Revert D135427 "[LTO] Make local linkage GlobalValue in non-prevailing COMDAT available_externally"

This reverts commit 8901635.

This change broke the following example and we need to check `if (GO->getComdat()->getName() == GO->getName())`
before `NonPrevailingComdats.insert(GO->getComdat());`
Revert for clarify.

```
// a.cc
template <typename T>
struct A final { virtual ~A() {} };
extern "C" void aa() { A<int> a; }
// b.cc
template <typename T>
struct A final { virtual ~A() {} };
template struct A<int>;
extern "C" void bb(A<int> *a) { delete a; }

clang -c -fpic -O0 -flto=thin a.cc && ld.lld -shared a.o b.o
```

Author: MaskRay

llvm/lib/LTO/LTO.cpp

@@ -712,11 +712,11 @@ handleNonPrevailingComdat(GlobalValue &GV,
   if (!NonPrevailingComdats.count(C))
     return;
 
-  // Additionally need to drop all global values from the comdat to
-  // available_externally, to satisfy the COMDAT requirement that all members
-  // are discarded as a unit. The non-local linkage global values avoid
-  // duplicate definition linker errors.
-  GV.setLinkage(GlobalValue::AvailableExternallyLinkage);
+  // Additionally need to drop externally visible global values from the comdat
+  // to available_externally, so that there aren't multiply defined linker
+  // errors.
+  if (!GV.hasLocalLinkage())
+    GV.setLinkage(GlobalValue::AvailableExternallyLinkage);
 
   if (auto GO = dyn_cast<GlobalObject>(&GV))
     GO->setComdat(nullptr);

llvm/lib/Transforms/IPO/FunctionImport.cpp

@@ -1051,7 +1051,6 @@ bool llvm::convertToDeclaration(GlobalValue &GV) {
 void llvm::thinLTOFinalizeInModule(Module &TheModule,
                                    const GVSummaryMapTy &DefinedGlobals,
                                    bool PropagateAttrs) {
-  DenseSet<Comdat *> NonPrevailingComdats;
   auto FinalizeInModule = [&](GlobalValue &GV, bool Propagate = false) {
     // See if the global summary analysis computed a new resolved linkage.
     const auto &GS = DefinedGlobals.find(GV.getGUID());
@@ -1129,10 +1128,8 @@ void llvm::thinLTOFinalizeInModule(Module &TheModule,
     // as this is a declaration for the linker, and will be dropped eventually.
     // It is illegal for comdats to contain declarations.
     auto *GO = dyn_cast_or_null<GlobalObject>(&GV);
-    if (GO && GO->isDeclarationForLinker() && GO->hasComdat()) {
-      NonPrevailingComdats.insert(GO->getComdat());
+    if (GO && GO->isDeclarationForLinker() && GO->hasComdat())
       GO->setComdat(nullptr);
-    }
   };
 
   // Process functions and global now
@@ -1142,36 +1139,6 @@ void llvm::thinLTOFinalizeInModule(Module &TheModule,
     FinalizeInModule(GV);
   for (auto &GV : TheModule.aliases())
     FinalizeInModule(GV);
-
-  // For a non-prevailing comdat, all its members must be available_externally.
-  // FinalizeInModule has handled non-local-linkage GlobalValues. Here we handle
-  // local linkage GlobalValues.
-  if (NonPrevailingComdats.empty())
-    return;
-  for (auto &GO : TheModule.global_objects()) {
-    if (auto *C = GO.getComdat(); C && NonPrevailingComdats.count(C)) {
-      GO.setComdat(nullptr);
-      GO.setLinkage(GlobalValue::AvailableExternallyLinkage);
-    }
-  }
-  bool Changed;
-  do {
-    Changed = false;
-    // If an alias references a GlobalValue in a non-prevailing comdat, change
-    // it to available_externally. For simplicity we only handle GlobalValue and
-    // ConstantExpr with a base object. ConstantExpr without a base object is
-    // unlikely used in a COMDAT.
-    for (auto &GA : TheModule.aliases()) {
-      if (GA.hasAvailableExternallyLinkage())
-        continue;
-      GlobalObject *Obj = GA.getAliaseeObject();
-      assert(Obj && "aliasee without an base object is unimplemented");
-      if (Obj->hasAvailableExternallyLinkage()) {
-        GA.setLinkage(GlobalValue::AvailableExternallyLinkage);
-        Changed = true;
-      }
-    }
-  } while (Changed);
 }
 
