diff --git a/clang/docs/ClangCommandLineReference.rst b/clang/docs/ClangCommandLineReference.rst
--- a/clang/docs/ClangCommandLineReference.rst
+++ b/clang/docs/ClangCommandLineReference.rst
@@ -18,9 +18,9 @@
 
 
 .. program:: clang
-.. option:: -B<dir>, --prefix <arg>, --prefix=<arg>
+.. option:: -B<prefix>, --prefix <arg>, --prefix=<arg>
 
-Add <dir> to search path for binaries and object files used implicitly
+Search $prefix/$triple-$file and $prefix$file for executables, libraries, includes, and data files used by the compiler. If $prefix is a directory, the directory is used to search for GCC installation on targets which commonly use GCC. The directory usually contains 'include' and 'lib'. Note: sysroot is also used to search for GCC installation and the largest version wins. Specify --gcc-toolchain to suppress sysroot. If $prefix is not a directory, the GCC installation detection is skipped but $prefix can still be used to find executables, typically for -nostdinc -nostdlib compiles
 
 .. option:: -F<arg>
 
@@ -256,7 +256,7 @@
 
 .. option:: --gcc-toolchain=<arg>, -gcc-toolchain <arg>
 
-Use the gcc toolchain at the given directory
+Search for GCC installation in the specified directory, similar to -B<dir>. The directory usually contains 'lib' and 'include'. If specified, sysroot is skipped for GCC detection. Note: executables (e.g. ld) used by the compiler are not overridden by the selected GCC installation
 
 .. option:: -gcodeview
 
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -599,8 +599,20 @@
 def _DASH_DASH : Option<["--"], "", KIND_REMAINING_ARGS>,
     Flags<[NoXarchOption, CoreOption]>;
 def A : JoinedOrSeparate<["-"], "A">, Flags<[RenderJoined]>, Group<gfortran_Group>;
-def B : JoinedOrSeparate<["-"], "B">, MetaVarName<"<dir>">,
-    HelpText<"Add <dir> to search path for binaries and object files used implicitly">;
+def B : JoinedOrSeparate<["-"], "B">, MetaVarName<"<prefix>">,
+    HelpText<"Search $prefix/$triple-$file and $prefix$file for executables, libraries, "
+    "includes, and data files used by the compiler. If $prefix is a directory, the "
+    "directory is used to search for GCC installation on targets which commonly use GCC. "
+    "The directory usually contains 'include' and 'lib'. Note: sysroot is also used to "
+    "search for GCC installation and the largest version wins. Specify --gcc-toolchain "
+    "to suppress sysroot. If $prefix is not a directory, the GCC installation detection "
+    "is skipped but $prefix can still be used to find executables, typically for -nostdinc "
+    "-nostdlib compiles">;
+def gcc_toolchain : Joined<["--"], "gcc-toolchain=">, Flags<[NoXarchOption]>,
+  HelpText<"Search for GCC installation in the specified directory, similar to -B<dir>. "
+  "The directory usually contains 'lib' and 'include'. If specified, sysroot is skipped "
+  "for GCC detection. Note: executables (e.g. ld) used by the compiler are not overridden "
+  "by the selected GCC installation">;
 def CC : Flag<["-"], "CC">, Flags<[CC1Option]>, Group<Preprocessor_Group>,
     HelpText<"Include comments from within macros in preprocessed output">,
     MarshallingInfoFlag<PreprocessorOutputOpts<"ShowMacroComments">>;
@@ -3666,8 +3678,6 @@
   MarshallingInfoFlag<FrontendOpts<"PrintSupportedCPUs">>;
 def mcpu_EQ_QUESTION : Flag<["-"], "mcpu=?">, Alias<print_supported_cpus>;
 def mtune_EQ_QUESTION : Flag<["-"], "mtune=?">, Alias<print_supported_cpus>;
-def gcc_toolchain : Joined<["--"], "gcc-toolchain=">, Flags<[NoXarchOption]>,
-  HelpText<"Use the gcc toolchain at the given directory">;
 def time : Flag<["-"], "time">,
   HelpText<"Time individual commands">;
 def traditional_cpp : Flag<["-", "--"], "traditional-cpp">, Flags<[CC1Option]>,
diff --git a/clang/test/Driver/gcc-toolchain.cpp b/clang/test/Driver/gcc-toolchain.cpp
--- a/clang/test/Driver/gcc-toolchain.cpp
+++ b/clang/test/Driver/gcc-toolchain.cpp
@@ -29,3 +29,23 @@
 // CHECK: "{{[^"]*}}/usr/lib/i386-linux-gnu/gcc/i686-linux-gnu/4.5{{/|\\\\}}crtbegin.o"
 // CHECK: "-L[[TOOLCHAIN]]/usr/lib/i386-linux-gnu/gcc/i686-linux-gnu/4.5"
 // CHECK: "-L[[TOOLCHAIN]]/usr/lib/i386-linux-gnu/gcc/i686-linux-gnu/4.5/../../../.."
+//
+// RUN: %clangxx -no-canonical-prefixes %s -### -o %t 2>&1 \
+// RUN:     --target=x86_64-linux-gnu -stdlib=libstdc++ \
+// RUN:     --gcc-toolchain=%S/Inputs/ubuntu_13.04_multiarch_tree/usr \
+// RUN:     --sysroot= | FileCheck %s --check-prefix=UBUNTU13
+
+/// If both --prefix and --gcc-toolchain are specified, the GCC installation with a larger version wins.
+// RUN: %clangxx -no-canonical-prefixes %s -### -o %t 2>&1 \
+// RUN:     --target=i386-unknown-linux -stdlib=libstdc++ \
+// RUN:     --prefix=%S/Inputs/ubuntu_13.04_multiarch_tree/usr \
+// RUN:     --gcc-toolchain=%S/Inputs/ubuntu_11.04_multiarch_tree/usr \
+// RUN:     --sysroot= | FileCheck %s --check-prefix=UBUNTU13
+
+// RUN: %clangxx -no-canonical-prefixes %s -### -o %t 2>&1 \
+// RUN:     --target=i386-unknown-linux -stdlib=libstdc++ \
+// RUN:     --prefix=%S/Inputs/ubuntu_11.04_multiarch_tree/usr \
+// RUN:     --gcc-toolchain=%S/Inputs/ubuntu_13.04_multiarch_tree/usr \
+// RUN:     --sysroot= | FileCheck %s --check-prefix=UBUNTU13
+
+// UBUNTU13: {{.*}}/ubuntu_13.04_multiarch_tree/usr/lib/gcc/x86_64-linux-gnu/4.7