From e0023883c6d2e6745a19e4b48e186ed156c1fca8 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 7 Apr 2024 17:19:27 +0200 Subject: Adding upstream version 2.11.2. Signed-off-by: Daniel Baumann --- gl/lib/rawmemchr.c | 125 +++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 125 insertions(+) create mode 100644 gl/lib/rawmemchr.c (limited to 'gl/lib/rawmemchr.c') diff --git a/gl/lib/rawmemchr.c b/gl/lib/rawmemchr.c new file mode 100644 index 0000000..ea68c1b --- /dev/null +++ b/gl/lib/rawmemchr.c @@ -0,0 +1,125 @@ +/* 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 . */ + +#include + +/* Specification. */ +#include + +/* A function definition is only needed if HAVE_RAWMEMCHR is not defined. */ +#if !HAVE_RAWMEMCHR + +# include +# include +# include + +# 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 -- cgit v1.2.3