[MinGW] Don't mark external variables as DSO local

[mstorsjo]

Summary

Since MinGW supports automatically importing external variables from DLLs even without the DLLImport attribute, we shouldn't mark them as DSO local unless we actually know them to be local for sure.

Keep marking thread local variables as DSO local.

[rnk]

lib/CodeGen/CodeGenModule.cpp

737

I think this linkage and declaration check should be GV->isDeclarationForLinker(), since that will catch available_externally globals as well. Those would come from a static data member of a class template that has an explicit instantiation declaration:

template <typename T>
struct Foo {
  static const int x = 42;
};
extern template struct Foo<int>;
const int *bar() {
  return &Foo<int>::x;
}

Yep, that does it on x86_64-windows-gnu. We probably want a stub for Foo<int>::x.

Can you add a test at clang/test/CodeGenCXX/dso-local.cpp for this?

[mstorsjo]

lib/CodeGen/CodeGenModule.cpp

737

Sure, will do.

[mstorsjo]

Changed the condition to GV->isDeclarationForLinker(), updated tests accordingly (weak_bar in CodeGen/dso-local-executable.c lost the dso_local flag) and added more tests in CodeGenCXX/dso-local-executable.cpp as requested.

[rnk]

lgtm

test/CodeGen/dso-local-executable.c

14

I take it that was a side effect of the isDeclarationForLinker change? I think that's correct.

[mstorsjo]

test/CodeGen/dso-local-executable.c

14

Yes, it changed due to that. It probably won't hurt at least.

lib/CodeGen/CodeGenModule.cpp

static bool shouldAssumeDSOLocal(const CodeGenModule &CGM,
   if (!GV->hasDefaultVisibility() && !GV->hasExternalWeakLinkage())
     return true;
 
   // DLLImport explicitly marks the GV as external.
   if (GV->hasDLLImportStorageClass())
     return false;
 
   const llvm::Triple &TT = CGM.getTriple();
+  if (TT.isWindowsGNUEnvironment()) {
+    // In MinGW, variables without DLLImport can still be automatically
+    // imported from a DLL by the linker; don't mark variables that
+    // potentially could come from another DLL as DSO local.
+    if (GV->hasExternalLinkage() && GV->isDeclaration() &&
+        isa<llvm::GlobalVariable>(GV) && !GV->isThreadLocal())
+      return false;
+  }
   // Every other GV is local on COFF.
   // Make an exception for windows OS in the triple: Some firmware builds use
   // *-win32-macho triples. This (accidentally?) produced windows relocations
   // without GOT tables in older clang versions; Keep this behaviour.
   // FIXME: even thread local variables?
   if (TT.isOSBinFormatCOFF() || (TT.isOSWindows() && TT.isOSBinFormatMachO()))
     return true;
 

test/CodeGen/dllimport.c

 
 // CHECK: @GlobalRedecl2 = external dllimport global i32
 __declspec(dllimport) int GlobalRedecl2;
 __declspec(dllimport) int GlobalRedecl2;
 USEVAR(GlobalRedecl2)
 
 // NB: MSVC issues a warning and makes GlobalRedecl3 dllexport. We follow GCC
 // and drop the dllimport with a warning.
-// CHECK: @GlobalRedecl3 = external dso_local global i32
+// MS: @GlobalRedecl3 = external dso_local global i32
+// GNU: @GlobalRedecl3 = external global i32
 __declspec(dllimport) extern int GlobalRedecl3;
                       extern int GlobalRedecl3; // dllimport ignored
 USEVAR(GlobalRedecl3)
 
 // Make sure this works even if the decl has been used before it's defined (PR20792).
 // MS: @GlobalRedecl4 = common dso_local dllexport global i32
 // GNU: @GlobalRedecl4 = common dso_local global i32
 __declspec(dllimport) extern int GlobalRedecl4;

test/CodeGen/dso-local-executable.c

 // RUN: %clang_cc1 -triple x86_64-pc-win32 -emit-llvm %s -o - | FileCheck -allow-deprecated-dag-overlap --check-prefix=COFF %s
 // COFF-DAG: @bar = external dso_local global i32
 // COFF-DAG: @weak_bar = extern_weak dso_local global i32
 // COFF-DAG: declare dso_local void @foo()
 // COFF-DAG: @baz = dso_local global i32 42
 // COFF-DAG: define dso_local i32* @zed()
 // COFF-DAG: @thread_var = external dso_local thread_local global i32
 // COFF-DAG: @local_thread_var = dso_local thread_local global i32 42
 // COFF-DAG: @import_var = external dllimport global i32
 // COFF-DAG: declare dllimport void @import_func()
 
+// RUN: %clang_cc1 -triple x86_64-w64-mingw32 -emit-llvm %s -o - | FileCheck -allow-deprecated-dag-overlap --check-prefix=MINGW %s
+// MINGW-DAG: @bar = external global i32
+// MINGW-DAG: @weak_bar = extern_weak dso_local global i32
+// MINGW-DAG: declare dso_local void @foo()
+// MINGW-DAG: @baz = dso_local global i32 42
+// MINGW-DAG: define dso_local i32* @zed()
+// MINGW-DAG: @thread_var = external dso_local thread_local global i32
+// MINGW-DAG: @local_thread_var = dso_local thread_local global i32 42
+// MINGW-DAG: @import_var = external dllimport global i32
+// MINGW-DAG: declare dllimport void @import_func()
+
 // RUN: %clang_cc1 -triple x86_64-pc-linux -emit-llvm -mrelocation-model static %s -o - | FileCheck -allow-deprecated-dag-overlap --check-prefix=STATIC %s
 // STATIC-DAG: @bar = external dso_local global i32
 // STATIC-DAG: @weak_bar = extern_weak dso_local global i32
 // STATIC-DAG: declare dso_local void @foo()
 // STATIC-DAG: @baz = dso_local global i32 42
 // STATIC-DAG: define dso_local i32* @zed()
 // STATIC-DAG: @thread_var = external thread_local global i32
 // STATIC-DAG: @local_thread_var = dso_local thread_local global i32 42

test/CodeGenCXX/dllexport.cpp

 
 // Declarations are not exported.
 // MSC-NOT: @"?ExternGlobalDecl@@3HA"
 // GNU-NOT: @ExternGlobalDecl
 __declspec(dllexport) extern int ExternGlobalDecl;
 
