[PowerPC][AIX] Warn when using pragma align(packed) on AIX.

With xlc and xlC pragma align(packed) will pack bitfields the same way
as pragma align(bit_packed). xlclang, xlclang++ and clang will
pack bitfields the same way as pragma pack(1). Issue a warning when
source code using pragma align(packed) is used to alert the user it
may not be compatable with xlc/xlC.

Differential Revision: https://reviews.llvm.org/D107506

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -916,6 +916,9 @@ def warn_pragma_options_align_reset_failed : Warning<
   InGroup<IgnoredPragmas>;
 def err_pragma_options_align_mac68k_target_unsupported : Error<
   "mac68k alignment pragma is not supported on this target">;
+def warn_pragma_align_not_xl_compatible : Warning<
+  "#pragma align(packed) may not be compatible with objects generated with AIX XL C/C++">,
+  InGroup<AIXCompat>;
 def warn_pragma_pack_invalid_alignment : Warning<
   "expected #pragma pack parameter to be '1', '2', '4', '8', or '16'">,
   InGroup<IgnoredPragmas>;

clang/lib/Sema/SemaDecl.cpp

@@ -16613,6 +16613,23 @@ void Sema::ActOnTagFinishDefinition(Scope *S, Decl *TagD,
   // Notify the consumer that we've defined a tag.
   if (!Tag->isInvalidDecl())
     Consumer.HandleTagDeclDefinition(Tag);
+
+  // Clangs implementation of #pragma align(packed) differs in bitfield layout
+  // from XLs and instead matches the XL #pragma pack(1) behavior.
+  if (Context.getTargetInfo().getTriple().isOSAIX() &&
+      AlignPackStack.hasValue()) {
+    AlignPackInfo APInfo = AlignPackStack.CurrentValue;
+    // Only diagnose #pragma align(packed).
+    if (!APInfo.IsAlignAttr() || APInfo.getAlignMode() != AlignPackInfo::Packed)
+      return;
+    const RecordDecl *RD = dyn_cast<RecordDecl>(Tag);
+    if (!RD)
+      return;
+    // Only warn if there is at least 1 bitfield member.
+    if (llvm::any_of(RD->fields(),
+                     [](const FieldDecl *FD) { return FD->isBitField(); }))
+      Diag(BraceRange.getBegin(), diag::warn_pragma_align_not_xl_compatible);
+  }
 }
 
 void Sema::ActOnObjCContainerFinishDefinition() {

clang/test/Sema/aix-pragma-align-packed-warn.c

@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -triple powerpc64-ibm-aix-xcoff  -fxl-pragma-pack -verify -fsyntax-only %s
+// RUN: %clang_cc1 -triple powerpc-ibm-aix-xcoff  -fxl-pragma-pack -verify -fsyntax-only %s
+
+#pragma align(packed)
+struct A {  // expected-warning {{#pragma align(packed) may not be compatible with objects generated with AIX XL C/C++}}
+  short s1;
+  int   : 0;
+  short s2;
+};
+
+struct B {  // expected-warning {{#pragma align(packed) may not be compatible with objects generated with AIX XL C/C++}}
+  short a : 8;
+  short b : 8;
+  int c;
+};
+
+struct C {
+  int x, y, z;
+};
+
+struct D {
+  double d;
+  struct A a;
+};
+#pragma align(reset)
+
+struct E {
+  int a : 28;
+  int   : 0;
+  int b : 16;
+};