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Diffstat (limited to 'sysdeps/powerpc/strlen.S')
| -rw-r--r-- | sysdeps/powerpc/strlen.S | 144 |
1 files changed, 144 insertions, 0 deletions
diff --git a/sysdeps/powerpc/strlen.S b/sysdeps/powerpc/strlen.S new file mode 100644 index 0000000000..dc6660b8fc --- /dev/null +++ b/sysdeps/powerpc/strlen.S @@ -0,0 +1,144 @@ +/* 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) |