improve DW_AT_const_value output

[gkeating at apple dot com (Geoffrey Keating)]

This patch permits more frequent DW_AT_const_value output.

If you change setup_one_parameter in tree-inline.c to set DECL_INITIAL
in the variable it creates, this gets you proper output of
DW_AT_const_value for parameters optimised out.  Unfortunately it
seems you also get a build failure in libstdc++ when something tries
to gimplify the DECL_INITIAL (of an unused variable, yet!).  I think
this patch will help in general, even without that change.

Bootstrapped & tested on powerpc-darwin.

-- 
- Geoffrey Keating <geoffk@apple.com>

===File ~/patches/gcc-dwarf-consts-1.patch==================
2006-03-26  Geoffrey Keating  <geoffk@apple.com>

	* dwarf2out.c (add_location_or_const_value_attribute): Call
	tree_add_const_value_attribute if nothing else works.
	(reference_to_unused): New.
	(rtl_for_decl_init): Use reference_to_unused to decide whether
	to output an initializer.

Index: gcc/dwarf2out.c
===================================================================
--- gcc/dwarf2out.c	(revision 112374)
+++ gcc/dwarf2out.c	(working copy)
@@ -9929,6 +9929,23 @@
 
 }
 
+/* Determine whether the evaluation of EXPR references any variables
+   or functions which aren't otherwise used (and therefore may not be
+   output).  */
+static tree
+reference_to_unused (tree * tp, int * walk_subtrees,
+		     void * data ATTRIBUTE_UNUSED)
+{
+  if (! EXPR_P (*tp) && ! CONSTANT_CLASS_P (*tp))
+    *walk_subtrees = 0;
+  
+  if (DECL_P (*tp) && ! TREE_PUBLIC (*tp) && ! TREE_USED (*tp)
+      && ! TREE_ASM_WRITTEN (*tp))
+    return *tp;
+  else
+    return NULL_TREE;
+}
+
 /* Generate an RTL constant from a decl initializer INIT with decl type TYPE,
    for use in a later add_const_value_attribute call.  */
 
@@ -9955,15 +9972,16 @@
 	rtl = gen_rtx_CONST_STRING (VOIDmode,
 				    ggc_strdup (TREE_STRING_POINTER (init)));
     }
+  /* Although DWARF could easily handle other kinds of aggregates, we
+     have no way to represent such values as RTL constants, so skip
+     those.  */
+  else if (AGGREGATE_TYPE_P (type))
+    ;
   /* If the initializer is something that we know will expand into an
-     immediate RTL constant, expand it now.  Expanding anything else
-     tends to produce unresolved symbols; see debug/5770 and c++/6381.  */
-  /* Aggregate, vector, and complex types may contain constructors that may
-     result in code being generated when expand_expr is called, so we can't
-     handle them here.  Integer and float are useful and safe types to handle
-     here.  */
-  else if ((INTEGRAL_TYPE_P (type) || SCALAR_FLOAT_TYPE_P (type))
-	   && initializer_constant_valid_p (init, type) == null_pointer_node)
+     immediate RTL constant, expand it now.  We must be careful not to
+     reference variables which won't be output.  */
+  else if (initializer_constant_valid_p (init, type)
+	   && ! walk_tree (&init, reference_to_unused, NULL, NULL))
     {
       rtl = expand_expr (init, NULL_RTX, VOIDmode, EXPAND_INITIALIZER);
 
@@ -10327,6 +10345,9 @@
       add_AT_location_description (die, attr, descr);
       return;
     }
+  /* None of that worked, so it must not really have a location;
+     try adding a constant value attribute from the DECL_INITIAL.  */
+  tree_add_const_value_attribute (die, decl);
 }
 
 /* If we don't have a copy of this variable in memory for some reason (such
============================================================