diff --git a/lld/COFF/Config.h b/lld/COFF/Config.h
--- a/lld/COFF/Config.h
+++ b/lld/COFF/Config.h
@@ -142,6 +142,7 @@
 
   // Used for /opt:lldlto=N
   unsigned ltoo = 2;
+  unsigned ltoos = 0;
 
   // Used for /opt:lldltojobs=N
   unsigned thinLTOJobs = 0;
diff --git a/lld/COFF/Driver.cpp b/lld/COFF/Driver.cpp
--- a/lld/COFF/Driver.cpp
+++ b/lld/COFF/Driver.cpp
@@ -1406,8 +1406,8 @@
         tailMerge = 0;
       } else if (s.startswith("lldlto=")) {
         StringRef optLevel = s.substr(7);
-        if (optLevel.getAsInteger(10, config->ltoo) || config->ltoo > 3)
-          error("/opt:lldlto: invalid optimization level: " + optLevel);
+        config->ltoo = CHECK(llvm::lto::getOptLevel(optLevel), "/opt:lldlto: ");
+        config->ltoos = check(llvm::lto::getSizeLevel(optLevel));
       } else if (s.startswith("lldltojobs=")) {
         StringRef jobs = s.substr(11);
         if (jobs.getAsInteger(10, config->thinLTOJobs) ||
diff --git a/lld/COFF/LTO.cpp b/lld/COFF/LTO.cpp
--- a/lld/COFF/LTO.cpp
+++ b/lld/COFF/LTO.cpp
@@ -79,6 +79,7 @@
   c.DisableVerify = true;
   c.DiagHandler = diagnosticHandler;
   c.OptLevel = config->ltoo;
+  c.SizeLevel = config->ltoos;
   c.CPU = getCPUStr();
   c.MAttrs = getMAttrs();
   c.CGOptLevel = args::getCGOptLevel(config->ltoo);
diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -238,6 +238,7 @@
   uint64_t zStackSize;
   unsigned ltoPartitions;
   unsigned ltoo;
+  unsigned ltoos;
   unsigned optimize;
   unsigned thinLTOJobs;
   int32_t splitStackAdjustSize;
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -895,7 +895,8 @@
   config->ltoDebugPassManager = args.hasArg(OPT_lto_debug_pass_manager);
   config->ltoNewPassManager = args.hasArg(OPT_lto_new_pass_manager);
   config->ltoNewPmPasses = args.getLastArgValue(OPT_lto_newpm_passes);
-  config->ltoo = args::getInteger(args, OPT_lto_O, 2);
+  config->ltoo = check(llvm::lto::getOptLevel(args.getLastArg(OPT_lto_O), 2));
+  config->ltoos = check(llvm::lto::getSizeLevel(args.getLastArg(OPT_lto_O), 0));
   config->ltoObjPath = args.getLastArgValue(OPT_lto_obj_path_eq);
   config->ltoPartitions = args::getInteger(args, OPT_lto_partitions, 1);
   config->ltoSampleProfile = args.getLastArgValue(OPT_lto_sample_profile);
@@ -1001,8 +1002,6 @@
   for (auto *arg : args.filtered(OPT_mllvm))
     parseClangOption(arg->getValue(), arg->getSpelling());
 
-  if (config->ltoo > 3)
-    error("invalid optimization level for LTO: " + Twine(config->ltoo));
   if (config->ltoPartitions == 0)
     error("--lto-partitions: number of threads must be > 0");
   if (config->thinLTOJobs == 0)
diff --git a/lld/ELF/LTO.cpp b/lld/ELF/LTO.cpp
--- a/lld/ELF/LTO.cpp
+++ b/lld/ELF/LTO.cpp
@@ -89,6 +89,7 @@
   c.DisableVerify = config->disableVerify;
   c.DiagHandler = diagnosticHandler;
   c.OptLevel = config->ltoo;
+  c.SizeLevel = config->ltoos;
   c.CPU = getCPUStr();
   c.MAttrs = getMAttrs();
   c.CGOptLevel = args::getCGOptLevel(config->ltoo);
diff --git a/lld/test/COFF/lto-opt-level.ll b/lld/test/COFF/lto-opt-level.ll
--- a/lld/test/COFF/lto-opt-level.ll
+++ b/lld/test/COFF/lto-opt-level.ll
@@ -4,6 +4,10 @@
 ; RUN: FileCheck --check-prefix=CHECK-O0 %s < %t0.map
 ; RUN: lld-link /out:%t2.exe /entry:main /subsystem:console /opt:lldlto=2 /lldmap:%t2.map %t.obj
 ; RUN: FileCheck --check-prefix=CHECK-O2 %s < %t2.map
+; RUN: lld-link /out:%ts.exe /entry:main /subsystem:console /opt:lldlto=s /lldmap:%ts.map %t.obj
+; RUN: FileCheck --check-prefix=CHECK-O2 %s < %ts.map
+; RUN: lld-link /out:%tz.exe /entry:main /subsystem:console /opt:lldlto=z /lldmap:%tz.map %t.obj
+; RUN: FileCheck --check-prefix=CHECK-O2 %s < %tz.map
 ; RUN: lld-link /out:%t2a.exe /entry:main /subsystem:console /lldmap:%t2a.map %t.obj
 ; RUN: FileCheck --check-prefix=CHECK-O2 %s < %t2a.map
 
