From 0e4cf807aeaf54a10e02176498a7df13ac722b37 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Storsj=C3=B6?= <martin@martin.st>
Date: Sun, 23 May 2021 01:47:04 +0300
Subject: [PATCH] [clang] [MinGW] Don't mark emutls variables as DSO local

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.

Differential Revision: https://reviews.llvm.org/D102970
---
 lib/CodeGen/CodeGenModule.cpp       | 7 ++++++-
 test/CodeGen/dso-local-executable.c | 6 ++++--
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/lib/CodeGen/CodeGenModule.cpp b/lib/CodeGen/CodeGenModule.cpp
index 744ccf0fe59f1..9b31ecdbd81a9 100644
--- a/lib/CodeGen/CodeGenModule.cpp
+++ b/lib/CodeGen/CodeGenModule.cpp
@@ -986,8 +986,13 @@ static bool shouldAssumeDSOLocal(const CodeGenModule &CGM,
     // 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. (Native TLS variables
+    // can't be dllimported at all, though.)
     if (GV->isDeclarationForLinker() && isa<llvm::GlobalVariable>(GV) &&
-        !GV->isThreadLocal())
+        (!GV->isThreadLocal() || CGM.getCodeGenOpts().EmulatedTLS))
       return false;
   }
 
diff --git a/test/CodeGen/dso-local-executable.c b/test/CodeGen/dso-local-executable.c
index 4c282a334b114..e16b8db9d4ede 100644
--- a/test/CodeGen/dso-local-executable.c
+++ b/test/CodeGen/dso-local-executable.c
@@ -9,13 +9,15 @@
 // 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()