Hi! 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. Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk and release branches? 2024-06-04 Jakub Jelinek <ja...@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. --- gcc/c/c-decl.cc.jj 2024-05-07 08:47:35.974836903 +0200 +++ gcc/c/c-decl.cc 2024-06-03 19:55:53.819586291 +0200 @@ -9446,6 +9446,17 @@ verify_counted_by_attribute (tree struct 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, t 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)) { --- gcc/testsuite/gcc.dg/pr114493-1.c.jj 2024-06-03 19:59:58.774336785 +0200 +++ gcc/testsuite/gcc.dg/pr114493-1.c 2024-06-03 19:59:12.931944923 +0200 @@ -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) {}; +} --- gcc/testsuite/gcc.dg/pr114493-2.c.jj 2024-06-03 19:59:58.774336785 +0200 +++ gcc/testsuite/gcc.dg/pr114493-2.c 2024-06-03 20:01:00.886512830 +0200 @@ -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) {}; +} Jakub