diff --git a/clang/lib/Basic/Targets/X86.h b/clang/lib/Basic/Targets/X86.h
--- a/clang/lib/Basic/Targets/X86.h
+++ b/clang/lib/Basic/Targets/X86.h
@@ -306,6 +306,9 @@
   }
 
   bool isValidTuneCPUName(StringRef Name) const override {
+    if (Name == "generic")
+      return true;
+
     // Allow 32-bit only CPUs regardless of 64-bit mode unlike isValidCPUName.
     // NOTE: gcc rejects 32-bit mtune CPUs in 64-bit mode. But being lenient
     // since mtune was ignored by clang for so long.
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -2074,20 +2074,27 @@
   }
 
   // Handle -mtune.
-  // FIXME: We should default to "generic" unless -march is set to match gcc.
+
+  // Default to "generic" unless -march is present.
+  std::string TuneCPU;
+  if (!Args.hasArg(clang::driver::options::OPT_march_EQ))
+    TuneCPU = "generic";
+
+  // Override based on -mtune.
   if (const Arg *A = Args.getLastArg(clang::driver::options::OPT_mtune_EQ)) {
     StringRef Name = A->getValue();
 
-    if (Name == "native")
+    if (Name == "native") {
       Name = llvm::sys::getHostCPUName();
+      if (!Name.empty())
+        TuneCPU = std::string(Name);
+    } else
+      TuneCPU = std::string(Name);
+  }
 
-    // Ignore generic either from getHostCPUName or from command line.
-    // FIXME: We need to support this eventually but isValidCPUName and the
-    // backend aren't ready for it yet.
-    if (Name != "generic") {
-      CmdArgs.push_back("-tune-cpu");
-      CmdArgs.push_back(Args.MakeArgString(Name));
-    }
+  if (!TuneCPU.empty()) {
+    CmdArgs.push_back("-tune-cpu");
+    CmdArgs.push_back(Args.MakeArgString(TuneCPU));
   }
 }
 
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -3877,7 +3877,7 @@
 
   // Extend the signature with the target options.
   code = hash_combine(code, TargetOpts->Triple, TargetOpts->CPU,
-                      TargetOpts->ABI);
+                      TargetOpts->TuneCPU, TargetOpts->ABI);
   for (const auto &FeatureAsWritten : TargetOpts->FeaturesAsWritten)
     code = hash_combine(code, FeatureAsWritten);
 
diff --git a/clang/lib/Frontend/FrontendActions.cpp b/clang/lib/Frontend/FrontendActions.cpp
--- a/clang/lib/Frontend/FrontendActions.cpp
+++ b/clang/lib/Frontend/FrontendActions.cpp
@@ -561,6 +561,7 @@
       Out.indent(2) << "Target options:\n";
       Out.indent(4) << "  Triple: " << TargetOpts.Triple << "\n";
       Out.indent(4) << "  CPU: " << TargetOpts.CPU << "\n";
+      Out.indent(4) << "  TuneCPU: " << TargetOpts.TuneCPU << "\n";
       Out.indent(4) << "  ABI: " << TargetOpts.ABI << "\n";
 
       if (!TargetOpts.FeaturesAsWritten.empty()) {
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -390,8 +390,10 @@
   // We can tolerate different CPUs in many cases, notably when one CPU
   // supports a strict superset of another. When allowing compatible
   // differences skip this check.
-  if (!AllowCompatibleDifferences)
+  if (!AllowCompatibleDifferences) {
     CHECK_TARGET_OPT(CPU, "target CPU");
+    CHECK_TARGET_OPT(TuneCPU, "tune CPU");
+  }
 
 #undef CHECK_TARGET_OPT
 
@@ -5779,6 +5781,7 @@
   TargetOptions TargetOpts;
   TargetOpts.Triple = ReadString(Record, Idx);
   TargetOpts.CPU = ReadString(Record, Idx);
+  TargetOpts.TuneCPU = ReadString(Record, Idx);
   TargetOpts.ABI = ReadString(Record, Idx);
   for (unsigned N = Record[Idx++]; N; --N) {
     TargetOpts.FeaturesAsWritten.push_back(ReadString(Record, Idx));
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -1272,6 +1272,7 @@
   const TargetOptions &TargetOpts = Target.getTargetOpts();
   AddString(TargetOpts.Triple, Record);
   AddString(TargetOpts.CPU, Record);
+  AddString(TargetOpts.TuneCPU, Record);
   AddString(TargetOpts.ABI, Record);
   Record.push_back(TargetOpts.FeaturesAsWritten.size());
   for (unsigned I = 0, N = TargetOpts.FeaturesAsWritten.size(); I != N; ++I) {
diff --git a/clang/test/Driver/x86-mtune.c b/clang/test/Driver/x86-mtune.c
--- a/clang/test/Driver/x86-mtune.c
+++ b/clang/test/Driver/x86-mtune.c
@@ -1,5 +1,14 @@
 // Ensure we support the -mtune flag.
-//
+
+// Default mtune should be generic.
+// RUN: %clang -target x86_64-unknown-unknown -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=notune
+// notune: "-tune-cpu" "generic"
+
+// RUN: %clang -target x86_64-unknown-unknown -c -### %s -mtune=generic 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=generic
+// generic: "-tune-cpu" "generic"
+
 // RUN: %clang -target x86_64-unknown-unknown -c -### %s -mtune=nocona 2>&1 \
 // RUN:   | FileCheck %s -check-prefix=nocona
 // nocona: "-tune-cpu" "nocona"
@@ -18,3 +27,16 @@
 // RUN:   | FileCheck %s -check-prefix=athlon
 // athlon: "-tune-cpu" "athlon"
 
+// Check interaction between march and mtune.
+
+// -march should remove default mtune generic.
+// RUN: %clang -target x86_64-unknown-unknown -c -### %s -march=core2 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=marchcore2
+// marchcore2: "-target-cpu" "core2"
+// marchcore2-NOT: "-tune-cpu"
+
+// -march should remove default mtune generic.
+// RUN: %clang -target x86_64-unknown-unknown -c -### %s -march=core2 -mtune=nehalem 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=marchmtune
+// marchmtune: "-target-cpu" "core2"
+// mmarchmtune: "-tune-cpu" "nehalem"
diff --git a/clang/test/Modules/module_file_info.m b/clang/test/Modules/module_file_info.m
--- a/clang/test/Modules/module_file_info.m
+++ b/clang/test/Modules/module_file_info.m
@@ -28,6 +28,7 @@
 // CHECK: Target options:
 // CHECK:     Triple:
 // CHECK:     CPU:
+// CHECK:     TuneCPU:
 // CHECK:     ABI:
 
 // CHECK: Header search options: