[gcc r15-1080] c: Fix up pointer types to may_alias structures [PR114493]

[Jakub Jelinek]

https://gcc.gnu.org/g:d5a3c6d43acb8b2211d9fb59d59482d74c010f01

commit r15-1080-gd5a3c6d43acb8b2211d9fb59d59482d74c010f01
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Jun 6 22:12:11 2024 +0200

    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.

Diff:
---
 gcc/c/c-decl.cc                   | 15 +++++++++++++++
 gcc/testsuite/gcc.dg/pr114493-1.c | 19 +++++++++++++++++++
 gcc/testsuite/gcc.dg/pr114493-2.c | 26 ++++++++++++++++++++++++++
 3 files changed, 60 insertions(+)

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 64924b87a91..6c09eb73128 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -9446,6 +9446,17 @@ verify_counted_by_attribute (tree struct_type, tree field_decl)
   return;
 }
 
+/* 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.
@@ -9791,6 +9802,10 @@ 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 && !C_TYPE_VARIABLE_SIZE (t))
     {
diff --git a/gcc/testsuite/gcc.dg/pr114493-1.c b/gcc/testsuite/gcc.dg/pr114493-1.c
new file mode 100644
index 00000000000..446f33eac3b
--- /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 00000000000..1b4a5792dc9
--- /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) {};
+}