diff --git a/lld/test/ELF/lto/opt-level.ll b/lld/test/ELF/lto/opt-level.ll
--- a/lld/test/ELF/lto/opt-level.ll
+++ b/lld/test/ELF/lto/opt-level.ll
@@ -6,6 +6,10 @@
 ; RUN: llvm-nm %t0 | FileCheck --check-prefix=CHECK-O0 %s
 ; RUN: ld.lld -o %t2 -e main --lto-O2 %t.o
 ; RUN: llvm-nm %t2 | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: ld.lld -o %ts -e main --lto-Os %t.o
+; RUN: llvm-nm %ts | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: ld.lld -o %tz -e main --lto-Oz %t.o
+; RUN: llvm-nm %tz | FileCheck --check-prefix=CHECK-O2 %s
 ; RUN: ld.lld -o %t2a -e main %t.o
 ; RUN: llvm-nm %t2a | FileCheck --check-prefix=CHECK-O2 %s
 ; RUN: ld.lld -o %t2 -e main %t.o --plugin-opt O2
@@ -19,14 +23,14 @@
 ; RUN:   FileCheck --check-prefix=INVALID1 %s
 ; RUN: not ld.lld -o %t3 -e main --plugin-opt=Ofoo %t.o 2>&1 | \
 ; RUN:   FileCheck --check-prefix=INVALID2 %s
-; INVALID2: --plugin-opt=Ofoo: number expected, but got 'foo'
+; INVALID2: invalid optimization level for LTO: foo
 
 ; RUN: not ld.lld -o %t3 -e main --lto-O-1 %t.o 2>&1 | \
 ; RUN:   FileCheck --check-prefix=INVALIDNEGATIVE1 %s
-; INVALIDNEGATIVE1: invalid optimization level for LTO: 4294967295
+; INVALIDNEGATIVE1: invalid optimization level for LTO: -1
 ; RUN: not ld.lld -o %t3 -e main --plugin-opt=O-1 %t.o 2>&1 | \
 ; RUN:   FileCheck --check-prefix=INVALIDNEGATIVE2 %s
-; INVALIDNEGATIVE2: invalid optimization level for LTO: 4294967295
+; INVALIDNEGATIVE2: invalid optimization level for LTO: -1
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
diff --git a/lld/test/wasm/lto/opt-level.ll b/lld/test/wasm/lto/opt-level.ll
--- a/lld/test/wasm/lto/opt-level.ll
+++ b/lld/test/wasm/lto/opt-level.ll
@@ -3,6 +3,10 @@
 ; RUN: obj2yaml %t0 | FileCheck --check-prefix=CHECK-O0 %s
 ; RUN: wasm-ld -o %t2 -e main --lto-O2 %t.o
 ; RUN: obj2yaml %t2 | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: wasm-ld -o %ts -e main --lto-Os %t.o
+; RUN: obj2yaml %ts | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: wasm-ld -o %tz -e main --lto-Oz %t.o
+; RUN: obj2yaml %tz | FileCheck --check-prefix=CHECK-O2 %s
 ; RUN: wasm-ld -o %t2a -e main %t.o
 ; RUN: obj2yaml %t2a | FileCheck --check-prefix=CHECK-O2 %s
 
diff --git a/lld/wasm/Config.h b/lld/wasm/Config.h
--- a/lld/wasm/Config.h
+++ b/lld/wasm/Config.h
@@ -52,6 +52,7 @@
   uint32_t zStackSize;
   unsigned ltoPartitions;
   unsigned ltoo;
+  unsigned ltoos;
   unsigned optimize;
   unsigned thinLTOJobs;
 
