Index: lib/Basic/Targets.cpp
===================================================================
--- lib/Basic/Targets.cpp
+++ lib/Basic/Targets.cpp
@@ -2730,6 +2730,14 @@
   case NoMMX3DNow:
     break;
   }
+
+  if (CPU >= CK_i486) {
+    Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_1");
+    Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2");
+    Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4");
+  }
+  if (CPU >= CK_i586)
+    Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_8");
 }
 
 bool X86TargetInfo::hasFeature(StringRef Feature) const {
Index: test/Preprocessor/predefined-macros.c
===================================================================
--- test/Preprocessor/predefined-macros.c
+++ test/Preprocessor/predefined-macros.c
@@ -26,3 +26,21 @@
 // RUN: %clang_cc1 %s -E -dM -o - \
 // RUN:   | FileCheck %s --check-prefix=CHECK-FINITE-MATH-FLAG-UNDEFINED
 // CHECK-FINITE-MATH-FLAG-UNDEFINED: #define __FINITE_MATH_ONLY__ 0
+//
+// RUN: %clang_cc1 %s -E -dM -o - -triple i686 -target-cpu i386 \
+// RUN:   | FileCheck %s --check-prefix=CHECK-SYNC_CAS_I386
+// CHECK-SYNC_CAS_I386-NOT: __GCC_HAVE_SYNC_COMPARE_AND_SWAP
+//
+// RUN: %clang_cc1 %s -E -dM -o - -triple i686 -target-cpu i486 \
+// RUN:   | FileCheck %s --check-prefix=CHECK-SYNC_CAS_I486
+// CHECK-SYNC_CAS_I486: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1
+// CHECK-SYNC_CAS_I486: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2
+// CHECK-SYNC_CAS_I486: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4
+// CHECK-SYNC_CAS_I486-NOT: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8
+//
+// RUN: %clang_cc1 %s -E -dM -o - -triple i686 -target-cpu i586 \
+// RUN:   | FileCheck %s --check-prefix=CHECK-SYNC_CAS_I586
+// CHECK-SYNC_CAS_I586: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1
+// CHECK-SYNC_CAS_I586: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2
+// CHECK-SYNC_CAS_I586: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4
+// CHECK-SYNC_CAS_I586: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8