src/libc/pagecopy.h

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/* Macros for copying by pages; used in memcpy, memmove.  Generic macros.
   Copyright (C) 1995, 1997 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, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

/* This file defines the macro:

   PAGE_COPY_FWD_MAYBE (dstp, srcp, nbytes_left, nbytes)

   which is invoked like WORD_COPY_FWD et al.  The pointers should be at
   least word aligned.  This will check if virtual copying by pages can and
   should be done and do it if so.

   System-specific pagecopy.h files should define these macros and then
   #include this file:

   PAGE_COPY_THRESHOLD
   -- Minimum size for which virtual copying by pages is worthwhile.

   PAGE_SIZE
   -- Size of a page.

   PAGE_COPY_FWD (dstp, srcp, nbytes_left, nbytes)
   -- Macro to perform the virtual copy operation.
   The pointers will be aligned to PAGE_SIZE bytes.
*/


#if PAGE_COPY_THRESHOLD

#include <assert.h>

#define PAGE_COPY_FWD_MAYBE(dstp, srcp, nbytes_left, nbytes)                  \
  do                                                                          \
    {                                                                         \
      if ((nbytes) >= PAGE_COPY_THRESHOLD &&                                  \
          PAGE_OFFSET ((dstp) - (srcp)) == 0)                                 \
        {                                                                     \
          /* The amount to copy is past the threshold for copying             \
             pages virtually with kernel VM operations, and the               \
             source and destination addresses have the same alignment.  */    \
          size_t nbytes_before = PAGE_OFFSET (-(dstp));                       \
          if (nbytes_before != 0)                                             \
            {                                                                 \
              /* First copy the words before the first page boundary.  */     \
              WORD_COPY_FWD (dstp, srcp, nbytes_left, nbytes_before);         \
              assert (nbytes_left == 0);                                      \
              nbytes -= nbytes_before;                                        \
            }                                                                 \
          PAGE_COPY_FWD (dstp, srcp, nbytes_left, nbytes);                    \
        }                                                                     \
    } while (0)

/* The page size is always a power of two, so we can avoid modulo division.  */
#define PAGE_OFFSET(n)  ((n) & (PAGE_SIZE - 1))

#else

#define PAGE_COPY_FWD_MAYBE(dstp, srcp, nbytes_left, nbytes) /* nada */

#endif

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