PR25575: Make GCC 4.4+ comatible layout for packed bit-fileds of char type, patch by D. Polukhin

This CL is for discussion how to better fix bit-filed layout compatibility issue with GCC (see PR25575 for test case and more details). Current clang behavior is compatible with GCC 4.1-4.3 series but it was fixed in 4.4+. Ignoring packed attribute looks very odd and because it was also fixed in GCC 4.4+, it makes sense also fix it in clang.
Differential Revision: http://reviews.llvm.org/D14872

llvm-svn: 254596

3 files changed, 32 insertions, 13 deletions

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 50122ecb8a4..e96c2338108 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2784,9 +2784,10 @@ def warn_int_to_void_pointer_cast : Warning<
   "cast to %1 from smaller integer type %0">,
   InGroup<IntToVoidPointerCast>;
 
-def warn_attribute_ignored_for_field_of_type : Warning<
-  "%0 attribute ignored for field of type %1">,
-  InGroup<IgnoredAttributes>;
+def warn_attribute_packed_for_bitfield : Warning<
+  "'packed' attribute was ignored on bit-fields with single-byte alignment "
+  "in older versions of GCC and Clang">,
+  InGroup<DiagGroup<"attribute-packed-for-bitfield">>;
 def warn_transparent_union_attribute_field_size_align : Warning<
   "%select{alignment|size}0 of field %1 (%2 bits) does not match the "
   "%select{alignment|size}0 of the first field in transparent union; "
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 93e9eb15b36..fed22c934ce 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -1073,17 +1073,14 @@ static void handlePackedAttr(Sema &S, Decl *D, const AttributeList &Attr) {
     TD->addAttr(::new (S.Context) PackedAttr(Attr.getRange(), S.Context,
                                         Attr.getAttributeSpellingListIndex()));
   else if (FieldDecl *FD = dyn_cast<FieldDecl>(D)) {
-    // If the alignment is less than or equal to 8 bits, the packed attribute
-    // has no effect.
+    // Report warning about changed offset in the newer compiler versions.
     if (!FD->getType()->isDependentType() &&
-        !FD->getType()->isIncompleteType() &&
+        !FD->getType()->isIncompleteType() && FD->isBitField() &&
         S.Context.getTypeAlign(FD->getType()) <= 8)
-      S.Diag(Attr.getLoc(), diag::warn_attribute_ignored_for_field_of_type)
-        << Attr.getName() << FD->getType();
-    else
-      FD->addAttr(::new (S.Context)
-                  PackedAttr(Attr.getRange(), S.Context,
-                             Attr.getAttributeSpellingListIndex()));
+      S.Diag(Attr.getLoc(), diag::warn_attribute_packed_for_bitfield);
+
+    FD->addAttr(::new (S.Context) PackedAttr(
+        Attr.getRange(), S.Context, Attr.getAttributeSpellingListIndex()));
   } else
     S.Diag(Attr.getLoc(), diag::warn_attribute_ignored) << Attr.getName();
 }
diff --git a/clang/test/Sema/struct-packed-align.c b/clang/test/Sema/struct-packed-align.c
index 417c30308ca..175866fbe97 100644
--- a/clang/test/Sema/struct-packed-align.c
+++ b/clang/test/Sema/struct-packed-align.c
@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 %s -fsyntax-only -verify
-// expected-no-diagnostics
+// RUN: %clang_cc1 %s -fsyntax-only -triple=x86_64-windows-coff -verify
 
 // Packed structs.
 struct s {
@@ -138,3 +138,24 @@ extern int n2[__alignof(struct nS) == 8 ? 1 : -1];
 extern int n1[sizeof(struct nS) == 9 ? 1 : -1];
 extern int n2[__alignof(struct nS) == 1 ? 1 : -1];
 #endif
+
+// Packed attribute shouldn't be ignored for bit-field of char types.
+// Note from GCC reference manual: The 4.1, 4.2 and 4.3 series of GCC ignore
+// the packed attribute on bit-fields of type char. This has been fixed in
+// GCC 4.4 but the change can lead to differences in the structure layout.
+// See the documentation of -Wpacked-bitfield-compat for more information.
+struct packed_chars {
+  char a:4;
+  char b:8 __attribute__ ((packed));
+  // expected-warning@-1 {{'packed' attribute was ignored on bit-fields with single-byte alignment in older versions of GCC and Clang}}
+  char c:4;
+};
+
+#if defined(_WIN32)
+// On Windows clang uses MSVC compatible layout in this case.
+extern int o1[sizeof(struct packed_chars) == 3 ? 1 : -1];
+extern int o2[__alignof(struct packed_chars) == 1 ? 1 : -1];
+#else
+extern int o1[sizeof(struct packed_chars) == 2 ? 1 : -1];
+extern int o2[__alignof(struct packed_chars) == 1 ? 1 : -1];
+#endif