RFA: lazily allocate bcache obstack

[Tom Tromey <tromey@redhat.com>]

Today I was looking at 'maint print statistics' for a large program
(openoffice, with 162 shared libraries).

In the output I noticed that in many cases there was an empty bcache
of some kind, each taking around 4k.

This patch changes bcache to lazily initialize its obstack.  According
to 'maint space 1', this saved 2355200 bytes on the openoffice test
case.

Built and regtested on x86-64 (compile farm).
Ok?

Tom

b/gdb/ChangeLog:
2008-08-05  Tom Tromey  <tromey@redhat.com>

	* bcache.c (deprecated_bcache_added): Initialize obstack.
	(bcache_xmalloc): Don't initialize obstack.
	(bcache_xfree): Conditionally free obstack.

diff --git a/gdb/bcache.c b/gdb/bcache.c
index 9f7a915..6235df5 100644
--- a/gdb/bcache.c
+++ b/gdb/bcache.c
@@ -231,6 +231,16 @@ deprecated_bcache_added (const void *addr, int length, struct bcache *bcache,
   if (added)
     *added = 0;
 
+  /* Lazily initialize the obstack.  This can save quite a bit of
+     memory in some cases.  */
+  if (bcache->total_count == 0)
+    {
+      /* We could use obstack_specify_allocation here instead, but
+	 gdb_obstack.h specifies the allocation/deallocation
+	 functions.  */
+      obstack_init (&bcache->cache);
+    }
+
   /* 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);
@@ -285,10 +295,6 @@ bcache_xmalloc (void)
 {
   /* Allocate the bcache pre-zeroed.  */
   struct bcache *b = XCALLOC (1, struct bcache);
-  /* We could use obstack_specify_allocation here instead, but
-     gdb_obstack.h specifies the allocation/deallocation
-     functions.  */
-  obstack_init (&b->cache);
   return b;
 }
 
@@ -298,7 +304,9 @@ bcache_xfree (struct bcache *bcache)
 {
   if (bcache == NULL)
     return;
-  obstack_free (&bcache->cache, 0);
+  /* Only free the obstack if we actually initialized it.  */
+  if (bcache->total_count > 0)
+    obstack_free (&bcache->cache, 0);
   xfree (bcache->bucket);
   xfree (bcache);
 }
@@ -457,5 +465,7 @@ print_bcache_statistics (struct bcache *c, char *type)
 int
 bcache_memory_used (struct bcache *bcache)
 {
+  if (bcache->total_count == 0)
+    return 0;
   return obstack_memory_used (&bcache->cache);
 }