Index: lld/trunk/ELF/Driver.cpp
===================================================================
--- lld/trunk/ELF/Driver.cpp
+++ lld/trunk/ELF/Driver.cpp
@@ -880,6 +880,21 @@
   return V;
 }
 
+// Parses --defsym=alias option.
+static std::vector<std::pair<StringRef, StringRef>>
+getDefsym(opt::InputArgList &Args) {
+  std::vector<std::pair<StringRef, StringRef>> Ret;
+  for (auto *Arg : Args.filtered(OPT_defsym)) {
+    StringRef From;
+    StringRef To;
+    std::tie(From, To) = StringRef(Arg->getValue()).split('=');
+    if (!isValidCIdentifier(To))
+      error("--defsym: symbol name expected, but got " + To);
+    Ret.push_back({From, To});
+  }
+  return Ret;
+}
+
 // Do actual linking. Note that when this function is called,
 // all linker scripts have already been parsed.
 template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
@@ -945,6 +960,10 @@
   for (auto *Arg : Args.filtered(OPT_wrap))
     Symtab.wrap(Arg->getValue());
 
+  // Handle --defsym=sym=alias option.
+  for (std::pair<StringRef, StringRef> &Def : getDefsym(Args))
+    Symtab.alias(Def.first, Def.second);
+
   // Now that we have a complete list of input files.
   // Beyond this point, no new files are added.
   // Aggregate all input sections into one place.
Index: lld/trunk/ELF/Options.td
===================================================================
--- lld/trunk/ELF/Options.td
+++ lld/trunk/ELF/Options.td
@@ -25,6 +25,8 @@
 def compress_debug_sections : J<"compress-debug-sections=">,
   HelpText<"Compress DWARF debug sections">;
 
+def defsym: J<"defsym=">, HelpText<"Define a symbol alias">;
+
 def L: JoinedOrSeparate<["-"], "L">, MetaVarName<"<dir>">,
   HelpText<"Add a directory to the library search path">;
 
Index: lld/trunk/ELF/SymbolTable.h
===================================================================
--- lld/trunk/ELF/SymbolTable.h
+++ lld/trunk/ELF/SymbolTable.h
@@ -86,6 +86,7 @@
 
   void trace(StringRef Name);
   void wrap(StringRef Name);
+  void alias(StringRef Alias, StringRef Name);
 
 private:
   std::vector<SymbolBody *> findByVersion(SymbolVersion Ver);
Index: lld/trunk/ELF/SymbolTable.cpp
===================================================================
--- lld/trunk/ELF/SymbolTable.cpp
+++ lld/trunk/ELF/SymbolTable.cpp
@@ -168,6 +168,19 @@
   memcpy(Sym->Body.buffer, Wrap->Body.buffer, sizeof(Wrap->Body));
 }
 
+// Creates alias for symbol. Used to implement --defsym=ALIAS=SYM.
+template <class ELFT>
+void SymbolTable<ELFT>::alias(StringRef Alias, StringRef Name) {
+  SymbolBody *B = find(Name);
+  if (!B) {
+    error("-defsym: undefined symbol: " + Name);
+    return;
+  }
+  Symbol *Sym = B->symbol();
+  Symbol *AliasSym = addUndefined(Alias);
+  memcpy(AliasSym->Body.buffer, Sym->Body.buffer, sizeof(AliasSym->Body));
+}
+
 static uint8_t getMinVisibility(uint8_t VA, uint8_t VB) {
   if (VA == STV_DEFAULT)
     return VB;
Index: lld/trunk/test/ELF/defsym.s
===================================================================
--- lld/trunk/test/ELF/defsym.s
+++ lld/trunk/test/ELF/defsym.s
@@ -0,0 +1,44 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld -o %t %t.o --defsym=foo2=foo1
+# RUN: llvm-readobj -t -s %t | FileCheck %s
+# RUN: llvm-objdump -d -print-imm-hex %t | FileCheck %s --check-prefix=USE
+
+## In compare with GNU linkers, symbol defined with --defsym does
+## not get aliased name in symbol table:
+# CHECK:      Symbol {
+# CHECK:        Name: foo1
+# CHECK-NEXT:   Value: 0x123
+# CHECK-NEXT:   Size:
+# CHECK-NEXT:   Binding: Global
+# CHECK-NEXT:   Type:
+# CHECK-NEXT:   Other:
+# CHECK-NEXT:   Section: Absolute
+# CHECK-NEXT: }
+# CHECK-NEXT: Symbol {
+# CHECK-NEXT:   Name: foo1
+# CHECK-NEXT:   Value: 0x123
+# CHECK-NEXT:   Size:
+# CHECK-NEXT:   Binding: Global
+# CHECK-NEXT:   Type:
+# CHECK-NEXT:   Other:
+# CHECK-NEXT:   Section: Absolute
+# CHECK-NEXT: }
+
+## Check we can use foo2 and it that it is an alias for foo1.
+# USE:       Disassembly of section .text:
+# USE-NEXT:  _start:
+# USE-NEXT:    movl $0x123, %edx
+
+# RUN: not ld.lld -o %t %t.o --defsym=foo2=1 2>&1 | FileCheck %s -check-prefix=ERR1
+# ERR1: error: --defsym: symbol name expected, but got 1
+
+# RUN: not ld.lld -o %t %t.o --defsym=foo2=und 2>&1 | FileCheck %s -check-prefix=ERR2
+# ERR2: error: -defsym: undefined symbol: und
+
+.globl foo1
+ foo1 = 0x123
+
+.global _start
+_start:
+  movl $foo2, %edx