Index: lib/Driver/ToolChains.cpp
===================================================================
--- lib/Driver/ToolChains.cpp
+++ lib/Driver/ToolChains.cpp
@@ -126,7 +126,7 @@
 }
 
 static const char *GetArmArchForMCpu(StringRef Value) {
-  return llvm::StringSwitch<const char *>(Value)
+  return llvm::StringSwitch<const char *>(Value.lower())
     .Cases("arm9e", "arm946e-s", "arm966e-s", "arm968e-s", "arm926ej-s","armv5")
     .Cases("arm10e", "arm10tdmi", "armv5")
     .Cases("arm1020t", "arm1020e", "arm1022e", "arm1026ej-s", "armv5")
Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -902,7 +902,7 @@
   if ((A = Args.getLastArg(options::OPT_mtune_EQ))) {
     CPU = A->getValue();
   } else if ((A = Args.getLastArg(options::OPT_mcpu_EQ))) {
-    StringRef Mcpu = A->getValue();
+    StringRef Mcpu = StringRef(A->getValue()).lower();
     CPU = Mcpu.split("+").first;
   }
 
@@ -1795,6 +1795,7 @@
 // decode CPU and feature.
 static bool DecodeAArch64Mcpu(const Driver &D, StringRef Mcpu, StringRef &CPU,
                               std::vector<const char *> &Features) {
+  Mcpu = Mcpu.lower();
   std::pair<StringRef, StringRef> Split = Mcpu.split("+");
   CPU = Split.first;
   if (CPU == "cyclone" || CPU == "cortex-a53" || CPU == "cortex-a57" || CPU == "cortex-a72") {
@@ -5641,7 +5642,7 @@
   // FIXME: Warn on inconsistent use of -mcpu and -march.
   // If we have -mcpu=, use that.
   if (Arg *A = Args.getLastArg(options::OPT_mcpu_EQ)) {
-    StringRef MCPU = A->getValue();
+    StringRef MCPU = StringRef(A->getValue()).lower();
     // Handle -mcpu=native.
     if (MCPU == "native")
       return llvm::sys::getHostCPUName();
@@ -7509,7 +7510,7 @@
     // march from being picked in the absence of a cpu flag.
     Arg *A;
     if ((A = Args.getLastArg(options::OPT_mcpu_EQ)) &&
-      StringRef(A->getValue()) == "krait")
+      StringRef(A->getValue()).lower() == "krait")
         CmdArgs.push_back("-march=armv7-a");
     else
       Args.AddLastArg(CmdArgs, options::OPT_mcpu_EQ);
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"
Index: test/Driver/arm-cortex-cpus.c
===================================================================
--- test/Driver/arm-cortex-cpus.c
+++ test/Driver/arm-cortex-cpus.c
@@ -351,3 +351,13 @@
 // RUN: %clang -target arm -mcpu=cortex-a57 -mbig-endian -mthumb -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-BE-CPUV8A-THUMB %s
 // RUN: %clang -target arm -mcpu=cortex-a72 -mbig-endian -mthumb -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-BE-CPUV8A-THUMB %s
 // CHECK-BE-CPUV8A-THUMB: "-cc1"{{.*}} "-triple" "thumbebv8-{{.*}}
+
+// ================== Check whether -mcpu accepts mixed-case values.
+// RUN: %clang -target arm-linux-gnueabi -mcpu=Cortex-a5 -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-CASE-INSENSITIVE-CPUV7A %s
+// RUN: %clang -target arm-linux-gnueabi -mcpu=cortex-A7 -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-CASE-INSENSITIVE-CPUV7A %s
+// RUN: %clang -target arm-linux-gnueabi -mcpu=CORTEX-a8 -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-CASE-INSENSITIVE-CPUV7A %s
+// RUN: %clang -target arm-linux-gnueabi -mcpu=Cortex-A9 -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-CASE-INSENSITIVE-CPUV7A %s
+// RUN: %clang -target arm-linux-gnueabi -mcpu=corteX-A12 -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-CASE-INSENSITIVE-CPUV7A %s
+// RUN: %clang -target arm-linux-gnueabi -mcpu=CorteX-a15 -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-CASE-INSENSITIVE-CPUV7A %s
+// RUN: %clang -target arm-linux-gnueabi -mcpu=CorteX-A17 -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-CASE-INSENSITIVE-CPUV7A %s
+// CHECK-CASE-INSENSITIVE-CPUV7A: "-cc1"{{.*}} "-triple" "armv7-{{.*}}