tree.h (TYPE_NONALIASED_COMPONENT): Expand comment.

	* tree.h (TYPE_NONALIASED_COMPONENT): Expand comment.
	(DECL_NONADDRESSABLE_P): Likewise.
	* alias.c (record_component_aliases): Fix comment.

From-SVN: r134868

gcc/ChangeLog

+2008-05-01  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* tree.h (TYPE_NONALIASED_COMPONENT): Expand comment.
+	(DECL_NONADDRESSABLE_P): Likewise.
+	* alias.c (record_component_aliases): Fix comment.
+
 2008-05-01  Simon Baldwin <simonb@google.com>
 
 	* c-common.h (warn_array_subscript_range): New function.

gcc/alias.c

@@ -740,9 +740,8 @@ record_alias_subset (alias_set_type superset, alias_set_type subset)
 
 /* Record that component types of TYPE, if any, are part of that type for
    aliasing purposes.  For record types, we only record component types
-   for fields that are marked addressable.  For array types, we always
-   record the component types, so the front end should not call this
-   function if the individual component aren't addressable.  */
+   for fields that are not marked non-addressable.  For array types, we
+   only record the component type if it is not marked non-aliased.  */
 
 void
 record_component_aliases (tree type)
@@ -756,7 +755,7 @@ record_component_aliases (tree type)
   switch (TREE_CODE (type))
     {
     case ARRAY_TYPE:
-      if (! TYPE_NONALIASED_COMPONENT (type))
+      if (!TYPE_NONALIASED_COMPONENT (type))
 	record_alias_subset (superset, get_alias_set (TREE_TYPE (type)));
       break;
 
@@ -775,7 +774,7 @@ record_component_aliases (tree type)
 				 get_alias_set (BINFO_TYPE (base_binfo)));
 	}
       for (field = TYPE_FIELDS (type); field != 0; field = TREE_CHAIN (field))
-	if (TREE_CODE (field) == FIELD_DECL && ! DECL_NONADDRESSABLE_P (field))
+	if (TREE_CODE (field) == FIELD_DECL && !DECL_NONADDRESSABLE_P (field))
 	  record_alias_subset (superset, get_alias_set (TREE_TYPE (field)));
       break;
 

gcc/tree.h

@@ -2264,8 +2264,9 @@ struct tree_block GTY(())
 #define TYPE_TRANSPARENT_UNION(NODE)  \
   (UNION_TYPE_CHECK (NODE)->type.transparent_union_flag)
 
-/* For an ARRAY_TYPE, indicates that it is not permitted to
-   take the address of a component of the type.  */
+/* For an ARRAY_TYPE, indicates that it is not permitted to take the
+   address of a component of the type.  This is the counterpart of
+   DECL_NONADDRESSABLE_P for arrays, see the definition of this flag.  */
 #define TYPE_NONALIASED_COMPONENT(NODE) \
   (ARRAY_TYPE_CHECK (NODE)->type.transparent_union_flag)
 
@@ -2896,7 +2897,20 @@ struct tree_decl_with_rtl GTY(())
 #define DECL_BIT_FIELD(NODE) (FIELD_DECL_CHECK (NODE)->decl_common.decl_flag_2)
 
 /* Used in a FIELD_DECL to indicate that we cannot form the address of
-   this component.  */
+   this component.  This makes it possible for Type-Based Alias Analysis
+   to disambiguate accesses to this field with indirect accesses using
+   the field's type:
+
+     struct S { int i; } s;
+     int *p;
+
+   If the flag is set on 'i', TBAA computes that s.i and *p never conflict.
+
+   From the implementation's viewpoint, the alias set of the type of the
+   field 'i' (int) will not be recorded as a subset of that of the type of
+   's' (struct S) in record_component_aliases.  The counterpart is that
+   accesses to s.i must not be given the alias set of the type of 'i'
+   (int) but instead directly that of the type of 's' (struct S).  */
 #define DECL_NONADDRESSABLE_P(NODE) \
   (FIELD_DECL_CHECK (NODE)->decl_common.decl_flag_3)