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31a02e9c5b
By adding a "landing area" at the end of the buffer, it becomes safe to parse more bytes at once. On 32-bit this makes fgets run about 4% faster but it does not save anything on 64-bit.
155 lines
4.1 KiB
C
155 lines
4.1 KiB
C
/*
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* fast fgets() replacement for log parsing
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*
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* Copyright 2000-2009 Willy Tarreau <w@1wt.eu>
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version
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* 2 of the License, or (at your option) any later version.
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*
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* This function manages its own buffer and returns a pointer to that buffer
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* in order to avoid expensive memory copies. It also checks for line breaks
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* 32 bits at a time. It could be improved a lot using mmap() but we would
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* not be allowed to replace trailing \n with zeroes and we would be limited
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* to small log files on 32-bit machines.
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*
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*/
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#include <stdlib.h>
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#include <string.h>
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#include <stdio.h>
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#include <unistd.h>
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// return non-zero if the integer contains at least one zero byte
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static inline unsigned int has_zero(unsigned int x)
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{
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unsigned int y;
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/* Principle: we want to perform 4 tests on one 32-bit int at once. For
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* this, we have to simulate an SIMD instruction which we don't have by
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* default. The principle is that a zero byte is the only one which
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* will cause a 1 to appear on the upper bit of a byte/word/etc... when
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* we subtract 1. So we can detect a zero byte if a one appears at any
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* of the bits 7, 15, 23 or 31 where it was not. It takes only one
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* instruction to test for the presence of any of these bits, but it is
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* still complex to check for their initial absence. Thus, we'll
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* proceed differently : we first save and clear only those bits, then
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* we check in the final result if one of them is present and was not.
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*/
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y = x;
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y -= 0x01010101; /* generate a carry */
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y &= ~x; /* clear the bits that were already set */
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return y & 0x80808080;
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}
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#define FGETS2_BUFSIZE (256*1024)
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const char *fgets2(FILE *stream)
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{
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static char buffer[FGETS2_BUFSIZE + 32];
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static char *end = buffer;
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static char *line = buffer;
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char *next;
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int ret;
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next = line;
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while (1) {
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/* this is a speed-up, we read 32 bits at once and check for an
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* LF character there. We stop if found then continue one at a
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* time.
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*/
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while (next < end && (((unsigned long)next) & 3) && *next != '\n')
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next++;
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/* Now next is multiple of 4 or equal to end. We know we can safely
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* read up to 32 bytes past end if needed because they're allocated.
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*/
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while (next < end) {
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if (has_zero(*(unsigned int *)next ^ 0x0A0A0A0A))
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break;
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next += 4;
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if (has_zero(*(unsigned int *)next ^ 0x0A0A0A0A))
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break;
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next += 4;
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if (has_zero(*(unsigned int *)next ^ 0x0A0A0A0A))
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break;
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next += 4;
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if (has_zero(*(unsigned int *)next ^ 0x0A0A0A0A))
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break;
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next += 4;
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if (has_zero(*(unsigned int *)next ^ 0x0A0A0A0A))
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break;
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next += 4;
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if (has_zero(*(unsigned int *)next ^ 0x0A0A0A0A))
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break;
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next += 4;
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if (has_zero(*(unsigned int *)next ^ 0x0A0A0A0A))
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break;
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next += 4;
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if (has_zero(*(unsigned int *)next ^ 0x0A0A0A0A))
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break;
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next += 4;
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}
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/* We finish if needed : if <next> is below <end>, it means we
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* found an LF in one of the 4 following bytes.
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*/
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while (next < end) {
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if (*next == '\n') {
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const char *start = line;
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*next = '\0';
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line = next + 1;
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return start;
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}
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next++;
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}
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/* we found an incomplete line. First, let's move the
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* remaining part of the buffer to the beginning, then
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* try to complete the buffer with a new read. We can't
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* rely on <next> anymore because it went past <end>.
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*/
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if (line > buffer) {
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if (end != line)
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memmove(buffer, line, end - line);
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end = buffer + (end - line);
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next = end;
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line = buffer;
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} else {
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if (end == buffer + FGETS2_BUFSIZE)
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return NULL;
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}
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ret = read(fileno(stream), end, buffer + FGETS2_BUFSIZE - end);
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if (ret <= 0) {
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if (end == line)
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return NULL;
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*end = '\0';
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end = line; /* ensure we stop next time */
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return line;
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}
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end += ret;
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*end = '\n'; /* make parser stop ASAP */
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/* search for '\n' again */
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}
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}
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#ifdef BENCHMARK
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int main() {
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const char *p;
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unsigned int lines = 0;
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while ((p=fgets2(stdin)))
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lines++;
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printf("lines=%d\n", lines);
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return 0;
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}
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#endif
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