mirror of
https://sourceware.org/git/binutils-gdb.git
synced 2024-12-27 04:52:05 +08:00
dc6c21dabf
This updates gnulib to a relatively recent commit. Most of this was done by the gnulib import script; the only change I made was to update-gnulib.sh. Tested on x86-64 Fedora 34. I also did a mingw cross build.
126 lines
4.8 KiB
C
126 lines
4.8 KiB
C
/* Searching in a string.
|
|
Copyright (C) 2008-2022 Free Software Foundation, Inc.
|
|
|
|
This file 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.
|
|
|
|
This file 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 this program. If not, see <https://www.gnu.org/licenses/>. */
|
|
|
|
#include <config.h>
|
|
|
|
/* Specification. */
|
|
#include <string.h>
|
|
|
|
/* A function definition is only needed if HAVE_RAWMEMCHR is not defined. */
|
|
#if !HAVE_RAWMEMCHR
|
|
|
|
# include <limits.h>
|
|
# include <stdalign.h>
|
|
# include <stdint.h>
|
|
|
|
# include "verify.h"
|
|
|
|
/* Find the first occurrence of C in S. */
|
|
void *
|
|
rawmemchr (const void *s, int c_in)
|
|
{
|
|
/* Change this typedef to experiment with performance. */
|
|
typedef uintptr_t longword;
|
|
/* If you change the "uintptr_t", you should change UINTPTR_WIDTH to match.
|
|
This verifies that the type does not have padding bits. */
|
|
verify (UINTPTR_WIDTH == UCHAR_WIDTH * sizeof (longword));
|
|
|
|
const unsigned char *char_ptr;
|
|
unsigned char c = c_in;
|
|
|
|
/* Handle the first few bytes by reading one byte at a time.
|
|
Do this until CHAR_PTR is aligned on a longword boundary. */
|
|
for (char_ptr = (const unsigned char *) s;
|
|
(uintptr_t) char_ptr % alignof (longword) != 0;
|
|
++char_ptr)
|
|
if (*char_ptr == c)
|
|
return (void *) char_ptr;
|
|
|
|
longword const *longword_ptr = s = char_ptr;
|
|
|
|
/* Compute auxiliary longword values:
|
|
repeated_one is a value which has a 1 in every byte.
|
|
repeated_c has c in every byte. */
|
|
longword repeated_one = (longword) -1 / UCHAR_MAX;
|
|
longword repeated_c = repeated_one * c;
|
|
longword repeated_hibit = repeated_one * (UCHAR_MAX / 2 + 1);
|
|
|
|
/* Instead of the traditional loop which tests each byte, we will
|
|
test a longword at a time. The tricky part is testing if any of
|
|
the bytes in the longword in question are equal to
|
|
c. We first use an xor with repeated_c. This reduces the task
|
|
to testing whether any of the bytes in longword1 is zero.
|
|
|
|
(The following comments assume 8-bit bytes, as POSIX requires;
|
|
the code's use of UCHAR_MAX should work even if bytes have more
|
|
than 8 bits.)
|
|
|
|
We compute tmp =
|
|
((longword1 - repeated_one) & ~longword1) & (repeated_one * 0x80).
|
|
That is, we perform the following operations:
|
|
1. Subtract repeated_one.
|
|
2. & ~longword1.
|
|
3. & a mask consisting of 0x80 in every byte.
|
|
Consider what happens in each byte:
|
|
- If a byte of longword1 is zero, step 1 and 2 transform it into 0xff,
|
|
and step 3 transforms it into 0x80. A carry can also be propagated
|
|
to more significant bytes.
|
|
- If a byte of longword1 is nonzero, let its lowest 1 bit be at
|
|
position k (0 <= k <= 7); so the lowest k bits are 0. After step 1,
|
|
the byte ends in a single bit of value 0 and k bits of value 1.
|
|
After step 2, the result is just k bits of value 1: 2^k - 1. After
|
|
step 3, the result is 0. And no carry is produced.
|
|
So, if longword1 has only non-zero bytes, tmp is zero.
|
|
Whereas if longword1 has a zero byte, call j the position of the least
|
|
significant zero byte. Then the result has a zero at positions 0, ...,
|
|
j-1 and a 0x80 at position j. We cannot predict the result at the more
|
|
significant bytes (positions j+1..3), but it does not matter since we
|
|
already have a non-zero bit at position 8*j+7.
|
|
|
|
The test whether any byte in longword1 is zero is equivalent
|
|
to testing whether tmp is nonzero.
|
|
|
|
This test can read beyond the end of a string, depending on where
|
|
C_IN is encountered. However, this is considered safe since the
|
|
initialization phase ensured that the read will be aligned,
|
|
therefore, the read will not cross page boundaries and will not
|
|
cause a fault. */
|
|
|
|
while (1)
|
|
{
|
|
longword longword1 = *longword_ptr ^ repeated_c;
|
|
|
|
if ((((longword1 - repeated_one) & ~longword1) & repeated_hibit) != 0)
|
|
break;
|
|
longword_ptr++;
|
|
}
|
|
|
|
char_ptr = s = longword_ptr;
|
|
|
|
/* At this point, we know that one of the sizeof (longword) bytes
|
|
starting at char_ptr is == c. If we knew endianness, we
|
|
could determine the first such byte without any further memory
|
|
accesses, just by looking at the tmp result from the last loop
|
|
iteration. However, the following simple and portable code does
|
|
not attempt this potential optimization. */
|
|
|
|
while (*char_ptr != c)
|
|
char_ptr++;
|
|
return (void *) char_ptr;
|
|
}
|
|
|
|
#endif
|