diff --git a/clang/CMakeLists.txt b/clang/CMakeLists.txt
--- a/clang/CMakeLists.txt
+++ b/clang/CMakeLists.txt
@@ -226,7 +226,7 @@
   "Colon separated list of directories clang will search for headers.")
 
 set(GCC_INSTALL_PREFIX "" CACHE PATH "Directory where gcc is installed." )
-set(DEFAULT_SYSROOT "" CACHE PATH
+set(DEFAULT_SYSROOT "" CACHE STRING
   "Default <path> to all compiler invocations for --sysroot=<path>." )
 
 set(ENABLE_LINKER_BUILD_ID OFF CACHE BOOL "pass --build-id to ld")
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -140,6 +140,13 @@
   Dir = std::string(llvm::sys::path::parent_path(ClangExecutable));
   InstalledDir = Dir; // Provide a sensible default installed dir.
 
+  if ((!SysRoot.empty()) && llvm::sys::path::is_relative(SysRoot)) {
+    // Prepend InstalledDir if SysRoot is relative
+    SmallString<128> P(InstalledDir);
+    llvm::sys::path::append(P, SysRoot);
+    SysRoot = std::string(P);
+  }
+
 #if defined(CLANG_CONFIG_FILE_SYSTEM_DIR)
   SystemConfigDir = CLANG_CONFIG_FILE_SYSTEM_DIR;
 #endif