1 /* Optimized strlen implementation for PowerPC.
2 Copyright (C) 1997, 1999 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. */
22 /* The algorithm here uses the following techniques:
24 1) Given a word 'x', we can test to see if it contains any 0 bytes
25 by subtracting 0x01010101, and seeing if any of the high bits of each
26 byte changed from 0 to 1. This works because the least significant
27 0 byte must have had no incoming carry (otherwise it's not the least
28 significant), so it is 0x00 - 0x01 == 0xff. For all other
29 byte values, either they have the high bit set initially, or when
30 1 is subtracted you get a value in the range 0x00-0x7f, none of which
31 have their high bit set. The expression here is
32 (x + 0xfefefeff) & ~(x | 0x7f7f7f7f), which gives 0x00000000 when
33 there were no 0x00 bytes in the word.
35 2) Given a word 'x', we can test to see _which_ byte was zero by
36 calculating ~(((x & 0x7f7f7f7f) + 0x7f7f7f7f) | x | 0x7f7f7f7f).
37 This produces 0x80 in each byte that was zero, and 0x00 in all
38 the other bytes. The '| 0x7f7f7f7f' clears the low 7 bits in each
39 byte, and the '| x' part ensures that bytes with the high bit set
40 produce 0x00. The addition will carry into the high bit of each byte
41 iff that byte had one of its low 7 bits set. We can then just see
42 which was the most significant bit set and divide by 8 to find how
43 many to add to the index.
44 This is from the book 'The PowerPC Compiler Writer's Guide',
45 by Steve Hoxey, Faraydon Karim, Bill Hay and Hank Warren.
47 We deal with strings not aligned to a word boundary by taking the
48 first word and ensuring that bytes not part of the string
49 are treated as nonzero. To allow for memory latency, we unroll the
50 loop a few times, being careful to ensure that we do not read ahead
51 across cache line boundaries.
54 1) How long are strings passed to strlen? If they're often really long,
55 we should probably use cache management instructions and/or unroll the
56 loop more. If they're often quite short, it might be better to use
57 fact (2) in the inner loop than have to recalculate it.
58 2) How popular are bytes with the high bit set? If they are very rare,
59 on some processors it might be useful to use the simpler expression
60 ~((x - 0x01010101) | 0x7f7f7f7f) (that is, on processors with only one
61 ALU), but this fails when any character has its high bit set. */
63 /* Some notes on register usage: Under the SVR4 ABI, we can use registers
64 0 and 3 through 12 (so long as we don't call any procedures) without
65 saving them. We can also use registers 14 through 31 if we save them.
66 We can't use r1 (it's the stack pointer), r2 nor r13 because the user
67 program may expect them to hold their usual value if we get sent
68 a signal. Integer parameters are passed in r3 through r10.
69 We can use condition registers cr0, cr1, cr5, cr6, and cr7 without saving
70 them, the others we must save. */
73 /* On entry, r3 points to the string, and it's left that way.
74 We use r6 to store 0xfefefeff, and r7 to store 0x7f7f7f7f.
75 r4 is used to keep the current index into the string; r5 holds
76 the number of padding bits we prepend to the string to make it
77 start at a word boundary. r8 holds the 'current' word.
78 r9-12 are temporaries. r0 is used as a temporary and for discarded
86 /* That's the setup done, now do the first pair of words.
87 We make an exception and use method (2) on the first two words, to reduce
99 /* Are we now aligned to a doubleword boundary? */
102 /* Handle second word of pair. */
135 /* When we get to here, r4 points to the first word in the string that
136 contains a zero byte, and the most significant set bit in r8 is in that