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.

2 files changed, 76 insertions, 3 deletions

diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index ad4c7ad..09b2c265 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -1172,6 +1172,7 @@ struct comptypes_data {
   bool different_types_p;
   bool warning_needed;
   bool anon_field;
+  bool pointedto;
   bool equiv;
 
   const struct tagged_tu_seen_cache* cache;
@@ -1235,8 +1236,36 @@ 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)
@@ -1357,6 +1386,7 @@ 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:
@@ -1375,7 +1405,7 @@ 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.  */
@@ -1515,6 +1545,12 @@ 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;
 
@@ -1610,6 +1646,7 @@ tagged_types_tu_compatible_p (const_tree t1, const_tree t2,
 	      return false;
 
 	    data->anon_field = !DECL_NAME (s1);
+	    data->pointedto = false;
 
 	    data->cache = &entry;
 	    if (!comptypes_internal (TREE_TYPE (s1), TREE_TYPE (s2), data))
diff --git a/gcc/testsuite/gcc.dg/c23-tag-incomplete-alias-1.c b/gcc/testsuite/gcc.dg/c23-tag-incomplete-alias-1.c
new file mode 100644
index 0000000..f9ec486
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-tag-incomplete-alias-1.c
@@ -0,0 +1,36 @@
+/* { dg-do run } 
+ * { dg-options "-std=c23 -O2" } */
+
+[[gnu::noinline]]
+void *alias(void *ap, void *bp, void *x, void *y)
+{
+	struct foo { struct bar *f; } *a = ap;
+	struct bar { long x; };
+
+	a->f = x;
+
+	{
+		struct bar;
+		struct foo { struct bar *f; } *b = bp;
+		struct bar { long x; };
+
+		// after completing bar, the two struct foo should be compatible 
+
+		b->f = y;
+	}
+
+
+	return a->f;
+}
+
+int main()
+{
+	struct bar { long x; };
+	struct foo { struct bar *f; } a;
+	struct bar x, y;
+	if (&y != alias(&a, &a, &x, &y))
+		__builtin_abort();
+
+	return 0;
+}
+