PR48030: Fix COMDAT-related linking problem with C++ thread_local static data members.

Previously when emitting a C++ guarded initializer, we tried to work out what
the enclosing function would be used for and added it to the COMDAT containing
the variable if we thought that doing so would be correct. But this was done
from a context in which we didn't -- and realistically couldn't -- correctly
infer how the enclosing function would be used.

Instead, add the initialization function to a COMDAT from the code that
creates it, in the case where it makes sense to do so: when we know that
the one and only reference to the initialization function is in
@llvm.global.ctors and that reference is in the same COMDAT.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D108680

clang/lib/CodeGen/CGDeclCXX.cpp

@@ -581,6 +581,16 @@ CodeGenModule::EmitCXXGlobalVarDeclInitFunc(const VarDecl *D,
       // llvm.used to prevent linker GC.
       addUsedGlobal(COMDATKey);
     }
+
+    // If we used a COMDAT key for the global ctor, the init function can be
+    // discarded if the global ctor entry is discarded.
+    // FIXME: Do we need to restrict this to ELF and Wasm?
+    llvm::Comdat *C = Addr->getComdat();
+    if (COMDATKey && C &&
+        (getTarget().getTriple().isOSBinFormatELF() ||
+         getTarget().getTriple().isOSBinFormatWasm())) {
+      Fn->setComdat(C);
+    }
   } else {
     I = DelayedCXXInitPosition.find(D); // Re-do lookup in case of re-hash.
     if (I == DelayedCXXInitPosition.end()) {

clang/lib/CodeGen/ItaniumCXXABI.cpp

@@ -2445,11 +2445,6 @@ void ItaniumCXXABI::EmitGuardedInit(CodeGenFunction &CGF,
         (CGM.getTarget().getTriple().isOSBinFormatELF() ||
          CGM.getTarget().getTriple().isOSBinFormatWasm())) {
       guard->setComdat(C);
-      // An inline variable's guard function is run from the per-TU
-      // initialization function, not via a dedicated global ctor function, so
-      // we can't put it in a comdat.
-      if (!NonTemplateInline)
-        CGF.CurFn->setComdat(C);
     } else if (CGM.supportsCOMDAT() && guard->isWeakForLinker()) {
       guard->setComdat(CGM.getModule().getOrInsertComdat(guard->getName()));
     }

clang/test/CodeGenCXX/cxx11-thread-local-instantiated.cpp

@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -std=c++11 -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s
+
+// PR48030
+
+template<typename T> struct TLS { static thread_local T *mData; };
+inline decltype(nullptr) non_constant_initializer() { return nullptr; }
+template<typename T> thread_local T *TLS<T>::mData = non_constant_initializer();
+struct S {};
+S *current() { return TLS<S>::mData; };
+
+// CHECK-DAG: @_ZN3TLSI1SE5mDataE = linkonce_odr thread_local global {{.*}}, comdat,
+// CHECK-DAG: @_ZGVN3TLSI1SE5mDataE = linkonce_odr thread_local global {{.*}}, comdat($_ZN3TLSI1SE5mDataE),
+// CHECK-DAG: @_ZTHN3TLSI1SE5mDataE = linkonce_odr alias {{.*}} @__cxx_global_var_init
+
+// CHECK-LABEL: define {{.*}} @_Z7currentv()
+// CHECK: call {{.*}} @_ZTWN3TLSI1SE5mDataE()
+
+// CHECK-LABEL: define weak_odr hidden {{.*}} @_ZTWN3TLSI1SE5mDataE() {{.*}} comdat {
+// CHECK: call void @_ZTHN3TLSI1SE5mDataE()
+// CHECK: ret {{.*}} @_ZN3TLSI1SE5mDataE
+
+// Unlike for a global, the global initialization function must not be in a
+// COMDAT with the variable, because it is referenced from the _ZTH function
+// which is outside that COMDAT.
+//
+// CHECK-NOT: define {{.*}} @__cxx_global_var_init{{.*}}comdat

clang/test/CodeGenCXX/cxx11-thread-local.cpp

@@ -179,8 +179,7 @@ int f() {
 
 // LINUX_AIX: define internal void @[[VF_M_INIT]]()
 // DARWIN: define internal cxx_fast_tlscc void @[[VF_M_INIT]]()
-// LINUX-SAME: comdat($_ZN1VIfE1mE)
-// DARWIN-NOT: comdat
+// CHECK-NOT: comdat
 // CHECK: load i8, i8* bitcast (i64* @_ZGVN1VIfE1mE to i8*)
 // CHECK: %[[VF_M_INITIALIZED:.*]] = icmp eq i8 %{{.*}}, 0
 // CHECK: br i1 %[[VF_M_INITIALIZED]],
@@ -192,8 +191,7 @@ int f() {
 
 // LINUX_AIX: define internal void @[[XF_M_INIT]]()
 // DARWIN: define internal cxx_fast_tlscc void @[[XF_M_INIT]]()
-// LINUX-SAME: comdat($_ZN1XIfE1mE)
-// DARWIN-NOT: comdat
+// CHECK-NOT: comdat
 // CHECK: load i8, i8* bitcast (i64* @_ZGVN1XIfE1mE to i8*)
 // CHECK: %[[XF_M_INITIALIZED:.*]] = icmp eq i8 %{{.*}}, 0
 // CHECK: br i1 %[[XF_M_INITIALIZED]],
@@ -313,8 +311,7 @@ void set_anon_i() {
 
