Index: test/wasm/lto/relocatable-undefined.ll
===================================================================
--- /dev/null
+++ test/wasm/lto/relocatable-undefined.ll
@@ -0,0 +1,36 @@
+; RUN: llvm-as %s -o %t.o
+; RUN: wasm-ld -r -o %t.wasm %t.o
+; RUN: obj2yaml %t.wasm | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+@missing_data = external global i32
+declare i32 @missing_func() local_unnamed_addr
+
+define i32 @foo() {
+entry:
+  %0 = call i32 @missing_func()
+  %1 = load i32, i32* @missing_data, align 4
+  ret i32 %1
+}
+
+
+; CHECK:        - Type:            CUSTOM
+; CHECK-NEXT:     Name:            linking
+; CHECK-NEXT:     Version:         2
+; CHECK-NEXT:     SymbolTable:     
+; CHECK-NEXT:       - Index:           0
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         Name:            foo
+; CHECK-NEXT:         Flags:           [  ]
+; CHECK-NEXT:         Function:        1
+; CHECK-NEXT:       - Index:           1
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         Name:            missing_func
+; CHECK-NEXT:         Flags:           [ UNDEFINED ]
+; CHECK-NEXT:         Function:        0
+; CHECK-NEXT:       - Index:           2
+; CHECK-NEXT:         Kind:            DATA
+; CHECK-NEXT:         Name:            missing_data
+; CHECK-NEXT:         Flags:           [ UNDEFINED ]
Index: wasm/InputFiles.cpp
===================================================================
--- wasm/InputFiles.cpp
+++ wasm/InputFiles.cpp
@@ -70,6 +70,7 @@
     assert(TypeIsUsed[Reloc.Index]);
     return TypeMap[Reloc.Index];
   }
+  LLVM_DEBUG(dbgs() << "calcNewIndex: " << toString(*Symbols[Reloc.Index]) << "\n");
   return Symbols[Reloc.Index]->getOutputSymbolIndex();
 }
 
Index: wasm/Writer.cpp
===================================================================
--- wasm/Writer.cpp
+++ wasm/Writer.cpp
@@ -935,6 +935,25 @@
     }
   }
 
+  // Undefined symbols from bitcode input files can still exist in the final
+  // output.  For defined symbols these get replaces with the symbols in the LTO
+  // object itself.
+  for (BitcodeFile *File : Symtab->BitcodeFiles) {
+    LLVM_DEBUG(dbgs() << "Symtab entries: " << File->getName() << "\n");
+    for (Symbol *Sym : File->getSymbols()) {
+      if (Sym->getFile() != File)
+        continue;
+
+      assert(Sym->isUndefined());
+
+      // (Since this is relocatable output, GC is not performed so symbols must
+      // be live.)
+      assert(Sym->isLive());
+      Sym->setOutputSymbolIndex(SymbolIndex++);
+      SymtabEntries.emplace_back(Sym);
+    }
+  }
+
   // For the moment, relocatable output doesn't contain any synthetic functions,
   // so no need to look through the Symtab for symbols not referenced by
   // Symtab->ObjectFiles.