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/* Copyright (C) 2010-2017 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library 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.
The GNU C Library 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 the GNU C Library. If not, see
<http://www.gnu.org/licenses/>. */
#include <string.h>
typedef unsigned long word;
static inline word
ldq_u(const void *s)
{
return *(const word *)((word)s & -8);
}
#define unlikely(X) __builtin_expect ((X), 0)
#define prefetch(X) __builtin_prefetch ((void *)(X), 0)
#define cmpbeq0(X) __builtin_alpha_cmpbge(0, (X))
#define find(X, Y) cmpbeq0 ((X) ^ (Y))
/* Search no more than N bytes of S for C. */
void *
__memchr (const void *s, int xc, size_t n)
{
const word *s_align;
word t, current, found, mask, offset;
if (unlikely (n == 0))
return 0;
current = ldq_u (s);
/* Replicate low byte of XC into all bytes of C. */
t = xc & 0xff; /* 0000000c */
t = (t << 8) | t; /* 000000cc */
t = (t << 16) | t; /* 0000cccc */
const word c = (t << 32) | t; /* cccccccc */
/* Align the source, and decrement the count by the number
of bytes searched in the first word. */
s_align = (const word *)((word)s & -8);
n += ((word)s & 7);
/* Deal with misalignment in the first word for the comparison. */
mask = (1ul << ((word)s & 7)) - 1;
/* If the entire string fits within one word, we may need masking
at both the front and the back of the string. */
if (unlikely (n <= 8))
{
mask |= -1ul << n;
goto last_quad;
}
found = find (current, c) & ~mask;
if (unlikely (found))
goto found_it;
s_align++;
n -= 8;
/* If the block is sufficiently large, align to cacheline and prefetch. */
if (unlikely (n >= 256))
{
/* Prefetch 3 cache lines beyond the one we're working on. */
prefetch (s_align + 8);
prefetch (s_align + 16);
prefetch (s_align + 24);
while ((word)s_align & 63)
{
current = *s_align;
found = find (current, c);
if (found)
goto found_it;
s_align++;
n -= 8;
}
/* Within each cacheline, advance the load for the next word
before the test for the previous word is complete. This
allows us to hide the 3 cycle L1 cache load latency. We
only perform this advance load within a cacheline to prevent
reading across page boundary. */
#define CACHELINE_LOOP \
do { \
word i, next = s_align[0]; \
for (i = 0; i < 7; ++i) \
{ \
current = next; \
next = s_align[1]; \
found = find (current, c); \
if (unlikely (found)) \
goto found_it; \
s_align++; \
} \
current = next; \
found = find (current, c); \
if (unlikely (found)) \
goto found_it; \
s_align++; \
n -= 64; \
} while (0)
/* While there's still lots more data to potentially be read,
continue issuing prefetches for the 4th cacheline out. */
while (n >= 256)
{
prefetch (s_align + 24);
CACHELINE_LOOP;
}
/* Up to 3 cache lines remaining. Continue issuing advanced
loads, but stop prefetching. */
while (n >= 64)
CACHELINE_LOOP;
/* We may have exhausted the buffer. */
if (n == 0)
return NULL;
}
/* Quadword aligned loop. */
current = *s_align;
while (n > 8)
{
found = find (current, c);
if (unlikely (found))
goto found_it;
current = *++s_align;
n -= 8;
}
/* The last word may need masking at the tail of the compare. */
mask = -1ul << n;
last_quad:
found = find (current, c) & ~mask;
if (found == 0)
return NULL;
found_it:
#ifdef __alpha_cix__
offset = __builtin_alpha_cttz (found);
#else
/* Extract LSB. */
found &= -found;
/* Binary search for the LSB. */
offset = (found & 0x0f ? 0 : 4);
offset += (found & 0x33 ? 0 : 2);
offset += (found & 0x55 ? 0 : 1);
#endif
return (void *)((word)s_align + offset);
}
#ifdef weak_alias
weak_alias (__memchr, memchr)
#endif
libc_hidden_builtin_def (memchr)
|
