sysdeps/alpha/memchr.c

   1 /* Copyright (C) 2010 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, write to the Free
  16    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  17    02111-1307 USA.  */
  18
  19 #include <string.h>
  20
  21 typedef unsigned long word;
  22
  23 static inline word
  24 ldq_u(const void *s)
  25 {
  26   return *(const word *)((word)s & -8);
  27 }
  28
  29 #define unlikely(X)     __builtin_expect ((X), 0)
  30 #define prefetch(X)     __builtin_prefetch ((void *)(X), 0)
  31
  32 #define cmpbeq0(X)      __builtin_alpha_cmpbge(0, (X))
  33 #define find(X, Y)      cmpbeq0 ((X) ^ (Y))
  34
  35 /* Search no more than N bytes of S for C.  */
  36
  37 void *
  38 __memchr (const void *s, int xc, size_t n)
  39 {
  40   const word *s_align;
  41   word t, current, found, mask, offset;
  42
  43   if (unlikely (n == 0))
  44     return 0;
  45
  46   current = ldq_u (s);
  47
  48   /* Replicate low byte of XC into all bytes of C.  */
  49   t = xc & 0xff;                        /* 0000000c */
  50   t = (t << 8) | t;                     /* 000000cc */
  51   t = (t << 16) | t;                    /* 0000cccc */
  52   const word c = (t << 32) | t;         /* cccccccc */
  53
  54   /* Align the source, and decrement the count by the number
  55      of bytes searched in the first word.  */
  56   s_align = (const word *)(s & -8);
  57   n += (s & 7);
  58
  59   /* Deal with misalignment in the first word for the comparison.  */
  60   mask = (1ul << (s & 7)) - 1;
  61
  62   /* If the entire string fits within one word, we may need masking
  63      at both the front and the back of the string.  */
  64   if (unlikely (n <= 8))
  65     {
  66       mask |= -1ul << n;
  67       goto last_quad;
  68     }
  69
  70   found = find (current, c) & ~mask;
  71   if (unlikely (found))
  72     goto found_it;
  73
  74   s_align++;
  75   n -= 8;
  76
  77   /* If the block is sufficiently large, align to cacheline and prefetch.  */
  78   if (unlikely (n >= 256))
  79     {
  80       /* Prefetch 3 cache lines beyond the one we're working on.  */
  81       prefetch (s_align + 8);
  82       prefetch (s_align + 16);
  83       prefetch (s_align + 24);
  84
  85       while ((word)s_align & 63)
  86         {
  87           current = *s_align;
  88           found = find (current, c);
  89           if (found)
  90             goto found_it;
  91           s_align++;
  92           n -= 8;
  93         }
  94
  95         /* Within each cacheline, advance the load for the next word
  96            before the test for the previous word is complete.  This
  97            allows us to hide the 3 cycle L1 cache load latency.  We
  98            only perform this advance load within a cacheline to prevent
  99            reading across page boundary.  */
 100 #define CACHELINE_LOOP                          \
 101         do {                                    \
 102           word i, next = s_align[0];            \
 103           for (i = 0; i < 7; ++i)               \
 104             {                                   \
 105               current = next;                   \
 106               next = s_align[1];                \
 107               found = find (current, c);        \
 108               if (unlikely (found))             \
 109                 goto found_it;                  \
 110               s_align++;                        \
 111             }                                   \
 112           current = next;                       \
 113           found = find (current, c);            \
 114           if (unlikely (found))                 \
 115             goto found_it;                      \
 116           s_align++;                            \
 117           n -= 64;                              \
 118         } while (0)
 119
 120       /* While there's still lots more data to potentially be read,
 121          continue issuing prefetches for the 4th cacheline out.  */
 122       while (n >= 256)
 123         {
 124           prefetch (s_align + 24);
 125           CACHELINE_LOOP;
 126         }
 127
 128       /* Up to 3 cache lines remaining.  Continue issuing advanced
 129          loads, but stop prefetching.  */
 130       while (n >= 64)
 131         CACHELINE_LOOP;
 132
 133       /* We may have exhausted the buffer.  */
 134       if (n == 0)
 135         return NULL;
 136     }
 137
 138   /* Quadword aligned loop.  */
 139   current = *s_align;
 140   while (n > 8)
 141     {
 142       found = find (current, c);
 143       if (unlikely (found))
 144         goto found_it;
 145       current = *++s_align;
 146       n -= 8;
 147     }
 148
 149   /* The last word may need masking at the tail of the compare.  */
 150   mask = -1ul << n;
 151  last_quad:
 152   found = find (current, c) & ~mask;
 153   if (found == 0)
 154     return NULL;
 155
 156  found_it:
 157 #ifdef __alpha_cix__
 158   offset = __builtin_alpha_cttz (found);
 159 #else
 160   /* Extract LSB.  */
 161   found &= -found;
 162
 163   /* Binary search for the LSB.  */
 164   offset  = (found & 0x0f ? 0 : 4);
 165   offset += (found & 0x33 ? 0 : 2);
 166   offset += (found & 0x55 ? 0 : 1);
 167 #endif
 168
 169   return (void *)((word)s_align + offset);
 170 }
 171
 172 #ifdef weak_alias
 173 weak_alias (__memchr, BP_SYM (memchr))
 174 #endif
 175 libc_hidden_builtin_def (memchr)