Treat instantiations of variable templates as definitions

[sepavloff]

Summary

Current version of clang cannot build the program:

template<int n> int var;
int main(int argc, char *argv[]) {
  return var<0>;
}

as linker does not find var<0>, codegen treats it as declaration only.
However this program must build succesfully, because the template
declaration is a definition as none of the conditions mentioned in
[basic.def]p2 is met. With this change codegen generates definitions
for file level variable template specialization even if the declaration
does not contain an initializer.

[rsmith]

lib/CodeGen/CodeGenModule.cpp

1612–1616

It's not appropriate to work around this in CodeGen. Instead, we need to fix the AST representation to distinguish between a variable template specialization definition with no initializer and a variable template specialization declaration. I'd suggest adding a flag to the AST to mark that a variable is not actually a definition despite looking like one, and use that to flag declaration-only instantiations of variable template specializations. See also D24508, where we have another use case for such a flag.

[sepavloff]

Fixed in r319605.

lib/CodeGen/CodeGenModule.cpp

if (const auto *FD = dyn_cast<FunctionDecl>(Global)) {
     assert(VD->isFileVarDecl() && "Cannot emit local var decl as global.");
     // We need to emit device-side global CUDA variables even if a
     // variable does not have a definition -- we still need to define
     // host-side shadow for it.
     bool MustEmitForCuda = LangOpts.CUDA && !LangOpts.CUDAIsDevice &&
                            !VD->hasDefinition() &&
                            (VD->hasAttr<CUDAConstantAttr>() ||
                             VD->hasAttr<CUDADeviceAttr>());
-    if (!MustEmitForCuda &&
+    bool MustEmitVarInst = isa<VarTemplateSpecializationDecl>(VD) &&
+                           !VD->isStaticDataMember() &&
+                           !VD->hasDefinition() &&
+                           !VD->hasAttr<DLLImportAttr>();
+    if (!(MustEmitForCuda || MustEmitVarInst) &&
         VD->isThisDeclarationADefinition() != VarDecl::Definition &&
         !Context.isMSStaticDataMemberInlineDefinition(VD)) {
       // If this declaration may have caused an inline variable definition to
       // change linkage, make sure that it's emitted.
       if (Context.getInlineVariableDefinitionKind(VD) ==
           ASTContext::InlineVariableDefinitionKind::Strong)
         GetAddrOfGlobalVar(VD);
       return;

test/CodeGenCXX/dllexport.cpp

 // Declarations are not exported.
 
 // dllexport implies a definition.
 // MSC-NOT: @"\01??$VarTmplDef@UExplicitInst_Exported@@@@3HA"
 // GNU-NOT: @_Z10VarTmplDefI21ExplicitInst_ExportedE
 template<typename T> __declspec(dllexport) int VarTmplDef;
 INSTVAR(VarTmplDef<ExplicitInst_Exported>)
 
-// MSC-DAG: @"\01??$VarTmplImplicitDef@UImplicitInst_Exported@@@@3HA" = external dllexport global
-// GNU-DAG: @_Z18VarTmplImplicitDefI21ImplicitInst_ExportedE          = external dllexport global
+// MSC-DAG: @"\01??$VarTmplImplicitDef@UImplicitInst_Exported@@@@3HA" = weak_odr dllexport global
+// GNU-DAG: @_Z18VarTmplImplicitDefI21ImplicitInst_ExportedE          = weak_odr dllexport global
 template<typename T> __declspec(dllexport) int VarTmplImplicitDef;
 USEVAR(VarTmplImplicitDef<ImplicitInst_Exported>)
 
 // Export definition.
 // MSC-DAG: @"\01??$VarTmplInit1@UExplicitInst_Exported@@@@3HA" = weak_odr dllexport global i32 1, comdat, align 4
 // GNU-DAG: @_Z12VarTmplInit1I21ExplicitInst_ExportedE          = weak_odr dllexport global i32 1, comdat, align 4
 template<typename T> __declspec(dllexport) int VarTmplInit1 = 1;
 INSTVAR(VarTmplInit1<ExplicitInst_Exported>)

test/CodeGenCXX/variable-templates.cpp

+// RUN: %clang_cc1 -std=c++14 %s -triple=x86_64-linux -emit-llvm -o - | FileCheck %s
+
+// Unused template
+// CHECK-NOT: _Z6var_00ILi0EE
+template<int n> int var_00;
+
+// Definition without initializer
+// CHECK: @_Z6var_01ILi0EE = linkonce_odr global i32 0, comdat
+template<int n> int var_01;
+int use_01a() {
+  return var_01<0>;
+}
+int use_01b() {
+  return var_01<0>;
+}
+
+// Definitions without initializer combined with extern declaration
+
+// CHECK: @_Z6var_02ILi0EE = linkonce_odr global i32 0, comdat
+template<int n> extern int var_02;
+template<int n> int var_02;
+int use_02() {
+  return var_02<0>;
+}
+
+// CHECK: @_Z6var_03ILi0EE = linkonce_odr global i32 0, comdat
+template<int n> int var_03;
+template<int n> extern int var_03;
+int use_03() {
+  return var_03<0>;
+}
+
+// CHECK: @_Z6var_04ILi0EE = linkonce_odr global i32 0, comdat
+template<int n> extern int var_04;
+int use_04() {
+  return var_04<0>;
+}
+template<int n> int var_04;
+
+// CHECK: @_Z6var_05ILi0EE = linkonce_odr global i32 0, comdat
+template<int n> int var_05;
+int use_05() {
+  return var_05<0>;
+}
+template<int n> extern int var_05;
+
+
+int main(int argc, char *argv[]) {
+  return 0;
+}