Index: clang/lib/Driver/ToolChains/AVR.cpp
===================================================================
--- clang/lib/Driver/ToolChains/AVR.cpp
+++ clang/lib/Driver/ToolChains/AVR.cpp
@@ -453,9 +453,19 @@
 }
 
 llvm::Optional<std::string> AVRToolChain::findAVRLibcInstallation() const {
+  // Search avr-libc installation according to avr-gcc installation.
+  std::string GCCParent(GCCInstallation.getParentLibPath());
+  std::string Path(GCCParent + "/avr/");
+  if (llvm::sys::fs::is_directory(Path))
+    return Optional<std::string>(Path);
+  Path = GCCParent + "/../avr/";
+  if (llvm::sys::fs::is_directory(Path))
+    return Optional<std::string>(Path);
+
+  // Search avr-libc installation from possible locations, and return the first
+  // one that exists, if there is no avr-gcc installed.
   for (StringRef PossiblePath : PossibleAVRLibcLocations) {
     std::string Path = getDriver().SysRoot + PossiblePath.str();
-    // Return the first avr-libc installation that exists.
     if (llvm::sys::fs::is_directory(Path))
       return Optional<std::string>(Path);
   }
Index: clang/test/Driver/avr-toolchain.c
===================================================================
--- clang/test/Driver/avr-toolchain.c
+++ clang/test/Driver/avr-toolchain.c
@@ -4,7 +4,7 @@
 // CC1: clang{{.*}} "-cc1" "-triple" "avr" {{.*}} "-fno-use-init-array"
 
 // RUN: %clang %s -### -no-canonical-prefixes -target avr --sysroot %S/Inputs/basic_avr_tree 2>&1 | FileCheck -check-prefix CC1A %s
-// CC1A: clang{{.*}} "-cc1" "-triple" "avr" {{.*}} "-internal-isystem" {{".*avr/include"}}
+// CC1A: clang{{.*}} "-cc1" "-triple" "avr" {{.*}} "-internal-isystem" {{".*../../../avr/include"}}
 
 // RUN: %clang %s -### -target avr --sysroot %S/Inputs/basic_avr_tree 2>&1 -nostdinc | FileCheck -check-prefix CC1B %s
 // CC1B-NOT: "-internal-isystem" {{".*avr/include"}}