Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -847,7 +847,7 @@
   std::string CPU;
   // If we have -mtune or -mcpu, use that.
   if ((A = Args.getLastArg(options::OPT_mtune_EQ))) {
-    CPU = A->getValue();
+    CPU = StringRef(A->getValue()).lower();
   } else if ((A = Args.getLastArg(options::OPT_mcpu_EQ))) {
     StringRef Mcpu = A->getValue();
     CPU = Mcpu.split("+").first.lower();
@@ -1884,10 +1884,11 @@
 getAArch64MicroArchFeaturesFromMtune(const Driver &D, StringRef Mtune,
                                      const ArgList &Args,
                                      std::vector<const char *> &Features) {
+  std::string MtuneLowerCase = Mtune.lower();
   // Handle CPU name is 'native'.
-  if (Mtune == "native")
-    Mtune = llvm::sys::getHostCPUName();
-  if (Mtune == "cyclone") {
+  if (MtuneLowerCase == "native")
+    MtuneLowerCase = llvm::sys::getHostCPUName();
+  if (MtuneLowerCase == "cyclone") {
     Features.push_back("+zcm");
     Features.push_back("+zcz");
   }
Index: test/Driver/aarch64-cpus.c
===================================================================
--- test/Driver/aarch64-cpus.c
+++ test/Driver/aarch64-cpus.c
@@ -118,15 +118,19 @@
 // RUN: %clang -target aarch64_be -mbig-endian -march=ARMV8.1-a -### -c %s 2>&1 | FileCheck -check-prefix=GENERICV81A-BE %s
 // GENERICV81A-BE: "-cc1"{{.*}} "-triple" "aarch64_be{{.*}}" "-target-cpu" "generic" "-target-feature" "+neon" "-target-feature" "+v8.1a"
 
-// ================== Check whether -mcpu accepts mixed-case values.
+// ================== Check whether -mcpu and -mtune accept mixed-case values.
 // RUN: %clang -target aarch64 -mcpu=Cortex-a53 -### -c %s 2>&1 | FileCheck -check-prefix=CASE-INSENSITIVE-CA53 %s
+// RUN: %clang -target aarch64 -mtune=Cortex-a53 -### -c %s 2>&1 | FileCheck -check-prefix=CASE-INSENSITIVE-CA53 %s
 // CASE-INSENSITIVE-CA53: "-cc1"{{.*}} "-triple" "aarch64{{.*}}" "-target-cpu" "cortex-a53"
 
 // RUN: %clang -target arm64 -mcpu=cortex-A53 -### -c %s 2>&1 | FileCheck -check-prefix=CASE-INSENSITIVE-ARM64-CA53 %s
+// RUN: %clang -target arm64 -mtune=cortex-A53 -### -c %s 2>&1 | FileCheck -check-prefix=CASE-INSENSITIVE-ARM64-CA53 %s
 // CASE-INSENSITIVE-ARM64-CA53: "-cc1"{{.*}} "-triple" "arm64{{.*}}" "-target-cpu" "cortex-a53"
 
 // RUN: %clang -target aarch64 -mcpu=CORTEX-A57 -### -c %s 2>&1 | FileCheck -check-prefix=CASE-INSENSITIVE-CA57 %s
+// RUN: %clang -target aarch64 -mtune=CORTEX-A57 -### -c %s 2>&1 | FileCheck -check-prefix=CASE-INSENSITIVE-CA57 %s
 // CASE-INSENSITIVE-CA57: "-cc1"{{.*}} "-triple" "aarch64{{.*}}" "-target-cpu" "cortex-a57"
 
 // RUN: %clang -target arm64 -mcpu=Cortex-A57 -### -c %s 2>&1 | FileCheck -check-prefix=CASE-INSENSITIVE-ARM64-CA57 %s
+// RUN: %clang -target arm64 -mtune=Cortex-A57 -### -c %s 2>&1 | FileCheck -check-prefix=CASE-INSENSITIVE-ARM64-CA57 %s
 // CASE-INSENSITIVE-ARM64-CA57: "-cc1"{{.*}} "-triple" "arm64{{.*}}" "-target-cpu" "cortex-a57"
Index: test/Preprocessor/aarch64-target-features.c
===================================================================
--- test/Preprocessor/aarch64-target-features.c
+++ test/Preprocessor/aarch64-target-features.c
@@ -64,6 +64,8 @@
 // CHECK-GENERIC: "-cc1"{{.*}} "-triple" "aarch64{{.*}}" "-target-feature" "+neon"
 
 // RUN: %clang -target aarch64 -mtune=cyclone -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-MTUNE-CYCLONE %s
+// ================== Check whether -mtune accepts mixed-case features.
+// RUN: %clang -target aarch64 -mtune=CYCLONE -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-MTUNE-CYCLONE %s
 // CHECK-MTUNE-CYCLONE: "-cc1"{{.*}} "-triple" "aarch64{{.*}}" "-target-feature" "+neon" "-target-feature" "+zcm" "-target-feature" "+zcz"
 
 // RUN: %clang -target aarch64 -mcpu=cyclone -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-MCPU-CYCLONE %s
@@ -108,6 +110,9 @@
 
 // RUN: %clang -target aarch64 -mcpu=cortex-a53 -mtune=cyclone -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-MCPU-MTUNE %s
 // RUN: %clang -target aarch64 -mtune=cyclone -mcpu=cortex-a53  -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-MCPU-MTUNE %s
+// ================== Check whether -mtune accepts mixed-case features.
+// RUN: %clang -target aarch64 -mcpu=cortex-a53 -mtune=CYCLONE -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-MCPU-MTUNE %s
+// RUN: %clang -target aarch64 -mtune=CyclonE -mcpu=cortex-a53  -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-MCPU-MTUNE %s
 // CHECK-MCPU-MTUNE: "-cc1"{{.*}} "-triple" "aarch64{{.*}}" "-target-feature" "+neon" "-target-feature" "+crc" "-target-feature" "+crypto" "-target-feature" "+zcm" "-target-feature" "+zcz"
 
 // RUN: %clang -target aarch64 -mcpu=generic+neon -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-ERROR-NEON %s