Index: ELF/Config.h
===================================================================
--- ELF/Config.h
+++ ELF/Config.h
@@ -91,6 +91,7 @@
   uint16_t EMachine = llvm::ELF::EM_NONE;
   uint64_t EntryAddr = -1;
   unsigned Optimize = 0;
+  unsigned LTOOptLevel = 2;
 };
 
 // The only instance of Configuration struct.
Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -280,6 +280,12 @@
       error("invalid optimization level");
   }
 
+  if (auto *Arg = Args.getLastArg(OPT_lto_O)) {
+    StringRef Val = Arg->getValue();
+    if (Val.getAsInteger(10, Config->LTOOptLevel))
+      error("invalid optimization level");
+  }
+
   if (auto *Arg = Args.getLastArg(OPT_hash_style)) {
     StringRef S = Arg->getValue();
     if (S == "gnu") {
Index: ELF/LTO.cpp
===================================================================
--- ELF/LTO.cpp
+++ ELF/LTO.cpp
@@ -52,7 +52,7 @@
 // Run LTO passes.
 // Note that the gold plugin has a similar piece of code, so
 // it is probably better to move this code to a common place.
-static void runLTOPasses(Module &M, TargetMachine &TM) {
+static void runLTOPasses(Module &M, TargetMachine &TM, unsigned OptLevel) {
   legacy::PassManager LtoPasses;
   LtoPasses.add(createTargetTransformInfoWrapperPass(TM.getTargetIRAnalysis()));
   PassManagerBuilder PMB;
@@ -62,7 +62,7 @@
   PMB.VerifyOutput = true;
   PMB.LoopVectorize = true;
   PMB.SLPVectorize = true;
-  PMB.OptLevel = 2; // FIXME: This should be an option.
+  PMB.OptLevel = OptLevel;
   PMB.populateLTOPassManager(LtoPasses);
   LtoPasses.run(M);
 
@@ -146,7 +146,7 @@
     saveBCFile(Combined, ".lto.bc");
 
   std::unique_ptr<TargetMachine> TM(getTargetMachine());
-  runLTOPasses(Combined, *TM);
+  runLTOPasses(Combined, *TM, Config->LTOOptLevel);
 
   raw_svector_ostream OS(OwningData);
   legacy::PassManager CodeGenPasses;
Index: ELF/Options.td
===================================================================
--- ELF/Options.td
+++ ELF/Options.td
@@ -75,6 +75,9 @@
 def l : JoinedOrSeparate<["-"], "l">, MetaVarName<"<libName>">,
   HelpText<"Root name of library to use">;
 
+def lto_O : Joined<["--"], "lto-O">, MetaVarName<"<opt-level>">,
+  HelpText<"Optimization level for LTO">;
+
 def m : JoinedOrSeparate<["-"], "m">,
   HelpText<"Set target emulation">;
 
Index: test/ELF/lto/opt-level.ll
===================================================================
--- /dev/null
+++ test/ELF/lto/opt-level.ll
@@ -0,0 +1,21 @@
+; RUN: llvm-as -o %t.o %s
+; RUN: ld.lld -o %t0 -m elf_x86_64 -e main --lto-O0 %t.o
+; RUN: llvm-nm %t0 | FileCheck --check-prefix=CHECK-O0 %s
+; RUN: ld.lld -o %t2 -m elf_x86_64 -e main --lto-O2 %t.o
+; RUN: llvm-nm %t2 | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: ld.lld -o %t2a -m elf_x86_64 -e main %t.o
+; RUN: llvm-nm %t2a | FileCheck --check-prefix=CHECK-O2 %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-O0: foo
+; CHECK-O2-NOT: foo
+define internal void @foo() {
+  ret void
+}
+
+define void @main() {
+  call void @foo()
+  ret void
+}