi  %r11,%r11,3
	add   %r3,%r0,%r11
	blr
0:
	.size	 strlen,0b-strlen