Index: clang/lib/Driver/ToolChains/AVR.h
===================================================================
--- clang/lib/Driver/ToolChains/AVR.h
+++ clang/lib/Driver/ToolChains/AVR.h
@@ -33,6 +33,7 @@
 
   llvm::Optional<std::string> findAVRLibcInstallation() const;
   StringRef getGCCInstallPath() const { return GCCInstallPath; }
+  std::string getCompilerRTPath() const override;
 
 protected:
   Tool *buildLinker() const override;
Index: clang/lib/Driver/ToolChains/AVR.cpp
===================================================================
--- clang/lib/Driver/ToolChains/AVR.cpp
+++ clang/lib/Driver/ToolChains/AVR.cpp
@@ -19,6 +19,7 @@
 #include "llvm/MC/SubtargetFeature.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Path.h"
 
 using namespace clang::driver;
 using namespace clang::driver::toolchains;
@@ -369,8 +370,7 @@
     : Generic_ELF(D, Triple, Args) {
   GCCInstallation.init(Triple, Args);
 
-  std::string CPU = getCPUName(D, Args, Triple);
-  if (CPU.empty())
+  if (getCPUName(D, Args, Triple).empty())
     D.Diag(diag::warn_drv_avr_mcu_not_specified);
 
   // Only add default libraries if the user hasn't explicitly opted out.
@@ -418,6 +418,14 @@
   return new tools::AVR::Linker(getTriple(), *this);
 }
 
+std::string AVRToolChain::getCompilerRTPath() const {
+  // Return the default path appended with '/avr', since AVR can never be a
+  // host environment.
+  SmallString<128> Path(ToolChain::getCompilerRTPath());
+  llvm::sys::path::append(Path, "avr");
+  return std::string(Path.str());
+}
+
 void AVR::Linker::ConstructJob(Compilation &C, const JobAction &JA,
                                const InputInfo &Output,
                                const InputInfoList &Inputs, const ArgList &Args,
@@ -449,6 +457,12 @@
   Args.AddAllArgs(CmdArgs, options::OPT_L);
   getToolChain().AddFilePathLibArgs(Args, CmdArgs);
 
+  // Currently we only support libgcc and compiler-rt.
+  auto RtLib = TC.GetRuntimeLibType(Args);
+  assert(
+      (RtLib == ToolChain::RLT_Libgcc || RtLib == ToolChain::RLT_CompilerRT) &&
+      "unknown runtime library");
+
   // Only add default libraries if the user hasn't explicitly opted out.
   bool LinkStdlib = false;
   if (!Args.hasArg(options::OPT_nostdlib) &&
@@ -464,10 +478,13 @@
         D.Diag(diag::warn_drv_avr_libc_not_found);
       } else {
         std::string SubPath = GetMCUSubPath(CPU);
+        // Add path of avr-libc.
         CmdArgs.push_back(
             Args.MakeArgString(Twine("-L") + *AVRLibcRoot + "/lib/" + SubPath));
-        CmdArgs.push_back(
-            Args.MakeArgString("-L" + TC.getGCCInstallPath() + "/" + SubPath));
+        // Add path of libgcc.
+        if (RtLib == ToolChain::RLT_Libgcc)
+          CmdArgs.push_back(Args.MakeArgString("-L" + TC.getGCCInstallPath() +
+                                               "/" + SubPath));
         LinkStdlib = true;
       }
     }
@@ -496,7 +513,11 @@
     std::string CrtFileName = std::string("-l:crt") + CPU + std::string(".o");
     CmdArgs.push_back(Args.MakeArgString(CrtFileName));
 
-    CmdArgs.push_back("-lgcc");
+    // Link to libgcc.
+    if (RtLib == ToolChain::RLT_Libgcc)
+      CmdArgs.push_back("-lgcc");
+
+    // Link to generic libraries of avr-libc.
     CmdArgs.push_back("-lm");
     CmdArgs.push_back("-lc");
 
@@ -505,6 +526,16 @@
 
     CmdArgs.push_back("--end-group");
 
+    // Link to compiler-rt. We directly put the libclang.builtins-avr.a
+    // as input file, other than '-lclang.builtins-avr'. This is a tradition
+    // from other platforms, such as x86 and arm.
+    if (RtLib == ToolChain::RLT_CompilerRT) {
+      std::string RtLib =
+          getToolChain().getCompilerRT(Args, "builtins", ToolChain::FT_Static);
+      if (llvm::sys::fs::exists(RtLib))
+        CmdArgs.push_back(Args.MakeArgString(RtLib));
+    }
+
     // Add user specified linker script.
     Args.AddAllArgs(CmdArgs, options::OPT_T);
 
Index: clang/test/Driver/avr-toolchain.c
===================================================================
--- clang/test/Driver/avr-toolchain.c
+++ clang/test/Driver/avr-toolchain.c
@@ -72,3 +72,18 @@
 // RUN: %clang -### --target=avr --sysroot=%S/Inputs/basic_avr_tree -mmcu=atmega328 %s -fuse-ld=%S/Inputs/basic_avr_tree/usr/bin/ld.lld -T avr.lds 2>&1 | FileCheck --check-prefix=LDS1 %s
 // LDS1: "-T" "avr.lds"
 // LDS1-NOT: "-mavr5"
+
+// RUN: %clang %s -### --target=avr -mmcu=atmega328 --sysroot=%S/Inputs/basic_avr_tree/ -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir 2>&1 | FileCheck --check-prefix=LIBGCC %s
+// RUN: %clang %s -### --target=avr -mmcu=atmega328 --sysroot=%S/Inputs/basic_avr_tree/ -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir --rtlib=libgcc 2>&1 | FileCheck --check-prefix=LIBGCC %s
+// LIBGCC: "-lgcc"
+// LIBGCC-NOT: libclang_rt
+
+// RUN: %clang %s -### --target=avr -mmcu=atmega328 --sysroot=%S/Inputs/basic_avr_tree/ -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir --rtlib=compiler-rt 2>&1 | FileCheck --check-prefix=COMRT %s
+// RUN: %clang %s -### --target=avr -mmcu=atmega328 --sysroot=%S/Inputs/basic_avr_tree/ -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir -fuse-ld=%S/Inputs/basic_avr_tree/usr/bin/ld.lld --rtlib=compiler-rt 2>&1 | FileCheck --check-prefix=COMRT %s
+// COMRT: avr/libclang_rt.builtins-avr.a
+// COMRT-NOT: "-lgcc"
+
+// RUN: %clang %s -### --target=avr --sysroot=%S/Inputs/basic_avr_tree/ -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir --rtlib=compiler-rt 2>&1 | FileCheck --check-prefix=NOMCU %s
+// RUN: %clang %s -### --target=avr --sysroot=%S/Inputs/basic_avr_tree/ -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir --rtlib=libgcc 2>&1 | FileCheck --check-prefix=NOMCU %s
+// NOMCU-NOT: libclang_rt
+// NOMCU-NOT: "-lgcc"