Index: cfe/trunk/docs/LanguageExtensions.rst
===================================================================
--- cfe/trunk/docs/LanguageExtensions.rst
+++ cfe/trunk/docs/LanguageExtensions.rst
@@ -2465,6 +2465,10 @@
           movl    %gs:(%eax), %eax
           ret
 
+You can also use the GCC compatibility macros ``__seg_fs`` and ``__seg_gs`` for
+the same purpose. The preprocessor symbols ``__SEG_FS`` and ``__SEG_GS``
+indicate their support.
+
 PowerPC Language Extensions
 ------------------------------
 
Index: cfe/trunk/lib/Basic/Targets/X86.cpp
===================================================================
--- cfe/trunk/lib/Basic/Targets/X86.cpp
+++ cfe/trunk/lib/Basic/Targets/X86.cpp
@@ -917,6 +917,11 @@
     DefineStd(Builder, "i386", Opts);
   }
 
+  Builder.defineMacro("__SEG_GS");
+  Builder.defineMacro("__SEG_FS");
+  Builder.defineMacro("__seg_gs", "__attribute__((address_space(256)))");
+  Builder.defineMacro("__seg_fs", "__attribute__((address_space(257)))");
+
   // Subtarget options.
   // FIXME: We are hard-coding the tune parameters based on the CPU, but they
   // truly should be based on -mtune options.
Index: cfe/trunk/test/Preprocessor/x86_seg_fs_gs.c
===================================================================
--- cfe/trunk/test/Preprocessor/x86_seg_fs_gs.c
+++ cfe/trunk/test/Preprocessor/x86_seg_fs_gs.c
@@ -0,0 +1,7 @@
+// RUN: %clang -target i386-unknown-unknown -x c -E -dM -o - %s | FileCheck -match-full-lines %s
+// RUN: %clang -target x86_64-unknown-unknown -x c -E -dM -o - %s | FileCheck -match-full-lines %s
+
+// CHECK: #define __SEG_FS 1
+// CHECK: #define __SEG_GS 1
+// CHECK: #define __seg_fs __attribute__((address_space(257)))
+// CHECK: #define __seg_gs __attribute__((address_space(256)))