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926cf114f7
[BZ #12019] The new implementation does not read too much data.
176 lines
4.5 KiB
C
176 lines
4.5 KiB
C
/* Copyright (C) 2010 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, write to the Free
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Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
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02111-1307 USA. */
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#include <string.h>
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typedef unsigned long word;
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static inline word
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ldq_u(const void *s)
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{
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return *(const word *)((word)s & -8);
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}
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#define unlikely(X) __builtin_expect ((X), 0)
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#define prefetch(X) __builtin_prefetch ((void *)(X), 0)
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#define cmpbeq0(X) __builtin_alpha_cmpbge(0, (X))
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#define find(X, Y) cmpbeq0 ((X) ^ (Y))
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/* Search no more than N bytes of S for C. */
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void *
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__memchr (const void *s, int xc, size_t n)
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{
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const word *s_align;
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word t, current, found, mask, offset;
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if (unlikely (n == 0))
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return 0;
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current = ldq_u (s);
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/* Replicate low byte of XC into all bytes of C. */
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t = xc & 0xff; /* 0000000c */
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t = (t << 8) | t; /* 000000cc */
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t = (t << 16) | t; /* 0000cccc */
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const word c = (t << 32) | t; /* cccccccc */
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/* Align the source, and decrement the count by the number
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of bytes searched in the first word. */
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s_align = (const word *)(s & -8);
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n += (s & 7);
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/* Deal with misalignment in the first word for the comparison. */
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mask = (1ul << (s & 7)) - 1;
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/* If the entire string fits within one word, we may need masking
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at both the front and the back of the string. */
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if (unlikely (n <= 8))
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{
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mask |= -1ul << n;
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goto last_quad;
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}
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found = find (current, c) & ~mask;
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if (unlikely (found))
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goto found_it;
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s_align++;
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n -= 8;
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/* If the block is sufficiently large, align to cacheline and prefetch. */
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if (unlikely (n >= 256))
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{
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/* Prefetch 3 cache lines beyond the one we're working on. */
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prefetch (s_align + 8);
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prefetch (s_align + 16);
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prefetch (s_align + 24);
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while ((word)s_align & 63)
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{
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current = *s_align;
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found = find (current, c);
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if (found)
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goto found_it;
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s_align++;
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n -= 8;
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}
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/* Within each cacheline, advance the load for the next word
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before the test for the previous word is complete. This
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allows us to hide the 3 cycle L1 cache load latency. We
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only perform this advance load within a cacheline to prevent
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reading across page boundary. */
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#define CACHELINE_LOOP \
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do { \
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word i, next = s_align[0]; \
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for (i = 0; i < 7; ++i) \
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{ \
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current = next; \
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next = s_align[1]; \
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found = find (current, c); \
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if (unlikely (found)) \
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goto found_it; \
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s_align++; \
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} \
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current = next; \
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found = find (current, c); \
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if (unlikely (found)) \
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goto found_it; \
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s_align++; \
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n -= 64; \
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} while (0)
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/* While there's still lots more data to potentially be read,
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continue issuing prefetches for the 4th cacheline out. */
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while (n >= 256)
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{
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prefetch (s_align + 24);
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CACHELINE_LOOP;
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}
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/* Up to 3 cache lines remaining. Continue issuing advanced
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loads, but stop prefetching. */
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while (n >= 64)
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CACHELINE_LOOP;
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/* We may have exhausted the buffer. */
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if (n == 0)
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return NULL;
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}
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/* Quadword aligned loop. */
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current = *s_align;
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while (n > 8)
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{
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found = find (current, c);
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if (unlikely (found))
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goto found_it;
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current = *++s_align;
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n -= 8;
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}
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/* The last word may need masking at the tail of the compare. */
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mask = -1ul << n;
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last_quad:
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found = find (current, c) & ~mask;
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if (found == 0)
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return NULL;
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found_it:
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#ifdef __alpha_cix__
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offset = __builtin_alpha_cttz (found);
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#else
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/* Extract LSB. */
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found &= -found;
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/* Binary search for the LSB. */
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offset = (found & 0x0f ? 0 : 4);
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offset += (found & 0x33 ? 0 : 2);
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offset += (found & 0x55 ? 0 : 1);
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#endif
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return (void *)((word)s_align + offset);
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}
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#ifdef weak_alias
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weak_alias (__memchr, BP_SYM (memchr))
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#endif
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libc_hidden_builtin_def (memchr)
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