This is the patch I sent previously, but I tried to improve the
description and added a long comment. This patch is needed so
that we do not have to update TYPE_CANONICAL of structures / unions
when a tagged type is completed that is (recursively) pointed to
by a member of the structure / union.
Bootstrapped and regression tested on x86_64.
C23: fix aliasing for structures/unions with incomplete types
When incomplete structure/union types are completed later, compatibility
of struct types that contain pointers to such types changes. When forming
equivalence classes for TYPE_CANONICAL, we therefor need to be conservative
and treat all structs with the same tag which are pointer targets as
equivalent for purposed of determining equivalency of structure/union
types which contain such types as member. This avoids having to update
TYPE_CANONICAL of such structure/unions recursively. The pointer types
themselves are updated in c_update_type_canonical.
gcc/c/
* c-typeck.cc (comptypes_internal): Add flag to track
whether a struct is the target of a pointer.
(tagged_types_tu_compatible): When forming equivalence
classes, treat nested pointed-to structs as equivalent.
gcc/testsuite/
* gcc.dg/c23-tag-incomplete-alias-1.c: New test.
diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index c07e2f2b5cf..7a14ef1868f 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -1194,6 +1194,7 @@ struct comptypes_data {
bool different_types_p;
bool warning_needed;
bool anon_field;
+ bool pointedto;
bool equiv;
const struct tagged_tu_seen_cache* cache;
@@ -1265,8 +1266,36 @@ comptypes_check_different_types (tree type1, tree type2,
}
-/* Like comptypes, but if it returns nonzero for struct and union
- types considered equivalent for aliasing purposes. */
+/* Like comptypes, but if it returns true for struct and union types
+ considered equivalent for aliasing purposes, i.e. for setting
+ TYPE_CANONICAL after completing a struct or union.
+
+ This function must return false only for types which are not
+ compatible according to C language semantics (cf. comptypes),
+ otherwise the middle-end would make incorrect aliasing decisions.
+ It may return true for some similar types that are not compatible
+ according to those stricter rules.
+
+ In particular, we ignore size expression in arrays so that the
+ following structs are in the same equivalence class:
+
+ struct foo { char (*buf)[]; };
+ struct foo { char (*buf)[3]; };
+ struct foo { char (*buf)[4]; };
+
+ We also treat unions / structs with members which are pointers to
+ structures or unions with the same tag as equivalent (if they are not
+ incompatible for other reasons). Although incomplete structure
+ or union types are not compatible to any other type, they may become
+ compatible to different types when completed. To avoid having to update
+ TYPE_CANONICAL at this point, we only consider the tag when forming
+ the equivalence classes. For example, the following types with tag
+ 'foo' are all considered equivalent:
+
+ struct bar;
+ struct foo { struct bar *x };
+ struct foo { struct bar { int a; } *x };
+ struct foo { struct bar { char b; } *x }; */
bool
comptypes_equiv_p (tree type1, tree type2)
@@ -1391,6 +1420,7 @@ comptypes_internal (const_tree type1, const_tree type2,
/* Do not remove mode information. */
if (TYPE_MODE (t1) != TYPE_MODE (t2))
return false;
+ data->pointedto = true;
return comptypes_internal (TREE_TYPE (t1), TREE_TYPE (t2), data);
case FUNCTION_TYPE:
@@ -1409,7 +1439,7 @@ comptypes_internal (const_tree type1, const_tree type2,
if ((d1 == NULL_TREE) != (d2 == NULL_TREE))
data->different_types_p = true;
- /* Ignore size mismatches. */
+ /* Ignore size mismatches when forming equivalence classes. */
if (data->equiv)
return true;
/* Sizes must match unless one is missing or variable. */
@@ -1549,6 +1579,12 @@ tagged_types_tu_compatible_p (const_tree t1, const_tree
t2,
if (TYPE_NAME (t1) != TYPE_NAME (t2))
return false;
+ /* When forming equivalence classes for TYPE_CANONICAL in C23, we treat
+ structs with the same tag as equivalent, but only when they are targets
+ of pointers inside other structs. */
+ if (data->equiv && data->pointedto)
+return true;
+
if (!data->anon_field && NULL_TREE == TYPE_NAME (t1))
return false;
@@ -1641,6 +1677,7 @@ tagged_types_tu_compatible_p (const_tree t1, const_tree
t2,
return false;
data->anon_field = !DECL_NAME (s1);
+ data->pointedto = false;
data->cache =
if (!comptypes_internal (TREE_TYPE (s1), TREE_TYPE (s2), data))
diff --git a/gcc/testsuite/gcc.dg/c23-tag-incomplete-alias-1.c
