Implement __declspec(selectany) under -fms-extensions

selectany only applies to externally visible global variables.  It has
the effect of making the data weak_odr.

The MSDN docs suggest that unused definitions can only be dropped at
linktime, so Clang uses weak instead of linkonce.  MSVC optimizes away
references to constant selectany data, so it must assume that there is
only one definition, hence weak_odr.

Reviewers: espindola

Differential Revision: http://llvm-reviews.chandlerc.com/D814

llvm-svn: 182266

7 files changed, 73 insertions, 1 deletions

diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index 7c59c16f148..04903148521 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -962,6 +962,10 @@ def ForceInline : InheritableAttr {
   let Spellings = [Keyword<"__forceinline">];
 }
 
+def SelectAny : InheritableAttr {
+  let Spellings = [Declspec<"selectany">];
+}
+
 def Win64 : InheritableAttr {
   let Spellings = [Keyword<"__w64">];
 }
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index a5b57e05ea0..55fae9ffedf 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1950,6 +1950,8 @@ def warn_weak_identifier_undeclared : Warning<
   "weak identifier %0 never declared">;
 def err_attribute_weak_static : Error<
   "weak declaration cannot have internal linkage">;
+def err_attribute_selectany_non_extern_data : Error<
+  "'selectany' can only be applied to data items with external linkage">;
 def warn_attribute_weak_import_invalid_on_definition : Warning<
   "'weak_import' attribute cannot be specified on a definition">,
   InGroup<IgnoredAttributes>;
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 07368790372..3a967c6fdc3 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -1937,7 +1937,13 @@ CodeGenModule::GetLLVMLinkageVarDefinition(const VarDecl *D,
     return llvm::Function::DLLImportLinkage;
   else if (D->hasAttr<DLLExportAttr>())
     return llvm::Function::DLLExportLinkage;
-  else if (D->hasAttr<WeakAttr>()) {
+  else if (D->hasAttr<SelectAnyAttr>()) {
+    // selectany symbols are externally visible, so use weak instead of
+    // linkonce.  MSVC optimizes away references to const selectany globals, so
+    // all definitions should be the same and ODR linkage should be used.
+    // http://msdn.microsoft.com/en-us/library/5tkz6s71.aspx
+    return llvm::GlobalVariable::WeakODRLinkage;
+  } else if (D->hasAttr<WeakAttr>()) {
     if (GV->isConstant())
       return llvm::GlobalVariable::WeakODRLinkage;
     else
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 885e8759422..50951a03f0c 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -4636,6 +4636,15 @@ static void checkAttributesAfterMerging(Sema &S, NamedDecl &ND) {
       ND.dropAttr<WeakRefAttr>();
     }
   }
+
+  // 'selectany' only applies to externally visible varable declarations.
+  // It does not apply to functions.
+  if (SelectAnyAttr *Attr = ND.getAttr<SelectAnyAttr>()) {
+    if (isa<FunctionDecl>(ND) || !ND.isExternallyVisible()) {
+      S.Diag(Attr->getLocation(), diag::err_attribute_selectany_non_extern_data);
+      ND.dropAttr<SelectAnyAttr>();
+    }
+  }
 }
 
 /// Given that we are within the definition of the given function,
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index c365bfc508c..627bc3d3649 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -4718,6 +4718,16 @@ static void handleForceInlineAttr(Sema &S, Decl *D, const AttributeList &Attr) {
                              Attr.getAttributeSpellingListIndex()));
 }
 
+static void handleSelectAnyAttr(Sema &S, Decl *D, const AttributeList &Attr) {
+  if (!checkMicrosoftExt(S, Attr))
+    return;
+  // Check linkage after possibly merging declaratinos.  See
+  // checkAttributesAfterMerging().
+  D->addAttr(::new (S.Context)
+             SelectAnyAttr(Attr.getRange(), S.Context,
+                           Attr.getAttributeSpellingListIndex()));
+}
+
 //===----------------------------------------------------------------------===//
 // Top Level Sema Entry Points
 //===----------------------------------------------------------------------===//
@@ -4944,6 +4954,9 @@ static void ProcessInheritableDeclAttr(Sema &S, Scope *scope, Decl *D,
   case AttributeList::AT_ForceInline:
     handleForceInlineAttr(S, D, Attr);
     break;
+  case AttributeList::AT_SelectAny:
+    handleSelectAnyAttr(S, D, Attr);
+    break;
 
   // Thread safety attributes:
   case AttributeList::AT_AssertExclusiveLock:
diff --git a/clang/test/CodeGen/ms-declspecs.c b/clang/test/CodeGen/ms-declspecs.c
index 26bdc58ebb7..5dc7787b8fb 100644
--- a/clang/test/CodeGen/ms-declspecs.c
+++ b/clang/test/CodeGen/ms-declspecs.c
@@ -1,5 +1,10 @@
 // RUN: %clang_cc1 -triple i386-pc-win32 %s -emit-llvm -fms-compatibility -o - | FileCheck %s
 
+__declspec(selectany) int x1 = 1;
+const __declspec(selectany) int x2 = 2;
+// CHECK: @x1 = weak_odr global i32 1, align 4
+// CHECK: @x2 = weak_odr constant i32 2, align 4
+
 struct __declspec(align(16)) S {
   char x;
 };
diff --git a/clang/test/SemaCXX/attr-selectany.cpp b/clang/test/SemaCXX/attr-selectany.cpp
new file mode 100644
index 00000000000..0f9776dbf5d
--- /dev/null
+++ b/clang/test/SemaCXX/attr-selectany.cpp
@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 -fms-extensions -fsyntax-only -verify %s
+// MSVC produces similar diagnostics.
+
+__declspec(selectany) void foo() { } // expected-error{{'selectany' can only be applied to data items with external linkage}}
+
+__declspec(selectany) int x1 = 1;
+
+const __declspec(selectany) int x2 = 2; // expected-error{{'selectany' can only be applied to data items with external linkage}}
+
+extern const __declspec(selectany) int x3 = 3;
+
+extern const int x4;
+const __declspec(selectany) int x4 = 4;
+
+// MSDN says this is incorrect, but MSVC doesn't diagnose it.
+extern __declspec(selectany) int x5;
+
+static __declspec(selectany) int x6 = 2; // expected-error{{'selectany' can only be applied to data items with external linkage}}
+
+// FIXME: MSVC accepts this and makes x7 externally visible and comdat, but keep
+// it as internal and not weak/linkonce.
+static int x7; // expected-note{{previous definition}}
+extern __declspec(selectany) int x7;  // expected-warning{{attribute declaration must precede definition}}
+
+int asdf() { return x7; }
+
+class X {
+ public:
+  X(int i) { i++; };
+  int i;
+};
+
+__declspec(selectany) X x(1);