 /// Run internalization on \p TheModule based on symmary analysis.

llvm/test/LTO/Resolution/X86/comdat-mixed-lto.ll

@@ -8,29 +8,16 @@
 ; The copy of C from this module is prevailing. The copy of C from the
 ; regular LTO module is not prevailing, and will be dropped to
 ; available_externally.
-; RUN: llvm-lto2 run -r=%t1.o,C,pl -r=%t2.o,C,l -r=%t1.o,testglobfunc,lxp -r=%t2.o,testglobfunc,lx -o %t3 %t1.o %t2.o -save-temps
+; RUN: llvm-lto2 run -r=%t1.o,C,pl -r=%t2.o,C,l -r=%t2.o,testglobfunc,lxp -r=%t1.o,testglobfunc,lx -o %t3 %t1.o %t2.o -save-temps
 
 ; The Input module (regular LTO) is %t3.0. Check to make sure that we removed
 ; __cxx_global_var_init and testglobfunc from comdat. Also check to ensure
 ; that testglobfunc was dropped to available_externally. Otherwise we would
 ; have linker multiply defined errors as it is no longer in a comdat and
 ; would clash with the copy from this module.
 ; RUN: llvm-dis %t3.0.0.preopt.bc -o - | FileCheck %s
-
-; CHECK: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 65535, ptr @__cxx_global_var_init, ptr @C }]
-; CHECK: @C = available_externally dso_local global %"class.Test::ptr" zeroinitializer, align 4
-; CHECK-NOT: declare
-; CHECK: declare dso_local void @__cxx_global_var_init() section ".text.startup"
-; CHECK-NOT: declare
-
-; Check the behavior with the prevailing testglobfunc in %t2.o.
-; RUN: llvm-lto2 run -r=%t1.o,C,pl -r=%t2.o,C,l -r=%t1.o,testglobfunc,lx -r=%t2.o,testglobfunc,plx -o %t4 %t1.o %t2.o -save-temps
-; RUN: llvm-dis %t4.0.0.preopt.bc -o - | FileCheck %s --check-prefix=CHECK2
-
-; CHECK2: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 65535, ptr @__cxx_global_var_init, ptr @C }]
-; CHECK2: @C = available_externally dso_local global %"class.Test::ptr" zeroinitializer, align 4
-; CHECK2: declare dso_local void @__cxx_global_var_init() section ".text.startup"
-; CHECK2: define available_externally dso_local void @testglobfunc() section ".text.startup" {
+; CHECK: define internal void @__cxx_global_var_init() section ".text.startup" {
+; CHECK: define available_externally dso_local void @testglobfunc() section ".text.startup" {
 
 ; ModuleID = 'comdat-mixed-lto.o'
 source_filename = "comdat-mixed-lto.cpp"

llvm/test/ThinLTO/X86/Inputs/linkonce_resolution_comdat.ll

@@ -1,24 +1,13 @@
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
-$f = comdat any
-$g = comdat any
+$c2 = comdat any
 
-@g_private = private global i32 41, comdat($g)
-
-define linkonce_odr i32 @f(i8*) unnamed_addr comdat($f) {
-    ret i32 41
-}
-
-define linkonce_odr i32 @g() unnamed_addr comdat($g) {
+define linkonce_odr i32 @f(i8*) unnamed_addr comdat($c2) {
     ret i32 41
 }
 
