strchr_32.c

/*
 * Copyright 2010 Tilera Corporation. All Rights Reserved.
 *
 *   This program is free software; you can redistribute it and/or
 *   modify it under the terms of the GNU General Public License
 *   as published by the Free Software Foundation, version 2.
 *
 *   This program 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, GOOD TITLE or
 *   NON INFRINGEMENT.  See the GNU General Public License for
 *   more details.
 */

#include <linux/types.h>
#include <linux/string.h>
#include <linux/module.h>

#undef strchr

char *strchr(const char *s, int c)
{
	int z, g;

	/* Get an aligned pointer. */
	const uintptr_t s_int = (uintptr_t) s;
	const uint32_t *p = (const uint32_t *)(s_int & -4);

	/* Create four copies of the byte for which we are looking. */
	const uint32_t goal = 0x01010101 * (uint8_t) c;

	/* Read the first aligned word, but force bytes before the string to
	 * match neither zero nor goal (we make sure the high bit of each
	 * byte is 1, and the low 7 bits are all the opposite of the goal
	 * byte).
	 *
	 * Note that this shift count expression works because we know shift
	 * counts are taken mod 32.
	 */
	const uint32_t before_mask = (1 << (s_int << 3)) - 1;
	uint32_t v = (*p | before_mask) ^ (goal & __insn_shrib(before_mask, 1));

	uint32_t zero_matches, goal_matches;
	while (1) {
		/* Look for a terminating '\0'. */
		zero_matches = __insn_seqb(v, 0);

		/* Look for the goal byte. */
		goal_matches = __insn_seqb(v, goal);

		if (__builtin_expect(zero_matches | goal_matches, 0))
			break;

		v = *++p;
	}

	z = __insn_ctz(zero_matches);
	g = __insn_ctz(goal_matches);

	/* If we found c before '\0' we got a match. Note that if c == '\0'
	 * then g == z, and we correctly return the address of the '\0'
	 * rather than NULL.
	 */
	return (g <= z) ? ((char *)p) + (g >> 3) : NULL;
}
EXPORT_SYMBOL(strchr);