 // LINUX_AIX: define internal void @[[V_M_INIT]]()
 // DARWIN: define internal cxx_fast_tlscc void @[[V_M_INIT]]()
-// LINUX-SAME: comdat($_ZN1VIiE1mE)
-// DARWIN-NOT: comdat
+// CHECK-NOT: comdat
 // CHECK: load i8, i8* bitcast (i64* @_ZGVN1VIiE1mE to i8*)
 // CHECK: %[[V_M_INITIALIZED:.*]] = icmp eq i8 %{{.*}}, 0
 // CHECK: br i1 %[[V_M_INITIALIZED]],
@@ -326,8 +323,7 @@ void set_anon_i() {
 
 // LINUX_AIX: define internal void @[[X_M_INIT]]()
 // DARWIN: define internal cxx_fast_tlscc void @[[X_M_INIT]]()
-// LINUX-SAME: comdat($_ZN1XIiE1mE)
-// DARWIN-NOT: comdat
+// CHECK-NOT: comdat
 // CHECK: load i8, i8* bitcast (i64* @_ZGVN1XIiE1mE to i8*)
 // CHECK: %[[X_M_INITIALIZED:.*]] = icmp eq i8 %{{.*}}, 0
 // CHECK: br i1 %[[X_M_INITIALIZED]],

clang/test/CodeGenCXX/cxx1z-inline-variables.cpp

@@ -90,9 +90,7 @@ const int &yib = Y<int>::b;
 // CHECK-LABEL: define {{.*}}global_var_init
 // CHECK: call i32 @_Z1fv
 
-// CHECK-LABEL: define {{.*}}global_var_init
-// CHECK-NOT: comdat
-// CHECK-SAME: {{$}}
+// CHECK-LABEL: define {{.*}}global_var_init{{.*}} comdat($b)
 // CHECK: load atomic {{.*}} acquire, align
 // CHECK: br
 // CHECK: __cxa_guard_acquire(i64* @_ZGV1b)

clang/test/OpenMP/threadprivate_codegen.cpp

@@ -3847,7 +3847,7 @@ int foobar() {
 //
 //
 // CHECK-TLS1-LABEL: define {{[^@]+}}@__cxx_global_var_init.3
-// CHECK-TLS1-SAME: () #[[ATTR0]] comdat($_ZN2STI2S4E2stE) {
+// CHECK-TLS1-SAME: () #[[ATTR0]] {
 // CHECK-TLS1-NEXT:  entry:
 // CHECK-TLS1-NEXT:    [[TMP0:%.*]] = load i8, i8* bitcast (i64* @_ZGVN2STI2S4E2stE to i8*), align 8
 // CHECK-TLS1-NEXT:    [[GUARD_UNINITIALIZED:%.*]] = icmp eq i8 [[TMP0]], 0
@@ -4373,7 +4373,7 @@ int foobar() {
 //
 //
 // CHECK-TLS2-LABEL: define {{[^@]+}}@__cxx_global_var_init.3
-// CHECK-TLS2-SAME: () #[[ATTR6]] comdat($_ZN2STI2S4E2stE) {
+// CHECK-TLS2-SAME: () #[[ATTR6]] {
 // CHECK-TLS2-NEXT:  entry:
 // CHECK-TLS2-NEXT:    [[TMP0:%.*]] = load i8, i8* bitcast (i64* @_ZGVN2STI2S4E2stE to i8*), align 8
 // CHECK-TLS2-NEXT:    [[GUARD_UNINITIALIZED:%.*]] = icmp eq i8 [[TMP0]], 0
@@ -4906,7 +4906,7 @@ int foobar() {
 //
 //
 // CHECK-TLS3-LABEL: define {{[^@]+}}@__cxx_global_var_init.3
-// CHECK-TLS3-SAME: () #[[ATTR0]] comdat($_ZN2STI2S4E2stE) !dbg [[DBG289:![0-9]+]] {
+// CHECK-TLS3-SAME: () #[[ATTR0]] !dbg [[DBG289:![0-9]+]] {
 // CHECK-TLS3-NEXT:  entry:
 // CHECK-TLS3-NEXT:    [[TMP0:%.*]] = load i8, i8* bitcast (i64* @_ZGVN2STI2S4E2stE to i8*), align 8, !dbg [[DBG290:![0-9]+]]
 // CHECK-TLS3-NEXT:    [[GUARD_UNINITIALIZED:%.*]] = icmp eq i8 [[TMP0]], 0, !dbg [[DBG290]]
@@ -5459,7 +5459,7 @@ int foobar() {
 //
 //
 // CHECK-TLS4-LABEL: define {{[^@]+}}@__cxx_global_var_init.3
-// CHECK-TLS4-SAME: () #[[ATTR7]] comdat($_ZN2STI2S4E2stE) !dbg [[DBG289:![0-9]+]] {
+// CHECK-TLS4-SAME: () #[[ATTR7]] !dbg [[DBG289:![0-9]+]] {
 // CHECK-TLS4-NEXT:  entry:
 // CHECK-TLS4-NEXT:    [[TMP0:%.*]] = load i8, i8* bitcast (i64* @_ZGVN2STI2S4E2stE to i8*), align 8, !dbg [[DBG290:![0-9]+]]
 // CHECK-TLS4-NEXT:    [[GUARD_UNINITIALIZED:%.*]] = icmp eq i8 [[TMP0]], 0, !dbg [[DBG290]]