diff --git a/lld/wasm/Driver.cpp b/lld/wasm/Driver.cpp
--- a/lld/wasm/Driver.cpp
+++ b/lld/wasm/Driver.cpp
@@ -321,7 +321,8 @@
   config->importMemory = args.hasArg(OPT_import_memory);
   config->sharedMemory = args.hasArg(OPT_shared_memory);
   config->importTable = args.hasArg(OPT_import_table);
-  config->ltoo = args::getInteger(args, OPT_lto_O, 2);
+  config->ltoo = check(llvm::lto::getOptLevel(args.getLastArg(OPT_lto_O), 2));
+  config->ltoos = check(llvm::lto::getSizeLevel(args.getLastArg(OPT_lto_O), 0));
   config->ltoPartitions = args::getInteger(args, OPT_lto_partitions, 1);
   config->optimize = args::getInteger(args, OPT_O, 0);
   config->outputFile = args.getLastArgValue(OPT_o);
@@ -394,8 +395,6 @@
     error("--compress-relocations is incompatible with output debug"
           " information. Please pass --strip-debug or --strip-all");
 
-  if (config->ltoo > 3)
-    error("invalid optimization level for LTO: " + Twine(config->ltoo));
   if (config->ltoPartitions == 0)
     error("--lto-partitions: number of threads must be > 0");
   if (config->thinLTOJobs == 0)
diff --git a/lld/wasm/LTO.cpp b/lld/wasm/LTO.cpp
--- a/lld/wasm/LTO.cpp
+++ b/lld/wasm/LTO.cpp
@@ -50,6 +50,7 @@
   c.DisableVerify = config->disableVerify;
   c.DiagHandler = diagnosticHandler;
   c.OptLevel = config->ltoo;
+  c.SizeLevel = config->ltoos;
   c.MAttrs = getMAttrs();
   c.CGOptLevel = args::getCGOptLevel(config->ltoo);
 
diff --git a/llvm/include/llvm/LTO/Config.h b/llvm/include/llvm/LTO/Config.h
--- a/llvm/include/llvm/LTO/Config.h
+++ b/llvm/include/llvm/LTO/Config.h
@@ -43,6 +43,7 @@
   CodeGenOpt::Level CGOptLevel = CodeGenOpt::Default;
   CodeGenFileType CGFileType = CGFT_ObjectFile;
   unsigned OptLevel = 2;
+  unsigned SizeLevel = 0;
   bool DisableVerify = false;
 
   /// Use the new pass manager
diff --git a/llvm/include/llvm/LTO/LTO.h b/llvm/include/llvm/LTO/LTO.h
--- a/llvm/include/llvm/LTO/LTO.h
+++ b/llvm/include/llvm/LTO/LTO.h
@@ -39,6 +39,9 @@
 class Module;
 class Target;
 class raw_pwrite_stream;
+namespace opt {
+class Arg;
+}
 
 /// Resolve linkage for prevailing symbols in the \p Index. Linkage changes
 /// recorded in the index and the ThinLTO backends must apply the changes to
@@ -450,6 +453,27 @@
   unsigned LinkerRedefined : 1;
 };
 
+class LTOOLevelError : public ErrorInfo<LTOOLevelError> {
+public:
+  static char ID;
+  StringRef Level;
+
+  LTOOLevelError(StringRef Level) : Level(Level) {}
+
+  void log(raw_ostream &OS) const override {
+    OS << "invalid optimization level for LTO: " << Level;
+  }
+
+  std::error_code convertToErrorCode() const override {
+    return llvm::inconvertibleErrorCode();
+  }
+};
+
+Expected<unsigned> getOptLevel(StringRef OArg);
+Expected<unsigned> getOptLevel(llvm::opt::Arg *OArg, unsigned Default);
+Expected<unsigned> getSizeLevel(StringRef OArg);
+Expected<unsigned> getSizeLevel(llvm::opt::Arg *OArg, unsigned Default);
+
 } // namespace lto
 } // namespace llvm
 
diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -29,6 +29,7 @@
 #include "llvm/LTO/SummaryBasedOptimizations.h"
 #include "llvm/Linker/IRMover.h"
 #include "llvm/Object/IRObjectFile.h"
+#include "llvm/Option/Arg.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/ManagedStatic.h"
@@ -136,6 +137,7 @@
   AddUnsigned(Conf.CGOptLevel);
   AddUnsigned(Conf.CGFileType);
   AddUnsigned(Conf.OptLevel);
+  AddUnsigned(Conf.SizeLevel);
   AddUnsigned(Conf.UseNewPM);
   AddUnsigned(Conf.Freestanding);
   AddString(Conf.OptPipeline);
@@ -1415,3 +1417,40 @@
   StatsFile->keep();
   return std::move(StatsFile);
 }
+
+char LTOOLevelError::ID;
+
+Expected<unsigned> lto::getOptLevel(StringRef S) {
+  if (S == "s" || S == "z" || S.empty())
+    return 2;
+
+  if (S == "g")
+    return 1;
+
+  int Res;
+  if (S.getAsInteger(10, Res) || Res < 0 || Res > 3)
+    return make_error<LTOOLevelError>(S);
+  return Res;
+}
+
+Expected<unsigned> lto::getOptLevel(opt::Arg *OArg, unsigned Default) {
+  if (!OArg)
+    return Default;
+  return lto::getOptLevel(OArg->getValue());
+}
+
+Expected<unsigned> lto::getSizeLevel(StringRef S) {
+  if (S[0] == 's')
+    return 1;
+  if (S[0] == 'z')
+    return 2;
+  if (S[0] >= '0' || S[0] <= '3' )
+    return 0;
+  return make_error<LTOOLevelError>(S);
+}
+
+Expected<unsigned> lto::getSizeLevel(opt::Arg *OArg, unsigned Default) {
+  if (!OArg)
+    return Default;
+  return lto::getSizeLevel(OArg->getValue());
+}
diff --git a/llvm/lib/LTO/LTOBackend.cpp b/llvm/lib/LTO/LTOBackend.cpp
--- a/llvm/lib/LTO/LTOBackend.cpp
+++ b/llvm/lib/LTO/LTOBackend.cpp
@@ -150,9 +150,39 @@
       CodeModel, Conf.CGOptLevel));
 }
 
+static PassBuilder::OptimizationLevel mapToLevel(const Config &Conf) {
+  switch (Conf.OptLevel) {
+  default:
+    llvm_unreachable("Invalid optimization level!");
+
+  case 0:
+    return PassBuilder::O0;
+
+  case 1:
+    return PassBuilder::O1;
+
+  case 2:
+    switch (Conf.SizeLevel) {
+    default:
+      llvm_unreachable("Invalid optimization level for size!");
+
+    case 0:
+      return PassBuilder::O2;
+
+    case 1:
+      return PassBuilder::Os;
+
+    case 2:
+      return PassBuilder::Oz;
+    }
+
+  case 3:
+    return PassBuilder::O3;
+  }
+}
+
 static void runNewPMPasses(Config &Conf, Module &Mod, TargetMachine *TM,
-                           unsigned OptLevel, bool IsThinLTO,
-                           ModuleSummaryIndex *ExportSummary,
+                           bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
                            const ModuleSummaryIndex *ImportSummary) {
   Optional<PGOOptions> PGOOpt;
   if (!Conf.SampleProfile.empty())
@@ -194,24 +224,7 @@
   ModulePassManager MPM(Conf.DebugPassManager);
   // FIXME (davide): verify the input.
 
-  PassBuilder::OptimizationLevel OL;
-
-  switch (OptLevel) {
-  default:
-    llvm_unreachable("Invalid optimization level");
-  case 0:
-    OL = PassBuilder::O0;
-    break;
-  case 1:
-    OL = PassBuilder::O1;
-    break;
-  case 2:
-    OL = PassBuilder::O2;
-    break;
-  case 3:
-    OL = PassBuilder::O3;
-    break;
-  }
+  PassBuilder::OptimizationLevel OL = mapToLevel(Conf);
 
   if (IsThinLTO)
     MPM = PB.buildThinLTODefaultPipeline(OL, Conf.DebugPassManager,
@@ -284,6 +297,7 @@
   PMB.LoopVectorize = true;
   PMB.SLPVectorize = true;
   PMB.OptLevel = Conf.OptLevel;
+  PMB.SizeLevel = Conf.SizeLevel;
   PMB.PGOSampleUse = Conf.SampleProfile;
   PMB.EnablePGOCSInstrGen = Conf.RunCSIRInstr;
   if (!Conf.RunCSIRInstr && !Conf.CSIRProfile.empty()) {
@@ -297,18 +311,17 @@
   passes.run(Mod);
 }
 
