glibc/sysdeps/powerpc/strlen.S

/* 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)