Index: include/llvm/LTO/LTO.h
===================================================================
--- include/llvm/LTO/LTO.h
+++ include/llvm/LTO/LTO.h
@@ -424,6 +424,9 @@
       /// This global is either used by more than one partition or has an
       /// external reference, and therefore cannot be internalized.
       External = -2u,
+
+      /// This global is undefined.
+      Undefined = -3u,
     };
   };
 
@@ -451,8 +454,8 @@
 /// each global symbol based on its internal resolution of that symbol.
 struct SymbolResolution {
   SymbolResolution()
-      : Prevailing(0), FinalDefinitionInLinkageUnit(0), VisibleToRegularObj(0) {
-  }
+      : Prevailing(0), FinalDefinitionInLinkageUnit(0), VisibleToRegularObj(0),
+        Undefined(0) {}
   /// The linker has chosen this definition of the symbol.
   unsigned Prevailing : 1;
 
@@ -462,6 +465,9 @@
 
   /// The definition of this symbol is visible outside of the LTO unit.
   unsigned VisibleToRegularObj : 1;
+
+  /// This symbol has no definition.
+  unsigned Undefined : 1;
 };
 
 } // namespace lto
Index: lib/LTO/LTO.cpp
===================================================================
--- lib/LTO/LTO.cpp
+++ lib/LTO/LTO.cpp
@@ -246,10 +246,12 @@
   auto &GlobalRes = GlobalResolutions[Sym.getName()];
   if (GV) {
     GlobalRes.UnnamedAddr &= GV->hasGlobalUnnamedAddr();
-    if (Res.Prevailing)
+    if (Res.Prevailing || Res.Undefined)
       GlobalRes.IRName = GV->getName();
   }
-  if (Res.VisibleToRegularObj || (GV && Used.count(GV)) ||
+  if (Res.Undefined && GV && GV->isDeclarationForLinker())
+    GlobalRes.Partition = GlobalResolution::Undefined;
+  else if (Res.VisibleToRegularObj || (GV && Used.count(GV)) ||
       (GlobalRes.Partition != GlobalResolution::Unknown &&
        GlobalRes.Partition != Partition))
     GlobalRes.Partition = GlobalResolution::External;
@@ -273,6 +275,8 @@
       OS << 'l';
     if (Res.VisibleToRegularObj)
       OS << 'x';
+    if (Res.Undefined)
+      OS << 'u';
     OS << '\n';
   }
   assert(ResI == Res.end());
@@ -461,11 +465,20 @@
       if (R.second.IRName.empty())
         continue;
       if (R.second.Partition != 0 &&
-          R.second.Partition != GlobalResolution::External)
+          R.second.Partition != GlobalResolution::External &&
+          R.second.Partition != GlobalResolution::Undefined)
         continue;
 
       GlobalValue *GV =
           RegularLTO.CombinedModule->getNamedValue(R.second.IRName);
+
+      if (GV && GV->hasExternalWeakLinkage() &&
+          R.second.Partition == GlobalResolution::Undefined) {
+        GV->replaceAllUsesWith(Constant::getNullValue(GV->getType()));
+        GV->eraseFromParent();
+        continue;
+      }
+
       // Ignore symbols defined in other partitions.
       if (!GV || GV->hasLocalLinkage())
         continue;
Index: test/LTO/Resolution/X86/weak.ll
===================================================================
--- /dev/null
+++ test/LTO/Resolution/X86/weak.ll
@@ -0,0 +1,22 @@
+; RUN: llvm-as %s -o %t.bc
+; RUN: llvm-lto2 %t.bc -r %t.bc,main,plx -r %t.bc,_Z1fv,u -o %t.o -save-temps
+; RUN: llvm-dis %t.o.0.2.internalize.bc -o - | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i32 @main() {
+entry:
+; weak symbol _Z1fv resolved as undefined, replaced with nullptr.
+; CHECK-NOT: _Z1fv
+  br i1 icmp ne (void ()* @_Z1fv, void ()* null), label %if.then, label %if.end
+
+if.then:                                          ; preds = %entry
+  ret i32 1
+  unreachable
+
+if.end:                                           ; preds = %entry
+  ret i32 0
+}
+
+declare extern_weak void @_Z1fv()
Index: test/tools/gold/X86/comdat.ll
===================================================================
--- test/tools/gold/X86/comdat.ll
+++ test/tools/gold/X86/comdat.ll
@@ -39,7 +39,7 @@
 ; RES: 1.o,a15,px{{$}}
 
