Index: clang/lib/Driver/ToolChains/AVR.cpp
===================================================================
--- clang/lib/Driver/ToolChains/AVR.cpp
+++ clang/lib/Driver/ToolChains/AVR.cpp
@@ -369,16 +369,18 @@
     : Generic_ELF(D, Triple, Args), LinkStdlib(false) {
   GCCInstallation.init(Triple, Args);
 
+  std::string CPU = getCPUName(D, Args, Triple);
+  if (CPU.empty())
+    D.Diag(diag::warn_drv_avr_mcu_not_specified);
+
   // Only add default libraries if the user hasn't explicitly opted out.
   if (!Args.hasArg(options::OPT_nostdlib) &&
       !Args.hasArg(options::OPT_nodefaultlibs) &&
+      !Args.hasArg(options::OPT_S) &&
       !Args.hasArg(options::OPT_c /* does not apply when not linking */)) {
-    std::string CPU = getCPUName(D, Args, Triple);
 
-    if (CPU.empty()) {
-      // We cannot link any standard libraries without an MCU specified.
-      D.Diag(diag::warn_drv_avr_mcu_not_specified);
-    } else {
+    // We cannot link any standard libraries without an MCU specified.
+    if (!CPU.empty()) {
       Optional<StringRef> FamilyName = GetMCUFamilyName(CPU);
       Optional<std::string> AVRLibcRoot = findAVRLibcInstallation();
 
Index: clang/test/Driver/avr-toolchain.c
===================================================================
--- clang/test/Driver/avr-toolchain.c
+++ clang/test/Driver/avr-toolchain.c
@@ -35,3 +35,30 @@
 // RUN: %clang %s -### -target avr --sysroot %S/Inputs/basic_avr_tree 2>&1 -nostdinc | FileCheck --check-prefix=NOSTDINC %s
 // RUN: %clang %s -### -target avr --sysroot %S/Inputs/basic_avr_tree 2>&1 -nostdlibinc | FileCheck --check-prefix=NOSTDINC %s
 // NOSTDINC-NOT: "-internal-isystem" {{".*avr/include"}}
+
+// RUN: %clang %s -### -target avr --sysroot %S/Inputs/basic_avr_tree -mmcu=atmega328 2>&1 | FileCheck --check-prefix=CHECK5 %s
+// CHECK5-NOT: warning: no target microcontroller specified on command line, cannot link standard libraries
+// CHECK5-NOT: warning: no avr-gcc installation can be found on the system, cannot link standard libraries
+// CHECK5-NOT: warning: standard library not linked and so no interrupt vector table or compiler runtime routines will be linked
+
+// RUN: %clang %s -### -target avr --sysroot %S/Inputs/ -mmcu=atmega328 2>&1 | FileCheck --check-prefix=CHECK6 %s
+// CHECK6-NOT: warning: no target microcontroller specified on command line, cannot link standard libraries
+// CHECK6: warning: no avr-gcc installation can be found on the system, cannot link standard libraries
+// CHECK6: warning: standard library not linked and so no interrupt vector table or compiler runtime routines will be linked
+
+// RUN: %clang %s -### -target avr --sysroot %S/Inputs/basic_avr_tree 2>&1 | FileCheck --check-prefix=CHECK7 %s
+// CHECK7: warning: no target microcontroller specified on command line, cannot link standard libraries
+// CHECK7-NOT: warning: no avr-gcc installation can be found on the system, cannot link standard libraries
+// CHECK7: warning: standard library not linked and so no interrupt vector table or compiler runtime routines will be linked
+
+// RUN: %clang %s -### -target avr --sysroot %S/Inputs/basic_avr_tree -c 2>&1 | FileCheck --check-prefix=CHECK8 %s
+// RUN: %clang %s -### -target avr --sysroot %S/Inputs/basic_avr_tree -S 2>&1 | FileCheck --check-prefix=CHECK8 %s
+// CHECK8: warning: no target microcontroller specified on command line, cannot link standard libraries
+// CHECK8-NOT: warning: no avr-gcc installation can be found on the system, cannot link standard libraries
+// CHECK8-NOT: warning: standard library not linked and so no interrupt vector table or compiler runtime routines will be linked
+
+// RUN: %clang %s -### -target avr --sysroot %S/Inputs/basic_avr_tree -mmcu=atmega328 -c 2>&1 | FileCheck --check-prefix=CHECK9 %s
+// RUN: %clang %s -### -target avr --sysroot %S/Inputs/basic_avr_tree -mmcu=atmega328 -S 2>&1 | FileCheck --check-prefix=CHECK9 %s
+// CHECK9-NOT: warning: no target microcontroller specified on command line, cannot link standard libraries
+// CHECK9-NOT: warning: no avr-gcc installation can be found on the system, cannot link standard libraries
+// CHECK9-NOT: warning: standard library not linked and so no interrupt vector table or compiler runtime routines will be linked