-bool opt(Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
+bool optimize(Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
          bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
          const ModuleSummaryIndex *ImportSummary) {
   // FIXME: Plumb the combined index into the new pass manager.
   if (!Conf.OptPipeline.empty())
     runNewPMCustomPasses(Mod, TM, Conf.OptPipeline, Conf.AAPipeline,
                          Conf.DisableVerify);
-  else if (Conf.UseNewPM)
-    runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
-                   ImportSummary);
-  else
-    runOldPMPasses(Conf, Mod, TM, IsThinLTO, ExportSummary, ImportSummary);
+  else {
+    auto runPasses = Conf.UseNewPM ? runNewPMPasses : runOldPMPasses;
+    runPasses(Conf, Mod, TM, IsThinLTO, ExportSummary, ImportSummary);
+  }
   return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
 }
 
@@ -443,7 +456,7 @@
   auto DiagnosticOutputFile = std::move(*DiagFileOrErr);
 
   if (!C.CodeGenOnly) {
-    if (!opt(C, TM.get(), 0, *Mod, /*IsThinLTO=*/false,
+    if (!optimize(C, TM.get(), 0, *Mod, /*IsThinLTO=*/false,
              /*ExportSummary=*/&CombinedIndex, /*ImportSummary=*/nullptr))
       return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
   }
@@ -538,7 +551,7 @@
   if (Conf.PostImportModuleHook && !Conf.PostImportModuleHook(Task, Mod))
     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 
-  if (!opt(Conf, TM.get(), Task, Mod, /*IsThinLTO=*/true,
+  if (!optimize(Conf, TM.get(), Task, Mod, /*IsThinLTO=*/true,
            /*ExportSummary=*/nullptr, /*ImportSummary=*/&CombinedIndex))
     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 
diff --git a/llvm/test/tools/gold/X86/opt-level.ll b/llvm/test/tools/gold/X86/opt-level.ll
--- a/llvm/test/tools/gold/X86/opt-level.ll
+++ b/llvm/test/tools/gold/X86/opt-level.ll
@@ -11,6 +11,14 @@
 ; RUN:    -m elf_x86_64 \
 ; RUN:    -plugin-opt=O2 -r -o %t.o %t.bc
 ; RUN: llvm-dis < %t.o.0.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext -plugin-opt=save-temps \
+; RUN:    -m elf_x86_64 \
+; RUN:    -plugin-opt=Os -r -o %t.o %t.bc
+; RUN: llvm-dis < %t.o.0.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext -plugin-opt=save-temps \
+; RUN:    -m elf_x86_64 \
+; RUN:    -plugin-opt=Oz -r -o %t.o %t.bc
+; RUN: llvm-dis < %t.o.0.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
 
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext -plugin-opt=save-temps \
 ; RUN:    -m elf_x86_64 --plugin-opt=new-pass-manager \
@@ -24,6 +32,14 @@
 ; RUN:    -m elf_x86_64 --plugin-opt=new-pass-manager \
 ; RUN:    -plugin-opt=O2 -r -o %t.o %t.bc
 ; RUN: llvm-dis < %t.o.0.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext -plugin-opt=save-temps \
+; RUN:    -m elf_x86_64 --plugin-opt=new-pass-manager \
+; RUN:    -plugin-opt=Os -r -o %t.o %t.bc
+; RUN: llvm-dis < %t.o.0.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext -plugin-opt=save-temps \
+; RUN:    -m elf_x86_64 --plugin-opt=new-pass-manager \
+; RUN:    -plugin-opt=Oz -r -o %t.o %t.bc
+; RUN: llvm-dis < %t.o.0.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
 
