mirror of
https://sourceware.org/git/glibc.git
synced 2024-12-26 20:51:11 +00:00
175 lines
6.6 KiB
ArmAsm
175 lines
6.6 KiB
ArmAsm
/* Optimized strlen implementation for PowerPC64.
|
|
Copyright (C) 1997, 1999, 2000, 2002, 2003, 2011 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 Lesser General Public
|
|
License as published by the Free Software Foundation; either
|
|
version 2.1 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
|
|
Lesser General Public License for more details.
|
|
|
|
You should have received a copy of the GNU Lesser General Public
|
|
License along with the GNU C Library; if not, see
|
|
<http://www.gnu.org/licenses/>. */
|
|
|
|
#include <sysdep.h>
|
|
#include <bp-sym.h>
|
|
#include <bp-asm.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.
|
|
|
|
Answer:
|
|
1) Added a Data Cache Block Touch early to prefetch the first 128
|
|
byte cache line. Adding dcbt instructions to the loop would not be
|
|
effective since most strings will be shorter than the cache line.*/
|
|
|
|
/* 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. */
|
|
|
|
/* int [r3] strlen (char *s [r3]) */
|
|
|
|
ENTRY (BP_SYM (strlen))
|
|
CALL_MCOUNT 1
|
|
|
|
#define rTMP1 r0
|
|
#define rRTN r3 /* incoming STR arg, outgoing result */
|
|
#define rSTR r4 /* current string position */
|
|
#define rPADN r5 /* number of padding bits we prepend to the
|
|
string to make it start at a word boundary */
|
|
#define rFEFE r6 /* constant 0xfefefefefefefeff (-0x0101010101010101) */
|
|
#define r7F7F r7 /* constant 0x7f7f7f7f7f7f7f7f */
|
|
#define rWORD1 r8 /* current string doubleword */
|
|
#define rWORD2 r9 /* next string doubleword */
|
|
#define rMASK r9 /* mask for first string doubleword */
|
|
#define rTMP2 r10
|
|
#define rTMP3 r11
|
|
#define rTMP4 r12
|
|
|
|
/* Note: The Bounded pointer support in this code is broken. This code
|
|
was inherited from PPC32 and that support was never completed.
|
|
Current PPC gcc does not support -fbounds-check or -fbounded-pointers.
|
|
These artifacts are left in the code as a reminder in case we need
|
|
bounded pointer support in the future. */
|
|
CHECK_BOUNDS_LOW (rRTN, rTMP1, rTMP2)
|
|
|
|
dcbt 0,rRTN
|
|
clrrdi rSTR, rRTN, 3
|
|
lis r7F7F, 0x7f7f
|
|
rlwinm rPADN, rRTN, 3, 26, 28
|
|
ld rWORD1, 0(rSTR)
|
|
addi r7F7F, r7F7F, 0x7f7f
|
|
li rMASK, -1
|
|
insrdi r7F7F, r7F7F, 32, 0
|
|
/* That's the setup done, now do the first pair of doublewords.
|
|
We make an exception and use method (2) on the first two doublewords,
|
|
to reduce overhead. */
|
|
srd rMASK, rMASK, rPADN
|
|
and rTMP1, r7F7F, rWORD1
|
|
or rTMP2, r7F7F, rWORD1
|
|
lis rFEFE, -0x101
|
|
add rTMP1, rTMP1, r7F7F
|
|
addi rFEFE, rFEFE, -0x101
|
|
nor rTMP1, rTMP2, rTMP1
|
|
and. rWORD1, rTMP1, rMASK
|
|
mtcrf 0x01, rRTN
|
|
bne L(done0)
|
|
sldi rTMP1, rFEFE, 32
|
|
add rFEFE, rFEFE, rTMP1
|
|
/* Are we now aligned to a doubleword boundary? */
|
|
bt 28, L(loop)
|
|
|
|
/* Handle second doubleword of pair. */
|
|
ldu rWORD1, 8(rSTR)
|
|
and rTMP1, r7F7F, rWORD1
|
|
or rTMP2, r7F7F, rWORD1
|
|
add rTMP1, rTMP1, r7F7F
|
|
nor. rWORD1, rTMP2, rTMP1
|
|
bne L(done0)
|
|
|
|
/* The loop. */
|
|
|
|
L(loop):
|
|
ld rWORD1, 8(rSTR)
|
|
ldu rWORD2, 16(rSTR)
|
|
add rTMP1, rFEFE, rWORD1
|
|
nor rTMP2, r7F7F, rWORD1
|
|
and. rTMP1, rTMP1, rTMP2
|
|
add rTMP3, rFEFE, rWORD2
|
|
nor rTMP4, r7F7F, rWORD2
|
|
bne L(done1)
|
|
and. rTMP1, rTMP3, rTMP4
|
|
beq L(loop)
|
|
|
|
and rTMP1, r7F7F, rWORD2
|
|
add rTMP1, rTMP1, r7F7F
|
|
andc rWORD1, rTMP4, rTMP1
|
|
b L(done0)
|
|
|
|
L(done1):
|
|
and rTMP1, r7F7F, rWORD1
|
|
subi rSTR, rSTR, 8
|
|
add rTMP1, rTMP1, r7F7F
|
|
andc rWORD1, rTMP2, rTMP1
|
|
|
|
/* When we get to here, rSTR points to the first doubleword in the string that
|
|
contains a zero byte, and the most significant set bit in rWORD1 is in that
|
|
byte. */
|
|
L(done0):
|
|
cntlzd rTMP3, rWORD1
|
|
subf rTMP1, rRTN, rSTR
|
|
srdi rTMP3, rTMP3, 3
|
|
add rRTN, rTMP1, rTMP3
|
|
/* GKM FIXME: check high bound. */
|
|
blr
|
|
END (BP_SYM (strlen))
|
|
libc_hidden_builtin_def (strlen)
|