Index: ELF/LTO.h
===================================================================
--- ELF/LTO.h
+++ ELF/LTO.h
@@ -23,6 +23,7 @@
 
 #include "lld/Core/LLVM.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Module.h"
 #include "llvm/Linker/IRMover.h"
@@ -46,6 +47,7 @@
   llvm::IRMover Mover{Combined};
   SmallString<0> OwningData;
   std::unique_ptr<MemoryBuffer> MB;
+  llvm::StringSet<> InternalizedSyms;
 };
 }
 }
Index: ELF/LTO.cpp
===================================================================
--- ELF/LTO.cpp
+++ ELF/LTO.cpp
@@ -22,6 +22,7 @@
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Transforms/IPO.h"
 #include "llvm/Transforms/IPO/PassManagerBuilder.h"
+#include "llvm/Transforms/Utils/ModuleUtils.h"
 
 using namespace llvm;
 using namespace llvm::object;
@@ -75,6 +76,7 @@
   unsigned BodyIndex = 0;
   ArrayRef<SymbolBody *> Bodies = F.getSymbols();
 
+  SmallPtrSet<GlobalValue *, 8> Used;
   for (const BasicSymbolRef &Sym : Obj->symbols()) {
     GlobalValue *GV = Obj->getSymbolGV(Sym.getRawDataRefImpl());
     assert(GV);
@@ -95,6 +97,18 @@
             GV->setLinkage(GlobalValue::WeakODRLinkage);
             break;
           }
+          // We collect the set of symbols we want to internalize here
+          // and change the linkage after the IRMover executed, i.e. after
+          // we imported the symbols and satisfied undefined references
+          // to it. We can't just change linkage here because otherwise
+          // the IRMover will just rename the symbol.
+
+          // Shared libraries need to be handled slightly differently.
+          // For now, let's be conservative and just never internalize
+          // symbols when creating a shared library.
+          if (!Config->Shared && !Used.count(GV) && !B->isUsedInRegularObj())
+            InternalizedSyms.insert(GV->getName());
+
           Keep.push_back(GV);
         }
     }
@@ -104,10 +118,22 @@
              [](GlobalValue &, IRMover::ValueAdder) {});
 }
 
+static void internalize(GlobalValue &GV) {
+  assert(!GV.hasLocalLinkage() &&
+      "Trying to internalize a symbol with local linkage!") ;
+  GV.setLinkage(GlobalValue::InternalLinkage);
+}
+
 // Merge all the bitcode files we have seen, codegen the result
 // and return the resulting ObjectFile.
 template <class ELFT>
 std::unique_ptr<elf::ObjectFile<ELFT>> BitcodeCompiler::compile() {
+  for (const auto &Name : InternalizedSyms) {
+    GlobalValue *GV = Combined.getNamedValue(Name.first());
+    assert(GV);
+    internalize(*GV);
+  }
+
   if (Config->SaveTemps)
     saveBCFile(Combined, ".lto.bc");
 
Index: ELF/SymbolTable.cpp
===================================================================
--- ELF/SymbolTable.cpp
+++ ELF/SymbolTable.cpp
@@ -214,6 +214,16 @@
 
   SymbolBody *Existing = Sym->Body;
 
+  // We want to internalize symbols for LTO to be effective,
+  // but we can internalize only those that are not referenced
+  // from regular objects.
+  if (auto *BC = dyn_cast<DefinedBitcode>(Existing))
+    if (auto *U = dyn_cast<Undefined>(New))
+      BC->setUsedInRegularObj();
+  if (auto *U = dyn_cast<Undefined>(Existing))
+    if (auto *BC = dyn_cast<DefinedBitcode>(New))
+      BC->setUsedInRegularObj();
+
   if (Lazy *L = dyn_cast<Lazy>(Existing)) {
     if (auto *Undef = dyn_cast<Undefined>(New)) {
       addMemberFile(Undef, L);
Index: ELF/Symbols.h
===================================================================
--- ELF/Symbols.h
+++ ELF/Symbols.h
@@ -92,6 +92,8 @@
   template <class ELFT>
   typename ELFT::uint getVA(typename ELFT::uint Addend = 0) const;
 
+  void setUsedInRegularObj() { IsUsedInRegularObj = true; }
+
   template <class ELFT> typename ELFT::uint getGotVA() const;
   template <class ELFT> typename ELFT::uint getGotPltVA() const;
   template <class ELFT> typename ELFT::uint getPltVA() const;
@@ -120,7 +122,8 @@
     IsFunc = Type == llvm::ELF::STT_FUNC;
     IsTls = Type == llvm::ELF::STT_TLS;
     IsGnuIFunc = Type == llvm::ELF::STT_GNU_IFUNC;
-    IsUsedInRegularObj = K != SharedKind && K != LazyKind;
+    IsUsedInRegularObj =
+        K != SharedKind && K != LazyKind && K != DefinedBitcodeKind;
   }
 
   const unsigned SymbolKind : 8;
@@ -318,7 +321,6 @@
   std::unique_ptr<InputFile> getMember();
 
   void setWeak() { IsWeak = true; }
-  void setUsedInRegularObj() { IsUsedInRegularObj = true; }
 
 private:
   ArchiveFile *File;
Index: test/ELF/lto/internalize-basic.ll
===================================================================
--- /dev/null
+++ test/ELF/lto/internalize-basic.ll
@@ -0,0 +1,18 @@
+; REQUIRES: x86
+; RUN: llvm-as %s -o %t.o
+; RUN: ld.lld -m elf_x86_64 %t.o -o %t2 -save-temps
+; RUN: llvm-dis %t2.lto.bc -o - | FileCheck %s
+
+target triple = "x86_64-unknown-linux-gnu"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @_start() {
+  ret void
+}
+
+define hidden void @foo() {
+  ret void
+}
+
+; Check that foo function is correctly internalized.
+; CHECK: define internal void @foo()