1 /* Optimized strlen implementation for PowerPC.
2 Copyright (C) 1997, 1999, 2000 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Library General Public License as
7 published by the Free Software Foundation; either version 2 of the
8 License, or (at your option) any later version.
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Library General Public License for more details.
15 You should have received a copy of the GNU Library General Public
16 License along with the GNU C Library; see the file COPYING.LIB. If not,
17 write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
18 Boston, MA 02111-1307, USA. */
24 /* The algorithm here uses the following techniques:
26 1) Given a word 'x', we can test to see if it contains any 0 bytes
27 by subtracting 0x01010101, and seeing if any of the high bits of each
28 byte changed from 0 to 1. This works because the least significant
29 0 byte must have had no incoming carry (otherwise it's not the least
30 significant), so it is 0x00 - 0x01 == 0xff. For all other
31 byte values, either they have the high bit set initially, or when
32 1 is subtracted you get a value in the range 0x00-0x7f, none of which
33 have their high bit set. The expression here is
34 (x + 0xfefefeff) & ~(x | 0x7f7f7f7f), which gives 0x00000000 when
35 there were no 0x00 bytes in the word.
37 2) Given a word 'x', we can test to see _which_ byte was zero by
38 calculating ~(((x & 0x7f7f7f7f) + 0x7f7f7f7f) | x | 0x7f7f7f7f).
39 This produces 0x80 in each byte that was zero, and 0x00 in all
40 the other bytes. The '| 0x7f7f7f7f' clears the low 7 bits in each
41 byte, and the '| x' part ensures that bytes with the high bit set
42 produce 0x00. The addition will carry into the high bit of each byte
43 iff that byte had one of its low 7 bits set. We can then just see
44 which was the most significant bit set and divide by 8 to find how
45 many to add to the index.
46 This is from the book 'The PowerPC Compiler Writer's Guide',
47 by Steve Hoxey, Faraydon Karim, Bill Hay and Hank Warren.
49 We deal with strings not aligned to a word boundary by taking the
50 first word and ensuring that bytes not part of the string
51 are treated as nonzero. To allow for memory latency, we unroll the
52 loop a few times, being careful to ensure that we do not read ahead
53 across cache line boundaries.
56 1) How long are strings passed to strlen? If they're often really long,
57 we should probably use cache management instructions and/or unroll the
58 loop more. If they're often quite short, it might be better to use
59 fact (2) in the inner loop than have to recalculate it.
60 2) How popular are bytes with the high bit set? If they are very rare,
61 on some processors it might be useful to use the simpler expression
62 ~((x - 0x01010101) | 0x7f7f7f7f) (that is, on processors with only one
63 ALU), but this fails when any character has its high bit set. */
65 /* Some notes on register usage: Under the SVR4 ABI, we can use registers
66 0 and 3 through 12 (so long as we don't call any procedures) without
67 saving them. We can also use registers 14 through 31 if we save them.
68 We can't use r1 (it's the stack pointer), r2 nor r13 because the user
69 program may expect them to hold their usual value if we get sent
70 a signal. Integer parameters are passed in r3 through r10.
71 We can use condition registers cr0, cr1, cr5, cr6, and cr7 without saving
72 them, the others we must save. */
74 /* int [r3] strlen (char *s [r3]) */
76 ENTRY (BP_SYM (strlen))
79 #define rRTN r3 /* incoming STR arg, outgoing result */
80 #define rSTR r4 /* current string position */
81 #define rPADN r5 /* number of padding bits we prepend to the
82 string to make it start at a word boundary */
83 #define rFEFE r6 /* constant 0xfefefeff (-0x01010101) */
84 #define r7F7F r7 /* constant 0x7f7f7f7f */
85 #define rWORD1 r8 /* current string word */
86 #define rWORD2 r9 /* next string word */
87 #define rMASK r9 /* mask for first string word */
92 CHECK_BOUNDS_LOW (rRTN, rTMP1, rTMP2)
96 rlwinm rPADN, rRTN, 3, 27, 28
99 addi r7F7F, r7F7F, 0x7f7f
100 /* That's the setup done, now do the first pair of words.
101 We make an exception and use method (2) on the first two words, to reduce
103 srw rMASK, rMASK, rPADN
104 and rTMP1, r7F7F, rWORD1
105 or rTMP2, r7F7F, rWORD1
106 add rTMP1, rTMP1, r7F7F
107 nor rTMP1, rTMP2, rTMP1
108 and. rWORD1, rTMP1, rMASK
112 addi rFEFE, rFEFE, -0x101
113 /* Are we now aligned to a doubleword boundary? */
116 /* Handle second word of pair. */
118 and rTMP1, r7F7F, rWORD1
119 or rTMP2, r7F7F, rWORD1
120 add rTMP1, rTMP1, r7F7F
121 nor. rWORD1, rTMP2, rTMP1
129 add rTMP1, rFEFE, rWORD1
130 nor rTMP2, r7F7F, rWORD1
131 and. rTMP1, rTMP1, rTMP2
132 add rTMP3, rFEFE, rWORD2
133 nor rTMP4, r7F7F, rWORD2
135 and. rTMP1, rTMP3, rTMP4
138 and rTMP1, r7F7F, rWORD2
139 add rTMP1, rTMP1, r7F7F
140 andc rWORD1, rTMP4, rTMP1
144 and rTMP1, r7F7F, rWORD1
146 add rTMP1, rTMP1, r7F7F
147 andc rWORD1, rTMP2, rTMP1
149 /* When we get to here, rSTR points to the first word in the string that
150 contains a zero byte, and the most significant set bit in rWORD1 is in that
154 subf rTMP1, rRTN, rSTR
156 add rRTN, rTMP1, rTMP3
157 /* GKM FIXME: check high bound. */
159 END (BP_SYM (strlen))