gdb/bcache.c

/* Implement a cached obstack.
   Written by Fred Fish <fnf@cygnus.com>
   Rewritten by Jim Blandy <jimb@cygnus.com>
   Copyright 1999 Free Software Foundation, Inc.

   This file is part of GDB.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2 of the License, or
   (at your option) any later version.

   This program 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 General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 59 Temple Place - Suite 330,
   Boston, MA 02111-1307, USA.  */

#include <stddef.h>
#include <stdlib.h>

#include "defs.h"
#include "obstack.h"
#include "bcache.h"
#include "gdb_string.h"		/* For memcpy declaration */



/* The hash function.  */

unsigned long
hash (void *addr, int length)
{
  /* If it's a short string, hash on every character.  Otherwise, sample
     characters from throughout the string.  */
  if (length <= 64)
    {
      char *byte = addr;
      unsigned long h = 0;
      int i;

      for (i = 0; i < length; i++)
	h = h * 65793 ^ (h >> (sizeof (h) * 8 - 6)) ^ byte[i];

      return h;
    }
  else
    {
      char *byte = addr;
      int n, i;
      unsigned long h = 0;

      for (n = i = 0; n < 64; n++)
	{
	  h = h * 65793 + (h >> (sizeof (h) * 8 - 6)) + byte[i];
	  i = h % length;
	}

      return h;
    }
}


/* Growing the bcache's hash table.  */

/* If the average chain length grows beyond this, then we want to
   resize our hash table.  */
#define CHAIN_LENGTH_THRESHOLD (5)

static void
expand_hash_table (struct bcache *bcache)
{
  /* A table of good hash table sizes.  Whenever we grow, we pick the
     next larger size from this table.  sizes[i] is close to 1 << (i+10),
     so we roughly double the table size each time.  After we fall off 
     the end of this table, we just double.  Don't laugh --- there have
     been executables sighted with a gigabyte of debug info.  */
  static unsigned long sizes[] = { 
    1021, 2053, 4099, 8191, 16381, 32771,
    65537, 131071, 262144, 524287, 1048573, 2097143,
    4194301, 8388617, 16777213, 33554467, 67108859, 134217757,
    268435459, 536870923, 1073741827, 2147483659UL
  };
  unsigned int new_num_buckets;
  struct bstring **new_buckets;
  unsigned int i;

  /* Find the next size.  */
  new_num_buckets = bcache->num_buckets * 2;
  for (i = 0; i < (sizeof (sizes) / sizeof (sizes[0])); i++)
    if (sizes[i] > bcache->num_buckets)
      {
	new_num_buckets = sizes[i];
	break;
      }

  /* Allocate the new table.  */
  {
    size_t new_size = new_num_buckets * sizeof (new_buckets[0]);
    new_buckets = (struct bstring **) xmalloc (new_size);
    memset (new_buckets, 0, new_size);

    bcache->structure_size -= (bcache->num_buckets
			       * sizeof (bcache->bucket[0]));
    bcache->structure_size += new_size;
  }

  /* Rehash all existing strings.  */
  for (i = 0; i < bcache->num_buckets; i++)
    {
      struct bstring *s, *next;

      for (s = bcache->bucket[i]; s; s = next)
	{
	  struct bstring **new_bucket;
	  next = s->next;

	  new_bucket = &new_buckets[(hash (&s->d.data, s->length)
				     % new_num_buckets)];
	  s->next = *new_bucket;
	  *new_bucket = s;
	}
    }

  /* Plug in the new table.  */
  if (bcache->bucket)
    free (bcache->bucket);
  bcache->bucket = new_buckets;
  bcache->num_buckets = new_num_buckets;
}


/* Looking up things in the bcache.  */

/* The number of bytes needed to allocate a struct bstring whose data
   is N bytes long.  */
#define BSTRING_SIZE(n) (offsetof (struct bstring, d.data) + (n))

/* Find a copy of the LENGTH bytes at ADDR in BCACHE.  If BCACHE has
   never seen those bytes before, add a copy of them to BCACHE.  In
   either case, return a pointer to BCACHE's copy of that string.  */
void *
bcache (void *addr, int length, struct bcache *bcache)
{
  int hash_index;
  struct bstring *s;

  /* If our average chain length is too high, expand the hash table.  */
  if (bcache->unique_count >= bcache->num_buckets * CHAIN_LENGTH_THRESHOLD)
    expand_hash_table (bcache);

  bcache->total_count++;
  bcache->total_size += length;

  hash_index = hash (addr, length) % bcache->num_buckets;