-define internal void @g_internal() unnamed_addr comdat($g) {
-    ret void
-}
-
-define i32 @h() {
+define i32 @g() {
     %i = call i32 @f(i8* null)
     ret i32 %i
 }

llvm/test/ThinLTO/X86/constructor-alias.ll

@@ -1,54 +1,33 @@
-; This test ensures that we drop the preempted copy of @f/@g from %t2.bc from their
-; comdats after making it available_externally. If not we would get a
-; verification error. g_internal/g_private are changed to available_externally
-; as well since it is in the same comdat of g.
+; This test ensures that we drop the preempted copy of @f from %t2.bc from its
+; comdat after making it available_externally. If not we would get a
+; verification error.
 ; RUN: opt -module-summary %s -o %t1.bc
 ; RUN: opt -module-summary %p/Inputs/linkonce_resolution_comdat.ll -o %t2.bc
-; RUN: llvm-lto -thinlto-action=run -disable-thinlto-funcattrs=0 %t1.bc %t2.bc -exported-symbol=f -exported-symbol=g -exported-symbol=h -thinlto-save-temps=%t3.
+; RUN: llvm-lto -thinlto-action=run -disable-thinlto-funcattrs=0 %t1.bc %t2.bc -exported-symbol=f -exported-symbol=g -thinlto-save-temps=%t3.
 
 ; RUN: llvm-dis %t3.0.3.imported.bc -o - | FileCheck %s --check-prefix=IMPORT1
 ; RUN: llvm-dis %t3.1.3.imported.bc -o - | FileCheck %s --check-prefix=IMPORT2
 ; Copy from first module is prevailing and converted to weak_odr, copy
 ; from second module is preempted and converted to available_externally and
 ; removed from comdat.
-; IMPORT1: @g_private = private global i32 43, comdat($g)
-; IMPORT1: define weak_odr i32 @f(i8* %0) unnamed_addr [[ATTR:#[0-9]+]] comdat {
-; IMPORT1: define weak_odr i32 @g() unnamed_addr [[ATTR]] comdat {
-; IMPORT1: define internal void @g_internal() unnamed_addr comdat($g) {
-
-; IMPORT2: @g_private = available_externally dso_local global i32 41{{$}}
+; IMPORT1: define weak_odr i32 @f(i8* %0) unnamed_addr [[ATTR:#[0-9]+]] comdat($c1) {
 ; IMPORT2: define available_externally i32 @f(i8* %0) unnamed_addr [[ATTR:#[0-9]+]] {
-; IMPORT2: define available_externally i32 @g() unnamed_addr [[ATTR]] {
-; IMPORT2: define available_externally dso_local void @g_internal() unnamed_addr {
 
 ; CHECK-DAG: attributes [[ATTR]] = { norecurse nounwind }
 
-; RUN: llvm-nm %t1.bc.thinlto.o | FileCheck %s --check-prefix=NM1
+; RUN: llvm-nm -o - < %t1.bc.thinlto.o | FileCheck %s --check-prefix=NM1
 ; NM1: W f
-; NM1: W g
 
-; RUN: llvm-nm %t2.bc.thinlto.o | FileCheck %s --check-prefix=NM2
+; RUN: llvm-nm -o - < %t2.bc.thinlto.o | FileCheck %s --check-prefix=NM2
 ; f() would have been turned into available_externally since it is preempted,
-; and inlined into h()
+; and inlined into g()
 ; NM2-NOT: f
-; NM2-NOT: g
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
-$f = comdat any
-$g = comdat any
-
-@g_private = private global i32 43, comdat($g)
+$c1 = comdat any
 
-define linkonce_odr i32 @f(i8*) unnamed_addr comdat {
+define linkonce_odr i32 @f(i8*) unnamed_addr comdat($c1) {
     ret i32 43
 }
-
-define linkonce_odr i32 @g() unnamed_addr comdat {
-    ret i32 43
-}
-
-define internal void @g_internal() unnamed_addr comdat($g) {
-    ret void
-}