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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 15:19:27 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 15:19:27 +0000
commite0023883c6d2e6745a19e4b48e186ed156c1fca8 (patch)
tree1a48b8056ec984385d0d862b683535d04d6ed215 /gl/lib/rawmemchr.c
parentInitial commit. (diff)
downloadman-db-e0023883c6d2e6745a19e4b48e186ed156c1fca8.tar.xz
man-db-e0023883c6d2e6745a19e4b48e186ed156c1fca8.zip
Adding upstream version 2.11.2.upstream/2.11.2upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'gl/lib/rawmemchr.c')
-rw-r--r--gl/lib/rawmemchr.c125
1 files changed, 125 insertions, 0 deletions
diff --git a/gl/lib/rawmemchr.c b/gl/lib/rawmemchr.c
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+/* 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