ThinLTOBitcodeWriter: Drop dead constant users before checking whether globals have no users.

[pcc]

Summary

With this change we can avoid promotion more often.

llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp

void promoteInternals(Module &ExportM, Module &ImportM, StringRef ModuleId,
                       SetVector<GlobalValue *> &PromoteExtra) {
   DenseMap<const Comdat *, Comdat *> RenamedComdats;
   for (auto &ExportGV : ExportM.global_values()) {
     if (!ExportGV.hasLocalLinkage())
       continue;
 
     auto Name = ExportGV.getName();
     GlobalValue *ImportGV = ImportM.getNamedValue(Name);
-    if ((!ImportGV || ImportGV->use_empty()) && !PromoteExtra.count(&ExportGV))
+    if (!PromoteExtra.count(&ExportGV)) {
+      if (!ImportGV)
         continue;
+      ImportGV->removeDeadConstantUsers();
+      if (ImportGV->use_empty())
+        continue;
+    }
 
     std::string NewName = (Name + ModuleId).str();
 
     if (const auto *C = ExportGV.getComdat())
       if (C->getName() == Name)
         RenamedComdats.try_emplace(C, ExportM.getOrInsertComdat(NewName));
 
     ExportGV.setName(NewName);

llvm/test/Transforms/ThinLTOBitcodeWriter/split-internal3.ll

+; RUN: opt -thinlto-bc -o %t %s
+; RUN: llvm-modextract -b -n 0 -o %t0 %t
+; RUN: llvm-modextract -b -n 1 -o %t1 %t
+; RUN: llvm-dis -o - %t0 | FileCheck --check-prefix=M0 %s
+; RUN: llvm-dis -o - %t1 | FileCheck --check-prefix=M1 %s
+
+; M0-NOT: @"g2$
+; M1: @g2 = internal
+@g1 = global i8* bitcast (i8** @g2 to i8*), !type !0
+@g2 = internal global i8* null, !type !0
+
+!0 = !{i32 0, !"typeid"}