 ; CHECK-O0: define internal void @foo(
 ; CHECK-O1: define internal void @foo(
diff --git a/llvm/test/tools/lto/opt-level.ll b/llvm/test/tools/lto/opt-level.ll
--- a/llvm/test/tools/lto/opt-level.ll
+++ b/llvm/test/tools/lto/opt-level.ll
@@ -3,6 +3,10 @@
 ; RUN: llvm-nm --no-llvm-bc %t.dylib | FileCheck --check-prefix=CHECK-O0 %s
 ; RUN: %ld64 -lto_library %llvmshlibdir/libLTO.dylib -arch x86_64 -dylib -mllvm -O2 -o %t.dylib %t.o
 ; RUN: llvm-nm --no-llvm-bc %t.dylib | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: %ld64 -lto_library %llvmshlibdir/libLTO.dylib -arch x86_64 -dylib -mllvm -Os -o %t.dylib %t.o
+; RUN: llvm-nm --no-llvm-bc %t.dylib | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: %ld64 -lto_library %llvmshlibdir/libLTO.dylib -arch x86_64 -dylib -mllvm -Oz -o %t.dylib %t.o
+; RUN: llvm-nm --no-llvm-bc %t.dylib | FileCheck --check-prefix=CHECK-O2 %s
 
 target triple = "x86_64-apple-macosx10.8.0"
 
diff --git a/llvm/tools/gold/gold-plugin.cpp b/llvm/tools/gold/gold-plugin.cpp
--- a/llvm/tools/gold/gold-plugin.cpp
+++ b/llvm/tools/gold/gold-plugin.cpp
@@ -132,6 +132,7 @@
   };
   static OutputType TheOutputType = OT_NORMAL;
   static unsigned OptLevel = 2;
+  static unsigned SizeLevel = 0;
   // Default parallelism of 0 used to indicate that user did not specify.
   // Actual parallelism default value depends on implementation.
   // Currently only affects ThinLTO, where the default is
@@ -261,9 +262,15 @@
     } else if (opt.startswith("cache-policy=")) {
       cache_policy = opt.substr(strlen("cache-policy="));
     } else if (opt.size() == 2 && opt[0] == 'O') {
-      if (opt[1] < '0' || opt[1] > '3')
-        message(LDPL_FATAL, "Optimization level must be between 0 and 3");
-      OptLevel = opt[1] - '0';
+      if (auto LevelOrErr = llvm::lto::getOptLevel(opt.substr(1, 1)))
+        OptLevel = *LevelOrErr;
+      else
+        message(LDPL_FATAL, toString(LevelOrErr.takeError()).c_str());
+
+      if (auto LevelOrErr = llvm::lto::getSizeLevel(opt.substr(1, 1)))
+        SizeLevel = *LevelOrErr;
+      else
+        message(LDPL_FATAL, toString(LevelOrErr.takeError()).c_str());
     } else if (opt.startswith("jobs=")) {
       if (StringRef(opt_ + 5).getAsInteger(10, Parallelism))
         message(LDPL_FATAL, "Invalid parallelism level: %s", opt_ + 5);
@@ -860,6 +867,7 @@
   Conf.CGOptLevel = getCGOptLevel();
   Conf.DisableVerify = options::DisableVerify;
   Conf.OptLevel = options::OptLevel;
+  Conf.SizeLevel = options::SizeLevel;
   if (options::Parallelism)
     Backend = createInProcessThinBackend(options::Parallelism);
   if (options::thinlto_index_only) {
diff --git a/llvm/tools/llvm-lto2/llvm-lto2.cpp b/llvm/tools/llvm-lto2/llvm-lto2.cpp
--- a/llvm/tools/llvm-lto2/llvm-lto2.cpp
+++ b/llvm/tools/llvm-lto2/llvm-lto2.cpp
@@ -29,10 +29,11 @@
 using namespace llvm;
 using namespace lto;
 
-static cl::opt<char>
-    OptLevel("O", cl::desc("Optimization level. [-O0, -O1, -O2, or -O3] "
-                           "(default = '-O2')"),
-             cl::Prefix, cl::ZeroOrMore, cl::init('2'));
+static cl::opt<std::string>
+    OptLevel("O",
+             cl::desc("Optimization level. [-O0, -O1, -O2, -Os, -Oz, or "
+                      "-O3] (default = '-O2')"),
+             cl::Prefix, cl::ZeroOrMore, cl::init("2"));
 
 static cl::opt<char> CGOptLevel(
     "cg-opt-level",
@@ -244,7 +245,10 @@
   Conf.OptPipeline = OptPipeline;
   Conf.AAPipeline = AAPipeline;
 
-  Conf.OptLevel = OptLevel - '0';
+  Conf.OptLevel =
+      check(llvm::lto::getOptLevel(OptLevel), "invalid optimization level");
+  Conf.SizeLevel =
+      check(llvm::lto::getSizeLevel(OptLevel), "invalid optimization level");
   Conf.UseNewPM = UseNewPM;
   switch (CGOptLevel) {
   case '0':