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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. */
+
+#include <sysdep.h>
+
+/* 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. */
+
+ENTRY(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 L(done0)
+ lis %r6,0xfeff
+ addi %r6,%r6,-0x101
+/* Are we now aligned to a doubleword boundary? */
+ bt 29,L(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 L(done0)
+
+/* The loop. */
+
+L(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 L(done1)
+ and. %r0,%r11,%r12
+ beq L(loop)
+
+ and %r0,%r7,%r9
+ add %r0,%r0,%r7
+ andc %r8,%r12,%r0
+ b L(done0)
+
+L(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. */
+L(done0):
+ cntlzw %r11,%r8
+ subf %r0,%r3,%r4
+ srwi %r11,%r11,3
+ add %r3,%r0,%r11
+ blr
+END(strlen)