Reduce global decl stream

[Jan Hubicka]

Hi,
this patch saves about 30% of global decl stream size in firefox.  While
implementing the lto sections for initializers I put very stupid heursitcs
to get_symbol_initial_value deciding whether the initializer is better streamed
inline or offline.  This ignores strings and may get bit out of hand.

With this patch and the compression, the largest ltrans unit is 
118479156 bytes and 103584016 out of that is a global decl stream.

Bootstrapped/regtested x86_64-linux OK?

Honza

        * lto-streamer-out.c (subtract_estimated_size): New function
        (get_symbol_initial_value): Use it.
Index: lto-streamer-out.c
===================================================================
--- lto-streamer-out.c  (revision 231546)
+++ lto-streamer-out.c  (working copy)
@@ -309,6 +309,33 @@ lto_is_streamable (tree expr)
             || TREE_CODE_CLASS (code) != tcc_statement);
 }
 
+/* Very rough estimate of streaming size of the initializer.  If we ignored
+   presence of strings, we could simply just count number of non-indexable
+   tree nodes and number of references to indexable nodes.  Strings however
+   may be very large and we do not want to dump them int othe global stream.
+
+   Count the size of initializer until the size in DATA is positive.  */
+
+static tree
+subtract_estimated_size (tree *tp, int *ws, void *data)
+{
+  long *sum = (long *)data;
+  if (tree_is_indexable (*tp))
+    {
+      *sum -= 4;
+      *ws = 0;
+    }
+  if (TREE_CODE (*tp) == STRING_CST)
+    *sum -= TREE_STRING_LENGTH (*tp) + 8;
+  if (TREE_CODE (*tp) == IDENTIFIER_NODE)
+    *sum -= IDENTIFIER_LENGTH (*tp) + 8;
+  else
+    *sum -= 16;
+  if (*sum < 0)
+    return *tp;
+  return NULL_TREE;
+}
+
 
 /* For EXPR lookup and return what we want to stream to OB as DECL_INITIAL.  */
 
@@ -329,10 +356,16 @@ get_symbol_initial_value (lto_symtab_enc
       varpool_node *vnode;
       /* Extra section needs about 30 bytes; do not produce it for simple
         scalar values.  */
-      if (TREE_CODE (DECL_INITIAL (expr)) == CONSTRUCTOR
-         || !(vnode = varpool_node::get (expr))
+      if (!(vnode = varpool_node::get (expr))
          || !lto_symtab_encoder_encode_initializer_p (encoder, vnode))
         initial = error_mark_node;
+      if (initial != error_mark_node)
+       {
+         long max_size = 30;
+         if (walk_tree (&initial, subtract_estimated_size, (void *)&max_size,
+                        NULL))
+           initial = error_mark_node;
+       }
     }
 
   return initial;