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- # 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), r2 nor r13 because the user
- # program may expect them to 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 %r7,0x7f7f
- rlwinm %r5,%r3,3,27,28
- lwz %r8,0(%r4)
- li %r9,-1
- addi %r7,%r7,0x7f7f
- # 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
- mtcrf 0x01,%r3
- bne done0
- lis %r6,0xfeff
- addi %r6,%r6,-0x101
- # Are we now aligned to a doubleword boundary?
- bt 29,loop
-
- # Handle second word of pair.
- 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