Index: ELF/Config.h
===================================================================
--- ELF/Config.h
+++ ELF/Config.h
@@ -23,6 +23,7 @@
   bool DiscardAll = false;
   bool DiscardLocals = false;
   bool ExportDynamic = false;
+  bool NoInhibitExec = false;
 };
 
 extern Configuration *Config;
Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -91,6 +91,9 @@
   if (Args.hasArg(OPT_export_dynamic))
     Config->ExportDynamic = true;
 
+  if (Args.hasArg(OPT_noinhibit_exec))
+    Config->NoInhibitExec = true;
+
   // Create a list of input files.
   std::vector<MemoryBufferRef> Inputs;
 
Index: ELF/InputSection.cpp
===================================================================
--- ELF/InputSection.cpp
+++ ELF/InputSection.cpp
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "InputSection.h"
+#include "Config.h"
 #include "Error.h"
 #include "InputFiles.h"
 #include "OutputSections.h"
@@ -72,7 +73,8 @@
           continue;
         break;
       case SymbolBody::UndefinedKind:
-        assert(Body->isWeak() && "Undefined symbol reached writer");
+        assert((Config->NoInhibitExec || Body->isWeak()) &&
+               "Undefined symbol reached writer");
         SymVA = 0;
         break;
       case SymbolBody::LazyKind:
Index: ELF/Options.td
===================================================================
--- ELF/Options.td
+++ ELF/Options.td
@@ -28,6 +28,10 @@
 def alias_discard_locals: Flag<["-"], "X">,
      Alias<discard_locals>;
 
+def noinhibit_exec : Flag<["--"], "noinhibit-exec">,
+     HelpText<"Retain the executable output file whenever"
+              " it is still usable">;
+
 //===----------------------------------------------------------------------===//
 /// Ignored options
 //===----------------------------------------------------------------------===//
Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -303,11 +303,14 @@
     if (&SymE > Syms.begin() && &SymE < Syms.end())
       SymFile = F.get();
   }
-  if (SymFile)
-    error(Twine("undefined symbol: ") + Sym.getName() + " in " +
-          SymFile->getName());
+
+  Twine Error =
+      Twine("undefined symbol: ") +
+      (SymFile ? (Sym.getName() + " in " + SymFile->getName()) : Sym.getName());
+  if (Config->NoInhibitExec)
+    warning(Error);
   else
-    error(Twine("undefined symbol: ") + Sym.getName());
+    error(Error);
 }
 
 // Create output section objects and add them to OutputSections.
Index: test/elf2/no-inhibit-exec.s
===================================================================
--- test/elf2/no-inhibit-exec.s
+++ test/elf2/no-inhibit-exec.s
@@ -0,0 +1,14 @@
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: lld -flavor gnu2 %t --noinhibit-exec -o %t2
+# RUN: not lld -flavor gnu2 %t -o %t2
+# REQUIRES: x86
+
+# next code will not link without noinhibit-exec flag 
+# because of undefined symbol _bar
+.globl _start;
+_start:
+  call _bar 
+  mov $60, %rax
+  mov $0, %rdi
+  syscall
+