[ThinLTO] Don't create a comdat group for a dropped def with initializer

[tejohnson]

Summary

Non-prevailing weak/linkonce odr symbols will be dropped to
available_externally when possible. If they had an initializer in the
global_ctors list, a comdat group was being created. This code
already had logic to skip available_externally defs, but now the
EliminateAvailableExternally pass will drop these symbols to
declarations earlier. Change the check to skip all declarations for
linker (which includes available_externally along with declarations).

[tejohnson]

Add llvm-lto and llvm-lto2 based tests

[mehdi_amini]

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

1653

Do we need a ThinLTO importing to expose this case?
Can't you have a "simple" llc test that test the CodeGen for this?

[tejohnson]

Replace ThinLTO-specific tests with a simple llc based test

[tejohnson]

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

1653

Good idea, done.

[mehdi_amini]

LGTM.

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

void AsmPrinter::EmitXXStructorList(const DataLayout &DL, const Constant *List,
   unsigned Align = Log2_32(DL.getPointerPrefAlignment());
   std::stable_sort(Structors.begin(), Structors.end(),
                    [](const Structor &L,
                       const Structor &R) { return L.Priority < R.Priority; });
   for (Structor &S : Structors) {
     const TargetLoweringObjectFile &Obj = getObjFileLowering();
     const MCSymbol *KeySym = nullptr;
     if (GlobalValue *GV = S.ComdatKey) {
-      if (GV->hasAvailableExternallyLinkage())
-        // If the associated variable is available_externally, some other TU
+      if (GV->isDeclarationForLinker())
+        // If the associated variable is not defined in this module
+        // (it might be available_externally, or have been an
+        // available_externally definition that was dropped by the
+        // EliminateAvailableExternally pass), some other TU
         // will provide its dynamic initializer.
         continue;
 
       KeySym = getSymbol(GV);
     }
     MCSection *OutputSection =
         (isCtor ? Obj.getStaticCtorSection(S.Priority, KeySym)
                 : Obj.getStaticDtorSection(S.Priority, KeySym));
     OutStreamer->SwitchSection(OutputSection);
     if (OutStreamer->getCurrentSection() != OutStreamer->getPreviousSection())

test/CodeGen/X86/dropped_constructor.ll

+; Test to ensure that a global value that was dropped to a declaration
+; (e.g. ThinLTO will drop non-prevailing weak to declarations) does not
+; provoke creation of a comdat when it had an initializer.
+; RUN: llc -mtriple x86_64-unknown-linux-gnu < %s | FileCheck %s
+; CHECK-NOT: comdat
+
+; ModuleID = 'dropped_constructor.o'
+source_filename = "dropped_constructor.c"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@fv = external global i8, align 8
[e-mail]_ctors = appending global [1 x { i32, void ()*, i8* }] [{ i32, void ()*, i8* } { i32 65535, void ()* @__cxx_global_var_init.33, i8* @fv }]
+
+; Function Attrs: norecurse nounwind
+define internal void @__cxx_global_var_init.33() section ".text.startup" {
+  store i8 1, i8* @fv, align 8
+  ret void
+}