 // M64-DAG: @__ImageBase = external dso_local constant i8
 
-// GNU-DAG: @_ZTVN10__cxxabiv117__class_type_infoE = external dso_local global
+// GNU-DAG: @_ZTVN10__cxxabiv117__class_type_infoE = external global
 
 // dllexport implies a definition.
 // MSC-DAG: @"?GlobalDef@@3HA" = dso_local dllexport global i32 0, align 4
 // GNU-DAG: @GlobalDef            = dso_local dllexport global i32 0, align 4
 __declspec(dllexport) int GlobalDef;
 
 // Export definition.
 // MSC-DAG: @"?GlobalInit1@@3HA" = dso_local dllexport global i32 1, align 4

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

test/CodeGenCXX/dllimport-members.cpp

 // MSC-DAG: @"??$ImportedStaticVar@UExplicitSpec_Imported@@@MemVarTmpl@@2HB" = external dllimport constant i32
 // GNU-DAG: @_ZN10MemVarTmpl17ImportedStaticVarI21ExplicitSpec_ImportedEE       = external dllimport constant i32
 template<> __declspec(dllimport) const int MemVarTmpl::ImportedStaticVar<ExplicitSpec_Imported>;
 USEMV(MemVarTmpl, ImportedStaticVar<ExplicitSpec_Imported>)
 
 // Not importing specialization of a member variable template without explicit
 // dllimport.
 // MSC-DAG: @"??$ImportedStaticVar@UExplicitSpec_NotImported@@@MemVarTmpl@@2HB" = external dso_local constant i32
-// GNU-DAG: @_ZN10MemVarTmpl17ImportedStaticVarI24ExplicitSpec_NotImportedEE       = external dso_local constant i32
+// GNU-DAG: @_ZN10MemVarTmpl17ImportedStaticVarI24ExplicitSpec_NotImportedEE       = external constant i32
 template<> const int MemVarTmpl::ImportedStaticVar<ExplicitSpec_NotImported>;
 USEMV(MemVarTmpl, ImportedStaticVar<ExplicitSpec_NotImported>)
 
 
 // Import explicit instantiation declaration of a non-imported member variable
 // template.
 // MSC-DAG: @"??$StaticVar@UExplicitDecl_Imported@@@MemVarTmpl@@2HB" = available_externally dllimport constant i32 1
 // GNU-DAG: @_ZN10MemVarTmpl9StaticVarI21ExplicitDecl_ImportedEE        = external dllimport constant i32

test/CodeGenCXX/dllimport.cpp

 // GNU-DAG: @GlobalRedecl2c            = external dllimport global i32
 int GlobalRedecl2c __attribute__((dllimport));
 int GlobalRedecl2c __attribute__((dllimport));
 USEVAR(GlobalRedecl2c)
 
 // NB: MSC issues a warning and makes GlobalRedecl3 dllexport. We follow GCC
 // and drop the dllimport with a warning.
 // MSC-DAG: @"?GlobalRedecl3@@3HA" = external dso_local global i32
-// GNU-DAG: @GlobalRedecl3            = external dso_local global i32
+// GNU-DAG: @GlobalRedecl3            = external global i32
 __declspec(dllimport) extern int GlobalRedecl3;
                       extern int GlobalRedecl3; // dllimport ignored
 USEVAR(GlobalRedecl3)
 
 // MSC-DAG: @"?ExternalGlobal@ns@@3HA" = external dllimport global i32
 // GNU-DAG: @_ZN2ns14ExternalGlobalE      = external dllimport global i32
 namespace ns { __declspec(dllimport) int ExternalGlobal; }
 USEVAR(ns::ExternalGlobal)

 
 // MSC-DAG: @"??$VarTmplRedecl2@UImplicitInst_Imported@@@@3HA" = external dllimport global i32
 // GNU-DAG: @_Z14VarTmplRedecl2I21ImplicitInst_ImportedE          = external dllimport global i32
 template<typename T> __declspec(dllimport) int VarTmplRedecl2;
 template<typename T> __declspec(dllimport) int VarTmplRedecl2;
 USEVAR(VarTmplRedecl2<ImplicitInst_Imported>)
 
 // MSC-DAG: @"??$VarTmplRedecl3@UImplicitInst_Imported@@@@3HA" = external dso_local global i32
-// GNU-DAG: @_Z14VarTmplRedecl3I21ImplicitInst_ImportedE          = external dso_local global i32
+// GNU-DAG: @_Z14VarTmplRedecl3I21ImplicitInst_ImportedE          = external global i32
 template<typename T> __declspec(dllimport) extern int VarTmplRedecl3;
 template<typename T>                       extern int VarTmplRedecl3; // dllimport ignored
 USEVAR(VarTmplRedecl3<ImplicitInst_Imported>)
 
 
 // MSC-DAG: @"??$ExternalVarTmpl@UImplicitInst_Imported@@@ns@@3HA" = external dllimport global i32
 // GNU-DAG: @_ZN2ns15ExternalVarTmplI21ImplicitInst_ImportedEE        = external dllimport global i32
 namespace ns { template<typename T> __declspec(dllimport) int ExternalVarTmpl; }