sysdeps/alpha/memchr.c

   1 /* Copyright (C) 2010-2024 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, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #include <string.h>
  19
  20 typedef unsigned long word;
  21
  22 static inline word
  23 ldq_u(const void *s)
  24 {
  25   return *(const word *)((word)s & -8);
  26 }
  27
  28 #define unlikely(X)     __builtin_expect ((X), 0)
  29 #define prefetch(X)     __builtin_prefetch ((void *)(X), 0)
  30
  31 #define cmpbeq0(X)      __builtin_alpha_cmpbge(0, (X))
  32 #define find(X, Y)      cmpbeq0 ((X) ^ (Y))
  33
  34 /* Search no more than N bytes of S for C.  */
  35
  36 void *
  37 __memchr (const void *s, int xc, size_t n)
  38 {
  39   const word *s_align;
  40   word t, current, found, mask, offset;
  41
  42   if (unlikely (n == 0))
  43     return 0;
  44
  45   current = ldq_u (s);
  46
  47   /* Replicate low byte of XC into all bytes of C.  */
  48   t = xc & 0xff;                        /* 0000000c */
  49   t = (t << 8) | t;                     /* 000000cc */
  50   t = (t << 16) | t;                    /* 0000cccc */
  51   const word c = (t << 32) | t;         /* cccccccc */
  52
  53   /* Align the source, and decrement the count by the number
  54      of bytes searched in the first word.  */
  55   s_align = (const word *)((word)s & -8);
  56   {
  57     size_t inc = n + ((word)s & 7);
  58     n = inc | -(inc < n);
  59   }
  60
  61   /* Deal with misalignment in the first word for the comparison.  */
  62   mask = (1ul << ((word)s & 7)) - 1;
  63
  64   /* If the entire string fits within one word, we may need masking
  65      at both the front and the back of the string.  */
  66   if (unlikely (n <= 8))
  67     {
  68       mask |= -1ul << n;
  69       goto last_quad;
  70     }
  71
  72   found = find (current, c) & ~mask;
  73   if (unlikely (found))
  74     goto found_it;
  75
  76   s_align++;
  77   n -= 8;
  78
  79   /* If the block is sufficiently large, align to cacheline and prefetch.  */
  80   if (unlikely (n >= 256))
  81     {
  82       /* Prefetch 3 cache lines beyond the one we're working on.  */
  83       prefetch (s_align + 8);
  84       prefetch (s_align + 16);
  85       prefetch (s_align + 24);
  86
  87       while ((word)s_align & 63)
  88         {
  89           current = *s_align;
  90           found = find (current, c);
  91           if (found)
  92             goto found_it;
  93           s_align++;
  94           n -= 8;
  95         }
  96
  97         /* Within each cacheline, advance the load for the next word
  98            before the test for the previous word is complete.  This
  99            allows us to hide the 3 cycle L1 cache load latency.  We
 100            only perform this advance load within a cacheline to prevent
 101            reading across page boundary.  */
 102 #define CACHELINE_LOOP                          \
 103         do {                                    \
 104           word i, next = s_align[0];            \
 105           for (i = 0; i < 7; ++i)               \
 106             {                                   \
 107               current = next;                   \
 108               next = s_align[1];                \
 109               found = find (current, c);        \
 110               if (unlikely (found))             \
 111                 goto found_it;                  \
 112               s_align++;                        \
 113             }                                   \
 114           current = next;                       \
 115           found = find (current, c);            \
 116           if (unlikely (found))                 \
 117             goto found_it;                      \
 118           s_align++;                            \
 119           n -= 64;                              \
 120         } while (0)
 121
 122       /* While there's still lots more data to potentially be read,
 123          continue issuing prefetches for the 4th cacheline out.  */
 124       while (n >= 256)
 125         {
 126           prefetch (s_align + 24);
 127           CACHELINE_LOOP;
 128         }
 129
 130       /* Up to 3 cache lines remaining.  Continue issuing advanced
 131          loads, but stop prefetching.  */
 132       while (n >= 64)
 133         CACHELINE_LOOP;
 134
 135       /* We may have exhausted the buffer.  */
 136       if (n == 0)
 137         return NULL;
 138     }
 139
 140   /* Quadword aligned loop.  */
 141   current = *s_align;
 142   while (n > 8)
 143     {
 144       found = find (current, c);
 145       if (unlikely (found))
 146         goto found_it;
 147       current = *++s_align;
 148       n -= 8;
 149     }
 150
 151   /* The last word may need masking at the tail of the compare.  */
 152   mask = -1ul << n;
 153  last_quad:
 154   found = find (current, c) & ~mask;
 155   if (found == 0)
 156     return NULL;
 157
 158  found_it:
 159 #ifdef __alpha_cix__
 160   offset = __builtin_alpha_cttz (found);
 161 #else
 162   /* Extract LSB.  */
 163   found &= -found;
 164
 165   /* Binary search for the LSB.  */
 166   offset  = (found & 0x0f ? 0 : 4);
 167   offset += (found & 0x33 ? 0 : 2);
 168   offset += (found & 0x55 ? 0 : 1);
 169 #endif
 170
 171   return (void *)((word)s_align + offset);
 172 }
 173
 174 #ifdef weak_alias
 175 weak_alias (__memchr, memchr)
 176 #endif
 177 libc_hidden_builtin_def (memchr)