c: Fix up pointer types to may_alias structures [PR114493]

The following testcase ICEs in ipa-free-lang, because the
fld_incomplete_type_of
          gcc_assert (TYPE_CANONICAL (t2) != t2
                      && TYPE_CANONICAL (t2) == TYPE_CANONICAL (TREE_TYPE (t)));
assertion doesn't hold.
This is because t is a struct S * type which was created while struct S
was still incomplete and without the may_alias attribute (and TYPE_CANONICAL
of a pointer type is a type created with can_alias_all = false argument),
while later on on the struct definition may_alias attribute was used.
fld_incomplete_type_of then creates an incomplete distinct copy of the
structure (but with the original attributes) but pointers created for it
are because of the "may_alias" attribute TYPE_REF_CAN_ALIAS_ALL, including
their TYPE_CANONICAL, because while that is created with !can_alias_all
argument, we later set it because of the "may_alias" attribute on the
to_type.

This doesn't ICE with C++ since PR70512 fix because the C++ FE sets
TYPE_REF_CAN_ALIAS_ALL on all pointer types to the class type (and its
variants) when the may_alias is added.

The following patch does that in the C FE as well.

2024-06-06  Jakub Jelinek  <jakub@redhat.com>

	PR c/114493
	* c-decl.cc (c_fixup_may_alias): New function.
	(finish_struct): Call it if "may_alias" attribute is
	specified.

	* gcc.dg/pr114493-1.c: New test.
	* gcc.dg/pr114493-2.c: New test.

(cherry picked from commit d5a3c6d43acb8b2211d9fb59d59482d74c010f01)

3 files changed, 60 insertions, 0 deletions

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 52af8f3..e63dab4 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -9393,6 +9393,17 @@ c_update_type_canonical (tree t)
     }
 }
 
+/* TYPE is a struct or union that we're applying may_alias to after the body is
+   parsed.  Fixup any POINTER_TO types.  */
+
+static void
+c_fixup_may_alias (tree type)
+{
+  for (tree t = TYPE_POINTER_TO (type); t; t = TYPE_NEXT_PTR_TO (t))
+    for (tree v = TYPE_MAIN_VARIANT (t); v; v = TYPE_NEXT_VARIANT (v))
+      TYPE_REF_CAN_ALIAS_ALL (v) = true;
+}
+
 /* Fill in the fields of a RECORD_TYPE or UNION_TYPE node, T.
    LOC is the location of the RECORD_TYPE or UNION_TYPE's definition.
    FIELDLIST is a chain of FIELD_DECL nodes for the fields.
@@ -9737,6 +9748,10 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
 
   C_TYPE_BEING_DEFINED (t) = 0;
 
+  if (lookup_attribute ("may_alias", TYPE_ATTRIBUTES (t)))
+    for (x = TYPE_MAIN_VARIANT (t); x; x = TYPE_NEXT_VARIANT (x))
+      c_fixup_may_alias (x);
+
   /* Set type canonical based on equivalence class.  */
   if (flag_isoc23)
     {
diff --git a/gcc/testsuite/gcc.dg/pr114493-1.c b/gcc/testsuite/gcc.dg/pr114493-1.c
new file mode 100644
index 0000000..446f33e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr114493-1.c
@@ -0,0 +1,19 @@
+/* PR c/114493 */
+/* { dg-do compile { target lto } } */
+/* { dg-options "-O2 -flto" } */
+
+void foo (void);
+struct S;
+struct S bar (struct S **);
+struct S qux (const struct S **);
+
+struct __attribute__((__may_alias__)) S {
+  int s;
+};
+
+struct S
+baz (void)
+{
+  foo ();
+  return (struct S) {};
+}
diff --git a/gcc/testsuite/gcc.dg/pr114493-2.c b/gcc/testsuite/gcc.dg/pr114493-2.c
new file mode 100644
index 0000000..1b4a579
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr114493-2.c
@@ -0,0 +1,26 @@
+/* PR c/114493 */
+/* { dg-do compile { target lto } } */
+/* { dg-options "-O2 -flto -std=c23" } */
+
+void foo (void);
+struct S;
+struct S bar (struct S **);
+struct S qux (const struct S **);
+
+void
+corge (void)
+{
+  struct S { int s; } s;
+  s.s = 0;
+}
+
+struct __attribute__((__may_alias__)) S {
+  int s;
+};
+
+struct S
+baz (void)
+{
+  foo ();
+  return (struct S) {};
+}