 ; RES: 2.o,f1,l{{$}}
-; RES: 2.o,will_be_undefined,{{$}}
+; RES: 2.o,will_be_undefined,u{{$}}
 ; RES: 2.o,v1,{{$}}
 ; RES: 2.o,r21,px{{$}}
 ; RES: 2.o,r22,px{{$}}
Index: test/tools/gold/X86/emit-llvm.ll
===================================================================
--- test/tools/gold/X86/emit-llvm.ll
+++ test/tools/gold/X86/emit-llvm.ll
@@ -43,7 +43,7 @@
 @g6 = linkonce_odr unnamed_addr global i32 32
 
 @g7 = extern_weak global i32
-; CHECK-DAG: @g7 = extern_weak global i32
+; CHECK-NOT: @g7
 
 @g8 = external global i32
 
@@ -110,7 +110,7 @@
 ; RES: .o,g4,px{{$}}
 ; RES: .o,g5,px{{$}}
 ; RES: .o,g6,p{{$}}
-; RES: .o,g7,{{$}}
-; RES: .o,g8,{{$}}
+; RES: .o,g7,u{{$}}
+; RES: .o,g8,u{{$}}
 ; RES: .o,g9,px{{$}}
 ; RES: .o,g10,px{{$}}
Index: test/tools/gold/X86/weak_undefined.ll
===================================================================
--- /dev/null
+++ test/tools/gold/X86/weak_undefined.ll
@@ -0,0 +1,24 @@
+; RUN: llvm-as %s -o %t.o
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
+; RUN:    --plugin-opt=emit-llvm \
+; RUN:    -shared %t.o -o %t2.o
+; RUN: llvm-dis %t2.o -o - | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i32 @main() {
+entry:
+; weak symbol _Z1fv resolved as undefined, replaced with nullptr.
+; CHECK-NOT: _Z1fv
+  br i1 icmp ne (void ()* @_Z1fv, void ()* null), label %if.then, label %if.end
+
+if.then:                                          ; preds = %entry
+  ret i32 1
+  unreachable
+
+if.end:                                           ; preds = %entry
+  ret i32 0
+}
+
+declare extern_weak void @_Z1fv()
Index: tools/gold/gold-plugin.cpp
===================================================================
--- tools/gold/gold-plugin.cpp
+++ tools/gold/gold-plugin.cpp
@@ -588,12 +588,15 @@
     case LDPR_UNKNOWN:
       llvm_unreachable("Unexpected resolution");
 
+    case LDPR_UNDEF:
+      R.Undefined = true;
+      break;
+
     case LDPR_RESOLVED_IR:
     case LDPR_RESOLVED_EXEC:
     case LDPR_RESOLVED_DYN:
     case LDPR_PREEMPTED_IR:
     case LDPR_PREEMPTED_REG:
-    case LDPR_UNDEF:
       break;
 
     case LDPR_PREVAILING_DEF_IRONLY:
Index: tools/llvm-lto2/llvm-lto2.cpp
===================================================================
--- tools/llvm-lto2/llvm-lto2.cpp
+++ tools/llvm-lto2/llvm-lto2.cpp
@@ -122,6 +122,8 @@
         Res.FinalDefinitionInLinkageUnit = true;
       else if (C == 'x')
         Res.VisibleToRegularObj = true;
+      else if (C == 'u')
+        Res.Undefined = true;
       else
         llvm::errs() << "invalid character " << C << " in resolution: " << R
                      << '\n';