[clang] [MinGW] Don't mark emutls variables as DSO local

[mstorsjo]

Summary

These actually can be automatically imported from another DLL. (This
works properly as long as the actual implementation of emutls is
linked dynamically from e.g. libgcc; if the implementation comes from
compiler-rt or a statically linked libgcc, it doesn't work as intended.)

This fixes PR50146 and https://github.com/msys2/MINGW-packages/issues/8706
(fixing calling std::call_once in a dynamically linked libstdc++);
since f73183958482602c4588b0f4a1c3a096e7542947 the dso_local attribute
on the TLS variable affected the actual generated code for accessing
the emutls variable.

The dso_local attribute on the emutls variable made those accesses to
use 32 bit relative addressing in code, which requires runtime pseudo
relocations in the text section, and breaks entirely if the actual
other variable ends up loaded too far away in the virtual address
space.

[mati865]

Wow, I haven't expected that.

[mstorsjo]

@rnk Does this seem ok to you too?

clang/lib/CodeGen/CodeGenModule.cpp

992

I guess that it’d be good to point out the reverse too, that native tls variables on windows can’t be dllimported.

[rnk]

lgtm

clang/lib/CodeGen/CodeGenModule.cpp

static bool shouldAssumeDSOLocal(const CodeGenModule &CGM,
   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.
+
+    // With EmulatedTLS, TLS variables can be autoimported from other DLLs
+    // (and this actually happens in the public interface of libstdc++), so
+    // such variables can't be marked as DSO local.
     if (GV->isDeclarationForLinker() && isa<llvm::GlobalVariable>(GV) &&
-        !GV->isThreadLocal())
+        (!GV->isThreadLocal() || CGM.getCodeGenOpts().EmulatedTLS))
       return false;
   }
 
   // On COFF, don't mark 'extern_weak' symbols as DSO local. If these symbols
   // remain unresolved in the link, they can be resolved to zero, which is
   // outside the current DSO.
   if (TT.isOSBinFormatCOFF() && GV->hasExternalWeakLinkage())
     return false;

clang/test/CodeGen/dso-local-executable.c

 // RUN: %clang_cc1 -triple x86_64-pc-win32 -emit-llvm %s -o - | FileCheck --check-prefix=COFF %s
 // COFF:      @baz = dso_local global i32 42
 // COFF-NEXT: @import_var = external dllimport global i32
 // COFF-NEXT: @weak_bar = extern_weak global i32
 // COFF-NEXT: @bar = external dso_local global i32
 // COFF-NEXT: @local_thread_var = dso_local thread_local global i32 42
 // COFF-NEXT: @thread_var = external dso_local thread_local global i32
 // COFF-DAG: declare dso_local void @foo()
 // COFF-DAG: define dso_local i32* @zed()
 // COFF-DAG: declare dllimport void @import_func()
 
-// RUN: %clang_cc1 -triple x86_64-w64-mingw32 -emit-llvm %s -o - | FileCheck --check-prefix=MINGW %s
+// RUN: %clang_cc1 -triple x86_64-w64-mingw32 -emit-llvm %s -o - | FileCheck --check-prefixes=MINGW,MINGW-NATIVE_TLS %s
+// RUN: %clang_cc1 -triple x86_64-w64-mingw32 -emit-llvm %s -o - -femulated-tls | FileCheck --check-prefixes=MINGW,MINGW-EMUTLS %s
 // MINGW:      @baz = dso_local global i32 42
 // MINGW-NEXT: @import_var = external dllimport global i32
 // MINGW-NEXT: @weak_bar = extern_weak global i32
 // MINGW-NEXT: @bar = external global i32
 // MINGW-NEXT: @local_thread_var = dso_local thread_local global i32 42
-// MINGW-NEXT: @thread_var = external dso_local thread_local global i32
+// MINGW-NATIVE_TLS-NEXT: @thread_var = external dso_local thread_local global i32
+// MINGW-EMUTLS-NEXT: @thread_var = external thread_local global i32
 // MINGW-DAG: declare dso_local void @foo()
 // MINGW-DAG: define dso_local i32* @zed()
 // MINGW-DAG: declare dllimport void @import_func()
 
 /// Static relocation model defaults to -fdirect-access-external-data and sets
 /// dso_local on most global objects.
 // RUN: %clang_cc1 -triple x86_64 -emit-llvm -mrelocation-model static %s -o - | FileCheck --check-prefix=STATIC %s
 // RUN: %clang_cc1 -triple x86_64 -emit-llvm -mrelocation-model static -fdirect-access-external-data %s -o - | FileCheck --check-prefix=STATIC %s