https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94707
--- Comment #15 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:575ac27fd5f18ffc9cfce8a99e987f52c5b898c9 commit r10-8027-g575ac27fd5f18ffc9cfce8a99e987f52c5b898c9 Author: Jakub Jelinek <ja...@redhat.com> Date: Wed Apr 29 09:01:49 2020 +0200 c++, middle-end, rs6000: Fix C++17 ABI incompatibilities during class layout and [[no_unique_address]] handling [PR94707] As reported by Iain and David, powerpc-darwin and powerpc-aix* have C++14 vs. C++17 ABI incompatibilities which are not fixed by mere adding of cxx17_empty_base_field_p calls. Unlike the issues that were seen on other targets where the artificial empty base field affected function argument passing or returning of values, on these two targets the difference is during class layout, not afterwards (e.g. struct empty_base {}; struct S : public empty_base { unsigned long long l[2]; }; will have different __alignof__ (S) between C++14 and C++17 (or possibly with double instead of unsigned long long too)). I've tried: struct X { }; struct Y { int : 0; }; struct Z { int : 0; Y y; }; struct U : public X { X q; }; struct A { float a, b, c, d; }; struct B : public X { float a, b, c, d; }; struct C : public Y { float a, b, c, d; }; struct D : public Z { float a, b, c, d; }; struct E : public U { float a, b, c, d; }; struct F { [[no_unique_address]] X x; float a, b, c, d; }; struct G { [[no_unique_address]] Y y; float a, b, c, d; }; struct H { [[no_unique_address]] Z z; float a, b, c, d; }; struct I { [[no_unique_address]] U u; float a, b, c, d; }; struct J { float a, b; [[no_unique_address]] X x; float c, d; }; struct K { float a, b; [[no_unique_address]] Y y; float c, d; }; struct L { float a, b; [[no_unique_address]] Z z; float c, d; }; struct M { float a, b; [[no_unique_address]] U u; float c, d; }; #define T(S, s) extern S s; extern void foo##s (S); int bar##s () { foo##s (s); return 0; } T (A, a) T (B, b) T (C, c) T (D, d) T (E, e) T (F, f) T (G, g) T (H, h) T (I, i) T (J, j) T (K, k) T (L, l) T (M, m) testcase on powerpc64-linux. Results: G++ 9 -std=c++14 A, B, C passed in fprs, the rest in gprs G++ 9 -std=c++17 A passed in fprs, the rest in gprs current trunk -std=c++14 & 17 A, B, C passed in fprs, the rest in gprs patched trunk -std=c++14 & 17 A, B, C, F, G, J, K passed in fprs, the rest in gprs clang++ [*] -std=c++14 & 17 A, B, C, F, G, J, K passed in fprs, the rest in gprs [*] clang version 11.0.0 (g...@github.com:llvm/llvm-project.git 5c352e69e76a26e4eda075e20aa6a9bb7686042c) Is that what we want? I think it matches the stated intent of P0840R2 or what Jason/Jonathan said, and doing something different like e.g. not treating C, G and K as homogenous because of the int : 0 in empty bases or in zero sized [[no_unique_address] fields would be quite hard to implement (because for C++14 the FIELD_DECL just isn't there). 2020-04-29 Jakub Jelinek <ja...@redhat.com> PR target/94707 * tree-core.h (tree_decl_common): Note decl_flag_0 used for DECL_FIELD_ABI_IGNORED. * tree.h (DECL_FIELD_ABI_IGNORED): Define. * calls.h (cxx17_empty_base_field_p): Change into a temporary macro, check DECL_FIELD_ABI_IGNORED flag with no "no_unique_address" attribute. * calls.c (cxx17_empty_base_field_p): Remove. * tree-streamer-out.c (pack_ts_decl_common_value_fields): Handle DECL_FIELD_ABI_IGNORED. * tree-streamer-in.c (unpack_ts_decl_common_value_fields): Likewise. * lto-streamer-out.c (hash_tree): Likewise. * config/rs6000/rs6000-call.c (rs6000_aggregate_candidate): Rename cxx17_empty_base_seen to empty_base_seen, change type to int *, adjust recursive calls, use DECL_FIELD_ABI_IGNORED instead of cxx17_empty_base_field_p, if "no_unique_address" attribute is present, propagate that to the caller too. (rs6000_discover_homogeneous_aggregate): Adjust rs6000_aggregate_candidate caller, emit different diagnostics when c++17 empty base fields are present and when empty [[no_unique_address]] fields are present. * config/rs6000/rs6000.c (rs6000_special_round_type_align, darwin_rs6000_special_round_type_align): Skip DECL_FIELD_ABI_IGNORED fields. * class.c (build_base_field): Set DECL_FIELD_ABI_IGNORED on C++17 empty base artificial FIELD_DECLs. (layout_class_type): Set DECL_FIELD_ABI_IGNORED on empty class field_poverlapping_p FIELD_DECLs. * lto-common.c (compare_tree_sccs_1): Handle DECL_FIELD_ABI_IGNORED. * g++.target/powerpc/pr94707-1.C: New test. * g++.target/powerpc/pr94707-2.C: New test. * g++.target/powerpc/pr94707-3.C: New test. * g++.target/powerpc/pr94707-4.C: New test. * g++.target/powerpc/pr94707-5.C: New test. * g++.target/powerpc/pr94707-4.C: New test.
