Move GCC-compatible pod-packing change to v16/old behavior available at v15 and below

Change matches D126334/e59f648d698e since this change got punted from
v15 too.

Author: dwblaikie

clang/docs/ReleaseNotes.rst

@@ -524,6 +524,12 @@ OpenCL C Language Changes in Clang
 ABI Changes in Clang
 --------------------
 
+- GCC doesn't pack non-POD members in packed structs unless the packed
+  attribute is also specified on the member. Clang historically did perform
+  such packing. Clang now matches the gcc behavior (except on Darwin and PS4).
+  You can switch back to the old ABI behavior with the flag:
+  ``-fclang-abi-compat=15.0``.
+
 OpenMP Support in Clang
 -----------------------
 

clang/include/clang/Basic/LangOptions.h

@@ -220,7 +220,6 @@ class LangOptions : public LangOptionsBase {
     /// Attempt to be ABI-compatible with code generated by Clang 14.0.x.
     /// This causes clang to:
     ///   - mangle dependent nested names incorrectly.
-    ///   - pack non-POD members of packed structs.
     ///   - make trivial only those defaulted copy constructors with a
     ///     parameter-type-list equivalent to the parameter-type-list of an
     ///     implicit declaration.
@@ -229,6 +228,7 @@ class LangOptions : public LangOptionsBase {
     /// Attempt to be ABI-compatible with code generated by Clang 15.0.x.
     /// This causes clang to:
     ///   - Reverse the implementation for DR692, DR1395 and DR1432.
+    ///   - pack non-POD members of packed structs.
     Ver15,
 
     /// Conform to the underlying platform's C and C++ ABIs as closely

clang/lib/AST/RecordLayoutBuilder.cpp

@@ -1892,7 +1892,7 @@ void ItaniumRecordLayoutBuilder::LayoutField(const FieldDecl *D,
   llvm::Triple Target = Context.getTargetInfo().getTriple();
   bool FieldPacked = (Packed && (!FieldClass || FieldClass->isPOD() ||
                                  Context.getLangOpts().getClangABICompat() <=
-                                     LangOptions::ClangABI::Ver14 ||
+                                     LangOptions::ClangABI::Ver15 ||
                                  Target.isPS() || Target.isOSDarwin())) ||
                      D->hasAttr<PackedAttr>();
 

clang/test/SemaCXX/class-layout.cpp

@@ -1,10 +1,10 @@
 // RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -fsyntax-only -verify -std=c++98 -Wno-inaccessible-base
 // RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -fsyntax-only -verify -std=c++11 -Wno-inaccessible-base
-// RUN: %clang_cc1 -triple x86_64-apple-darwin    %s -fsyntax-only -verify -std=c++11 -Wno-inaccessible-base -DCLANG_ABI_COMPAT=14
+// RUN: %clang_cc1 -triple x86_64-apple-darwin    %s -fsyntax-only -verify -std=c++11 -Wno-inaccessible-base -DCLANG_ABI_COMPAT=15
 // RUN: %clang_cc1 -triple x86_64-scei-ps4        %s -fsyntax-only -verify -std=c++11 -Wno-inaccessible-base -DCLANG_ABI_COMPAT=6
 // RUN: %clang_cc1 -triple x86_64-sie-ps5         %s -fsyntax-only -verify -std=c++11 -Wno-inaccessible-base -DCLANG_ABI_COMPAT=6
 // RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -fsyntax-only -verify -std=c++11 -Wno-inaccessible-base -fclang-abi-compat=6 -DCLANG_ABI_COMPAT=6
-// RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -fsyntax-only -verify -std=c++11 -Wno-inaccessible-base -fclang-abi-compat=14 -DCLANG_ABI_COMPAT=14
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -fsyntax-only -verify -std=c++11 -Wno-inaccessible-base -fclang-abi-compat=15 -DCLANG_ABI_COMPAT=15
 // expected-no-diagnostics
 
 #define SA(n, p) int a##n[(p) ? 1 : -1]
@@ -621,7 +621,7 @@ struct t2 {
   char c2;
   t1 v1;
 } __attribute__((packed));
-#if defined(CLANG_ABI_COMPAT) && CLANG_ABI_COMPAT <= 14
+#if defined(CLANG_ABI_COMPAT) && CLANG_ABI_COMPAT <= 15
 _Static_assert(_Alignof(t1) == 4, "");
 _Static_assert(_Alignof(t2) == 1, "");
 #else