Index: ELF/Config.h
===================================================================
--- ELF/Config.h
+++ ELF/Config.h
@@ -182,6 +182,7 @@
   bool WarnCommon;
   bool WarnIfuncTextrel;
   bool WarnMissingEntry;
+  bool WarnOnce;
   bool WarnSymbolOrdering;
   bool WriteAddends;
   bool ZCombreloc;
Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -868,6 +868,7 @@
       Args.hasFlag(OPT_warn_ifunc_textrel, OPT_no_warn_ifunc_textrel, false);
   Config->WarnSymbolOrdering =
       Args.hasFlag(OPT_warn_symbol_ordering, OPT_no_warn_symbol_ordering, true);
+  Config->WarnOnce = Args.hasArg(OPT_warn_once);
   Config->ZCombreloc = getZFlag(Args, "combreloc", "nocombreloc", true);
   Config->ZCopyreloc = getZFlag(Args, "copyreloc", "nocopyreloc", true);
   Config->ZExecstack = getZFlag(Args, "execstack", "noexecstack", false);
Index: ELF/Options.td
===================================================================
--- ELF/Options.td
+++ ELF/Options.td
@@ -379,6 +379,9 @@
 
 def warn_unresolved_symbols: F<"warn-unresolved-symbols">,
   HelpText<"Report unresolved symbols as warnings">;
+  
+def warn_once: F<"warn-once">,
+  HelpText<"Warn only once for each undefined symbol">;
 
 defm whole_archive: B<"whole-archive",
     "Force load of all members in a static library",
@@ -510,7 +513,6 @@
 def: F<"sort-common">;
 def: F<"stats">;
 def: F<"warn-execstack">;
-def: F<"warn-once">;
 def: F<"warn-shared-textrel">;
 def: F<"EB">;
 def: F<"EL">;
Index: ELF/Relocations.cpp
===================================================================
--- ELF/Relocations.cpp
+++ ELF/Relocations.cpp
@@ -654,6 +654,12 @@
   if (Config->UnresolvedSymbols == UnresolvedPolicy::Ignore && CanBeExternal)
     return false;
 
+  if (Config->WarnOnce) {
+    if (Sym.Warned)
+      return false;
+    Sym.Warned = true;
+  }
+
   std::string Msg =
       "undefined symbol: " + toString(Sym) + "\n>>> referenced by ";
 
Index: ELF/Symbols.h
===================================================================
--- ELF/Symbols.h
+++ ELF/Symbols.h
@@ -183,7 +183,7 @@
         Type(Type), StOther(StOther), SymbolKind(K), NeedsPltAddr(false),
         IsInIplt(false), IsInIgot(false), IsPreemptible(false),
         Used(!Config->GcSections), NeedsTocRestore(false),
-        ScriptDefined(false) {}
+        ScriptDefined(false), Warned(false) {}
 
 public:
   // True the symbol should point to its PLT entry.
@@ -209,6 +209,10 @@
   // True if this symbol is defined by a linker script.
   unsigned ScriptDefined : 1;
 
+  // True if this symbol is already reported as undefined. This
+  // bit is used for --warn-once option.
+  unsigned Warned : 1;
+
   bool isSection() const { return Type == llvm::ELF::STT_SECTION; }
   bool isTls() const { return Type == llvm::ELF::STT_TLS; }
   bool isFunc() const { return Type == llvm::ELF::STT_FUNC; }
Index: test/ELF/warn-once.s
===================================================================
--- test/ELF/warn-once.s
+++ test/ELF/warn-once.s
@@ -0,0 +1,13 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: not ld.lld -warn-once %t.o -o %t.exe 2>&1 | FileCheck %s
+
+# CHECK-COUNT-1: error: undefined symbol: foo
+# CHECK-COUNT-1: >>> referenced by warn-once.s
+
+.file "warn-once.s"
+
+  .globl _start
+_start:
+  call foo
+  call foo