Index: clang/lib/Driver/ToolChains/CommonArgs.cpp
===================================================================
--- clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -382,6 +382,10 @@
   }
 
   case llvm::Triple::avr:
+    // We encourage users to use '-mcpu' as their first choice,
+    // rather than the old '-mmcu' (inherited from avr-gcc).
+    if (const Arg *A = Args.getLastArg(options::OPT_mcpu_EQ))
+      return A->getValue();
     if (const Arg *A = Args.getLastArg(options::OPT_mmcu_EQ))
       return A->getValue();
     return "";
Index: clang/test/Driver/avr-mmcu.c
===================================================================
--- clang/test/Driver/avr-mmcu.c
+++ clang/test/Driver/avr-mmcu.c
@@ -1,13 +1,16 @@
 // A test for the propagation of the -mmcu option to -cc1 and -cc1as
 
+// RUN: %clang -### --target=avr -mcpu=attiny11 -save-temps %s 2>&1 | FileCheck -check-prefix=CHECK0 %s
 // RUN: %clang -### --target=avr -mmcu=attiny11 -save-temps %s 2>&1 | FileCheck -check-prefix=CHECK0 %s
 // CHECK0: "-cc1" {{.*}} "-target-cpu" "attiny11"
 // CHECK0: "-cc1as" {{.*}} "-target-cpu" "attiny11"
 
+// RUN: %clang -### --target=avr -mcpu=at90s2313 -save-temps %s 2>&1 | FileCheck -check-prefix=CHECK1 %s
 // RUN: %clang -### --target=avr -mmcu=at90s2313 -save-temps %s 2>&1 | FileCheck -check-prefix=CHECK1 %s
 // CHECK1: "-cc1" {{.*}} "-target-cpu" "at90s2313"
 // CHECK1: "-cc1as" {{.*}} "-target-cpu" "at90s2313"
 
+// RUN: %clang -### --target=avr -mcpu=at90s8515 -save-temps %s 2>&1 | FileCheck -check-prefix=CHECK2 %s
 // RUN: %clang -### --target=avr -mmcu=at90s8515 -save-temps %s 2>&1 | FileCheck -check-prefix=CHECK2 %s
 // CHECK2: "-cc1" {{.*}} "-target-cpu" "at90s8515"
 // CHECK2: "-cc1as" {{.*}} "-target-cpu" "at90s8515"
@@ -72,10 +75,15 @@
 // CHECKh: "-cc1" {{.*}} "-target-cpu" "atxmega64a1u"
 // CHECKh: "-cc1as" {{.*}} "-target-cpu" "atxmega64a1u"
 
+// RUN: %clang -### --target=avr -mcpu=atxmega128a3u -save-temps %s 2>&1 | FileCheck -check-prefix=CHECKj %s
 // RUN: %clang -### --target=avr -mmcu=atxmega128a3u -save-temps %s 2>&1 | FileCheck -check-prefix=CHECKj %s
 // CHECKj: "-cc1" {{.*}} "-target-cpu" "atxmega128a3u"
 // CHECKj: "-cc1as" {{.*}} "-target-cpu" "atxmega128a3u"
 
+// RUN: %clang -### --target=avr -mcpu=atxmega128a4u -save-temps %s 2>&1 | FileCheck -check-prefix=CHECKi %s
 // RUN: %clang -### --target=avr -mmcu=atxmega128a4u -save-temps %s 2>&1 | FileCheck -check-prefix=CHECKi %s
 // CHECKi: "-cc1" {{.*}} "-target-cpu" "atxmega128a4u"
 // CHECKi: "-cc1as" {{.*}} "-target-cpu" "atxmega128a4u"
+
+// RUN: %clang --target=avr -mcpu=atmega328 -mmcu=atmega168 -c %s 2>&1 | FileCheck -check-prefix=CHECKz %s
+// CHECKz: warning: argument unused during compilation: '-mmcu=atmega168'