  /* Search the hash bucket for a string identical to the caller's.  */
  for (s = bcache->bucket[hash_index]; s; s = s->next)
    if (s->length == length
	&& ! memcmp (&s->d.data, addr, length))
      return &s->d.data;

  /* The user's string isn't in the list.  Insert it after *ps.  */
  {
    struct bstring *new
      = obstack_alloc (&bcache->cache, BSTRING_SIZE (length));
    memcpy (&new->d.data, addr, length);
    new->length = length;
    new->next = bcache->bucket[hash_index];
    bcache->bucket[hash_index] = new;

    bcache->unique_count++;
    bcache->unique_size += length;
    bcache->structure_size += BSTRING_SIZE (length);

    return &new->d.data;
  }
}


/* Freeing bcaches.  */

/* Free all the storage associated with BCACHE.  */
void
free_bcache (struct bcache *bcache)
{
  obstack_free (&bcache->cache, 0);
  if (bcache->bucket)
    free (bcache->bucket);

  /* This isn't necessary, but at least the bcache is always in a
     consistent state.  */
  memset (bcache, 0, sizeof (*bcache));
}



/* Printing statistics.  */

static int
compare_ints (const void *ap, const void *bp)
{
  /* Because we know we're comparing two ints which are positive,
     there's no danger of overflow here.  */
  return * (int *) ap - * (int *) bp;
}


static void
print_percentage (int portion, int total)
{
  if (total == 0)
    printf_filtered ("(not applicable)\n");
  else
    printf_filtered ("%3d%%\n", portion * 100 / total);
}


/* Print statistics on BCACHE's memory usage and efficacity at
   eliminating duplication.  NAME should describe the kind of data
   BCACHE holds.  Statistics are printed using `printf_filtered' and
   its ilk.  */
void
print_bcache_statistics (struct bcache *c, char *type)
{
  int occupied_buckets;
  int max_chain_length;
  int median_chain_length;

  /* Count the number of occupied buckets, and measure chain lengths.  */
  {
    unsigned int b;
    int *chain_length
      = (int *) alloca (c->num_buckets * sizeof (*chain_length));

    occupied_buckets = 0;

    for (b = 0; b < c->num_buckets; b++)
      {
	struct bstring *s = c->bucket[b];

	chain_length[b] = 0;

	if (s)
	  {
	    occupied_buckets++;
	    
	    while (s)
	      {
		chain_length[b]++;
		s = s->next;
	      }
	  }
      }

    /* To compute the median, we need the set of chain lengths sorted.  */
    qsort (chain_length, c->num_buckets, sizeof (chain_length[0]),
	   compare_ints);

    if (c->num_buckets > 0)
      {
	max_chain_length = chain_length[c->num_buckets - 1];
	median_chain_length = chain_length[c->num_buckets / 2];
      }
    else
      {
	max_chain_length = 0;
	median_chain_length = 0;
      }
  }

  printf_filtered ("  Cached '%s' statistics:\n", type);
  printf_filtered ("    Total object count:  %ld\n", c->total_count);
  printf_filtered ("    Unique object count: %lu\n", c->unique_count);
  printf_filtered ("    Percentage of duplicates, by count: ");
  print_percentage (c->total_count - c->unique_count, c->total_count);
  printf_filtered ("\n");

  printf_filtered ("    Total object size:   %ld\n", c->total_size);
  printf_filtered ("    Unique object size:  %ld\n", c->unique_size);
  printf_filtered ("    Percentage of duplicates, by size:  ");
  print_percentage (c->total_size - c->unique_size, c->total_size);
  printf_filtered ("\n");

  printf_filtered ("    Total memory used by bcache, including overhead: %ld\n",
		   c->structure_size);
  printf_filtered ("    Percentage memory overhead: ");
  print_percentage (c->structure_size - c->unique_size, c->unique_size);
  printf_filtered ("    Net memory savings:         ");
  print_percentage (c->total_size - c->structure_size, c->total_size);
  printf_filtered ("\n");

  printf_filtered ("    Hash table size:           %3d\n", c->num_buckets);
  printf_filtered ("    Hash table population:     ");
  print_percentage (occupied_buckets, c->num_buckets);
  printf_filtered ("    Median hash chain length:  %3d\n",
		   median_chain_length);
  printf_filtered ("    Average hash chain length: ");
  if (c->num_buckets > 0)
    printf_filtered ("%3lu\n", c->unique_count / c->num_buckets);
  else
    printf_filtered ("(not applicable)\n");
  printf_filtered ("    Maximum hash chain length: %3d\n", max_chain_length);
  printf_filtered ("\n");
}