Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -850,7 +850,7 @@
     CPU = A->getValue();
   } else if ((A = Args.getLastArg(options::OPT_mcpu_EQ))) {
     StringRef Mcpu = A->getValue();
-    CPU = Mcpu.split("+").first;
+    CPU = Mcpu.split("+").first.lower();
   }
 
   // Handle CPU name is 'native'.
@@ -1893,7 +1893,8 @@
                                const ArgList &Args,
                                std::vector<const char *> &Features) {
   StringRef CPU;
-  if (!DecodeAArch64Mcpu(D, Mcpu, CPU, Features))
+  std::string McpuLowerCase = Mcpu.lower();
+  if (!DecodeAArch64Mcpu(D, McpuLowerCase, CPU, Features))
     return false;
 
   return true;
@@ -1919,7 +1920,8 @@
                                     std::vector<const char *> &Features) {
   StringRef CPU;
   std::vector<const char *> DecodedFeature;
-  if (!DecodeAArch64Mcpu(D, Mcpu, CPU, DecodedFeature))
+  std::string McpuLowerCase = Mcpu.lower();
+  if (!DecodeAArch64Mcpu(D, McpuLowerCase, CPU, DecodedFeature))
     return false;
 
   return getAArch64MicroArchFeaturesFromMtune(D, CPU, Args, Features);
Index: test/Driver/aarch64-cpus.c
===================================================================
--- test/Driver/aarch64-cpus.c
+++ test/Driver/aarch64-cpus.c
@@ -111,3 +111,15 @@
 // 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.
+// RUN: %clang -target aarch64 -mcpu=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
+// 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
+// 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
+// CASE-INSENSITIVE-ARM64-CA57: "-cc1"{{.*}} "-triple" "arm64{{.*}}" "-target-cpu" "cortex-a57"