GCC ABI Compatibility: Preserve alignment of non-pod members in packed structs

This matches GCC: https://godbolt.org/z/sM5q95PGY

I realize this is an API break for clang+clang - so I'm totally open to
discussing how we should deal with that. If Apple wants to keep the
Clang layout indefinitely, if we want to put a flag on this so non-Apple
folks can opt out of this fix/new behavior.

Differential Revision: https://reviews.llvm.org/D117616

Author: dwblaikie

clang/docs/ReleaseNotes.rst

@@ -236,6 +236,12 @@ ABI Changes in Clang
   is still in the process of being stabilized, so this type should not yet be
   used in interfaces that require ABI stability.
 
+- 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=13.0``.
+
 OpenMP Support in Clang
 -----------------------
 

clang/include/clang/Basic/LangOptions.h

@@ -181,6 +181,10 @@ class LangOptions : public LangOptionsBase {
     /// global-scope inline variables incorrectly.
     Ver12,
 
+    /// Attempt to be ABI-compatible with code generated by Clang 13.0.x.
+    /// This causes clang to not pack non-POD members of packed structs.
+    Ver13,
+
     /// Conform to the underlying platform's C and C++ ABIs as closely
     /// as we can.
     Latest

clang/lib/AST/RecordLayoutBuilder.cpp

@@ -1887,7 +1887,12 @@ void ItaniumRecordLayoutBuilder::LayoutField(const FieldDecl *D,
   UnfilledBitsInLastUnit = 0;
   LastBitfieldStorageUnitSize = 0;
 
-  bool FieldPacked = Packed || D->hasAttr<PackedAttr>();
+  llvm::Triple Target = Context.getTargetInfo().getTriple();
+  bool FieldPacked = (Packed && (!FieldClass || FieldClass->isPOD() ||
+                                 Context.getLangOpts().getClangABICompat() <=
+                                     LangOptions::ClangABI::Ver13 ||
+                                 Target.isPS4() || Target.isOSDarwin())) ||
+                     D->hasAttr<PackedAttr>();
 
   AlignRequirementKind AlignRequirement = AlignRequirementKind::None;
   CharUnits FieldSize;

clang/lib/Frontend/CompilerInvocation.cpp

@@ -3560,6 +3560,8 @@ void CompilerInvocation::GenerateLangArgs(const LangOptions &Opts,
     GenerateArg(Args, OPT_fclang_abi_compat_EQ, "11.0", SA);
   else if (Opts.getClangABICompat() == LangOptions::ClangABI::Ver12)
     GenerateArg(Args, OPT_fclang_abi_compat_EQ, "12.0", SA);
+  else if (Opts.getClangABICompat() == LangOptions::ClangABI::Ver13)
+    GenerateArg(Args, OPT_fclang_abi_compat_EQ, "13.0", SA);
 
   if (Opts.getSignReturnAddressScope() ==
       LangOptions::SignReturnAddressScopeKind::All)
@@ -4062,6 +4064,8 @@ bool CompilerInvocation::ParseLangArgs(LangOptions &Opts, ArgList &Args,
         Opts.setClangABICompat(LangOptions::ClangABI::Ver11);
       else if (Major <= 12)
         Opts.setClangABICompat(LangOptions::ClangABI::Ver12);
+      else if (Major <= 13)
+        Opts.setClangABICompat(LangOptions::ClangABI::Ver13);
     } else if (Ver != "latest") {
       Diags.Report(diag::err_drv_invalid_value)
           << A->getAsString(Args) << A->getValue();

clang/test/SemaCXX/class-layout.cpp

@@ -1,6 +1,9 @@
 // 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=13
+// 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-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=13 -DCLANG_ABI_COMPAT=13
 // expected-no-diagnostics
 
 #define SA(n, p) int a##n[(p) ? 1 : -1]
@@ -604,3 +607,37 @@ namespace PR37275 {
 #endif
 #pragma pack(pop)
 }
+
+namespace non_pod {
+struct t1 {
+protected:
+  int a;
+};
+// GCC prints warning: ignoring packed attribute because of unpacked non-POD field 't1 t2::v1'`
+struct t2 {
+  char c1;
+  short s1;
+  char c2;
+  t1 v1;
+} __attribute__((packed));
+#if defined(CLANG_ABI_COMPAT) && CLANG_ABI_COMPAT <= 13
+_Static_assert(_Alignof(t1) == 4, "");
+_Static_assert(_Alignof(t2) == 1, "");
+#else
+_Static_assert(_Alignof(t1) == 4, "");
+_Static_assert(_Alignof(t2) == 4, "");
+#endif
+_Static_assert(sizeof(t2) == 8, ""); // it's still packing the rest of the struct
+} // namespace non_pod
+
+namespace non_pod_packed {
+struct t1 {
+protected:
+  int a;
+} __attribute__((packed));
+struct t2 {
+  t1 v1;
+} __attribute__((packed));
+_Static_assert(_Alignof(t1) == 1, "");
+_Static_assert(_Alignof(t2) == 1, "");
+} // namespace non_pod_packed