mono/metadata/gc-memfuncs.c

   1 /*
   2  * test-sgen-qsort.c: Our own bzero/memmove.
   3  *
   4  * Copyright (C) 2013 Xamarin Inc
   5  *
   6  * This library is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU Library General Public
   8  * License 2.0 as published by the Free Software Foundation;
   9  *
  10  * This library is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13  * Library General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU Library General Public
  16  * License 2.0 along with this library; if not, write to the Free
  17  * Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  18  */
  19
  20 /*
  21  * SGen cannot deal with invalid pointers on the heap or in registered roots.  Sometimes we
  22  * need to copy or zero out memory in code that might be interrupted by collections.  To
  23  * guarantee that those operations will not result in invalid pointers, we must do it
  24  * word-atomically.
  25  *
  26  * libc's bzero() and memcpy()/memmove() functions do not guarantee word-atomicity, even in
  27  * cases where one would assume so.  For instance, some implementations (like Darwin's on
  28  * x86) have variants of memcpy() using vector instructions.  Those may copy bytewise for
  29  * the region preceding the first vector-aligned address.  That region could be
  30  * word-aligned, but it would still be copied byte-wise.
  31  *
  32  * All our memory writes here are to "volatile" locations.  This is so that C compilers
  33  * don't "optimize" our code back to calls to bzero()/memmove().  LLVM, specifically, will
  34  * do that.
  35  */
  36
  37 #include <config.h>
  38
  39 #include "metadata/gc-internal.h"
  40
  41 #define ptr_mask ((sizeof (void*) - 1))
  42 #define _toi(ptr) ((size_t)ptr)
  43 #define unaligned_bytes(ptr) (_toi(ptr) & ptr_mask)
  44 #define align_down(ptr) ((void*)(_toi(ptr) & ~ptr_mask))
  45 #define align_up(ptr) ((void*) ((_toi(ptr) + ptr_mask) & ~ptr_mask))
  46 #if SIZEOF_VOID_P == 4
  47 #define bytes_to_words(n)       ((size_t)(n) >> 2)
  48 #elif SIZEOF_VOID_P == 8
  49 #define bytes_to_words(n)       ((size_t)(n) >> 3)
  50 #else
  51 #error We only support 32 and 64 bit architectures.
  52 #endif
  53
  54 #define BZERO_WORDS(dest,words) do {                    \
  55                 void * volatile *__d = (void* volatile*)(dest);         \
  56                 int __n = (words);                      \
  57                 int __i;                                \
  58                 for (__i = 0; __i < __n; ++__i)         \
  59                         __d [__i] = NULL;               \
  60         } while (0)
  61
  62
  63 /**
  64  * mono_gc_bzero_aligned:
  65  * @dest: address to start to clear
  66  * @size: size of the region to clear
  67  *
  68  * Zero @size bytes starting at @dest.
  69  * The address of @dest MUST be aligned to word boundaries
  70  *
  71  * FIXME borrow faster code from some BSD libc or bionic
  72  */
  73 void
  74 mono_gc_bzero_aligned (void *dest, size_t size)
  75 {
  76         volatile char *d = (char*)dest;
  77         size_t tail_bytes, word_bytes;
  78
  79         g_assert (unaligned_bytes (dest) == 0);
  80
  81         /* copy all words with memmove */
  82         word_bytes = (size_t)align_down (size);
  83         switch (word_bytes) {
  84         case sizeof (void*) * 1:
  85                 BZERO_WORDS (d, 1);
  86                 break;
  87         case sizeof (void*) * 2:
  88                 BZERO_WORDS (d, 2);
  89                 break;
  90         case sizeof (void*) * 3:
  91                 BZERO_WORDS (d, 3);
  92                 break;
  93         case sizeof (void*) * 4:
  94                 BZERO_WORDS (d, 4);
  95                 break;
  96         default:
  97                 BZERO_WORDS (d, bytes_to_words (word_bytes));
  98         }
  99
 100         tail_bytes = unaligned_bytes (size);
 101         if (tail_bytes) {
 102                 d += word_bytes;
 103                 do {
 104                         *d++ = 0;
 105                 } while (--tail_bytes);
 106         }
 107 }
 108
 109 /**
 110  * mono_gc_bzero_atomic:
 111  * @dest: address to start to clear
 112  * @size: size of the region to clear
 113  *
 114  * Zero @size bytes starting at @dest.
 115  *
 116  * Use this to zero memory without word tearing when dest is aligned.
