Index: lld/trunk/ELF/Config.h
===================================================================
--- lld/trunk/ELF/Config.h
+++ lld/trunk/ELF/Config.h
@@ -78,6 +78,7 @@
   bool SysvHash = true;
   bool Threads;
   bool Verbose;
+  bool WarnCommon;
   bool ZExecStack;
   bool ZNodelete;
   bool ZNow;
Index: lld/trunk/ELF/Driver.cpp
===================================================================
--- lld/trunk/ELF/Driver.cpp
+++ lld/trunk/ELF/Driver.cpp
@@ -244,6 +244,7 @@
   Config->StripAll = Args.hasArg(OPT_strip_all);
   Config->Threads = Args.hasArg(OPT_threads);
   Config->Verbose = Args.hasArg(OPT_verbose);
+  Config->WarnCommon = Args.hasArg(OPT_warn_common);
 
   Config->DynamicLinker = getString(Args, OPT_dynamic_linker);
   Config->Entry = getString(Args, OPT_entry);
Index: lld/trunk/ELF/Options.td
===================================================================
--- lld/trunk/ELF/Options.td
+++ lld/trunk/ELF/Options.td
@@ -131,6 +131,9 @@
 def version : Flag<["--", "-"], "version">,
   HelpText<"Display the version number">;
 
+def warn_common : Flag<["--", "-"], "warn-common">,
+  HelpText<"Warn about duplicate common symbols">;
+
 def whole_archive : Flag<["--", "-"], "whole-archive">,
   HelpText<"Force load of all members in a static library">;
 
@@ -178,9 +181,9 @@
 def fatal_warnings : Flag<["--"], "fatal-warnings">;
 def no_add_needed : Flag<["--"], "no-add-needed">;
 def no_fatal_warnings : Flag<["--"], "no-fatal-warnings">;
+def no_warn_common : Flag<["--", "-"], "no-warn-common">;
 def no_warn_mismatch : Flag<["--"], "no-warn-mismatch">;
 def version_script : Separate<["--"], "version-script">;
-def warn_common : Flag<["--"], "warn-common">;
 def warn_shared_textrel : Flag<["--"], "warn-shared-textrel">;
 def G : Separate<["-"], "G">;
 
Index: lld/trunk/ELF/Symbols.cpp
===================================================================
--- lld/trunk/ELF/Symbols.cpp
+++ lld/trunk/ELF/Symbols.cpp
@@ -159,6 +159,8 @@
 }
 
 static int compareCommons(DefinedCommon *A, DefinedCommon *B) {
+  if (Config->WarnCommon)
+    warning("multiple common of " + A->getName());
   A->Alignment = B->Alignment = std::max(A->Alignment, B->Alignment);
   if (A->Size < B->Size)
     return -1;
@@ -199,6 +201,8 @@
   if (isCommon() && Other->isCommon())
     return compareCommons(cast<DefinedCommon>(this),
                           cast<DefinedCommon>(Other));
+  if (Config->WarnCommon)
+    warning("common " + this->getName() + " is overridden");
   return isCommon() ? -1 : 1;
 }
 
Index: lld/trunk/test/ELF/Inputs/warn-common.s
===================================================================
--- lld/trunk/test/ELF/Inputs/warn-common.s
+++ lld/trunk/test/ELF/Inputs/warn-common.s
@@ -0,0 +1,2 @@
+.type arr,@object
+.comm arr,8,4
Index: lld/trunk/test/ELF/Inputs/warn-common2.s
===================================================================
--- lld/trunk/test/ELF/Inputs/warn-common2.s
+++ lld/trunk/test/ELF/Inputs/warn-common2.s
@@ -0,0 +1,8 @@
+.type arr,@object
+.data
+.globl arr
+.p2align 2
+arr:
+ .long 1
+ .long 0
+ .size arr, 8
Index: lld/trunk/test/ELF/warn-common.s
===================================================================
--- lld/trunk/test/ELF/warn-common.s
+++ lld/trunk/test/ELF/warn-common.s
@@ -0,0 +1,25 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t1.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %p/Inputs/warn-common.s -o %t2.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %p/Inputs/warn-common2.s -o %t3.o
+
+## Report multiple commons if warn-common is specified
+# RUN: ld.lld --warn-common %t1.o %t2.o -o %t.out 2>&1 | FileCheck %s --check-prefix=WARN
+# WARN: multiple common of arr
+
+## no-warn-common is ignored
+# RUN: ld.lld --no-warn-common %t1.o %t2.o -o %t.out
+# RUN: llvm-readobj %t.out > /dev/null
+
+## Report if common is overridden
+# RUN: ld.lld --warn-common %t1.o %t3.o -o %t.out 2>&1 | FileCheck %s --check-prefix=OVER
+# OVER: common arr is overridden
+
+## Report if common is overridden, but in different order
+# RUN: ld.lld --warn-common %t3.o %t1.o -o %t.out 2>&1 | FileCheck %s --check-prefix=OVER
+
+.globl _start
+_start:
+
+.type arr,@object
+.comm arr,4,4