 117  */
 118 void
 119 mono_gc_bzero_atomic (void *dest, size_t size)
 120 {
 121         if (unaligned_bytes (dest))
 122                 memset (dest, 0, size);
 123         else
 124                 mono_gc_bzero_aligned (dest, size);
 125 }
 126
 127 #define MEMMOVE_WORDS_UPWARD(dest,src,words) do {       \
 128                 void * volatile *__d = (void* volatile*)(dest);         \
 129                 void **__s = (void**)(src);             \
 130                 int __n = (int)(words);                 \
 131                 int __i;                                \
 132                 for (__i = 0; __i < __n; ++__i)         \
 133                         __d [__i] = __s [__i];          \
 134         } while (0)
 135
 136 #define MEMMOVE_WORDS_DOWNWARD(dest,src,words) do {     \
 137                 void * volatile *__d = (void* volatile*)(dest);         \
 138                 void **__s = (void**)(src);             \
 139                 int __n = (int)(words);                 \
 140                 int __i;                                \
 141                 for (__i = __n - 1; __i >= 0; --__i)    \
 142                         __d [__i] = __s [__i];          \
 143         } while (0)
 144
 145
 146 /**
 147  * mono_gc_memmove_aligned:
 148  * @dest: destination of the move
 149  * @src: source
 150  * @size: size of the block to move
 151  *
 152  * Move @size bytes from @src to @dest.
 153  *
 154  * Use this to copy memory without word tearing when both pointers are aligned
 155  */void
 156 mono_gc_memmove_aligned (void *dest, const void *src, size_t size)
 157 {
 158         g_assert (unaligned_bytes (dest) == 0);
 159         g_assert (unaligned_bytes (src) == 0);
 160
 161         /*
 162         If we're copying less than a word we don't need to worry about word tearing
 163         so we bailout to memmove early.
 164         */
 165         if (size < sizeof(void*)) {
 166                 memmove (dest, src, size);
 167                 return;
 168         }
 169
 170         /*
 171          * A bit of explanation on why we align only dest before doing word copies.
 172          * Pointers to managed objects must always be stored in word aligned addresses, so
 173          * even if dest is misaligned, src will be by the same amount - this ensure proper atomicity of reads.
 174          *
 175          * We don't need to case when source and destination have different alignments since we only do word stores
 176          * using memmove, which must handle it.
 177          */
 178         if (dest > src && ((size_t)((char*)dest - (char*)src) < size)) { /*backward copy*/
 179                         volatile char *p = (char*)dest + size;
 180                         char *s = (char*)src + size;
 181                         char *start = (char*)dest;
 182                         char *align_end = MAX((char*)dest, (char*)align_down (p));
 183                         char *word_start;
 184                         size_t bytes_to_memmove;
 185
 186                         while (p > align_end)
 187                                 *--p = *--s;
 188
 189                         word_start = align_up (start);
 190                         bytes_to_memmove = p - word_start;
 191                         p -= bytes_to_memmove;
 192                         s -= bytes_to_memmove;
 193                         MEMMOVE_WORDS_DOWNWARD (p, s, bytes_to_words (bytes_to_memmove));
 194         } else {
 195                 volatile char *d = (char*)dest;
 196                 const char *s = (const char*)src;
 197                 size_t tail_bytes;
 198
 199                 /* copy all words with memmove */
 200                 MEMMOVE_WORDS_UPWARD (d, s, bytes_to_words (align_down (size)));
 201
 202                 tail_bytes = unaligned_bytes (size);
 203                 if (tail_bytes) {
 204                         d += (size_t)align_down (size);
 205                         s += (size_t)align_down (size);
 206                         do {
 207                                 *d++ = *s++;
 208                         } while (--tail_bytes);
 209                 }
 210         }
 211 }
 212
 213 /**
 214  * mono_gc_memmove_atomic:
 215  * @dest: destination of the move
 216  * @src: source
 217  * @size: size of the block to move
 218  *
 219  * Move @size bytes from @src to @dest.
 220  *
 221  * Use this to copy memory without word tearing when both pointers are aligned
 222  */
 223 void
 224 mono_gc_memmove_atomic (void *dest, const void *src, size_t size)
 225 {
 226         if (unaligned_bytes (_toi (dest) | _toi (src)))
 227                 memmove (dest, src, size);
 228         else
 229                 mono_gc_memmove_aligned (dest, src, size);
 230 }