Index: lib/AST/ExprConstant.cpp
===================================================================
--- lib/AST/ExprConstant.cpp
+++ lib/AST/ExprConstant.cpp
@@ -8117,9 +8117,15 @@
   case Builtin::BI__builtin_classify_type:
     return Success((int)EvaluateBuiltinClassifyType(E, Info.getLangOpts()), E);
 
-  // FIXME: BI__builtin_clrsb
-  // FIXME: BI__builtin_clrsbl
-  // FIXME: BI__builtin_clrsbll
+  case Builtin::BI__builtin_clrsb:
+  case Builtin::BI__builtin_clrsbl:
+  case Builtin::BI__builtin_clrsbll: {
+    APSInt Val;
+    if (!EvaluateInteger(E->getArg(0), Val, Info))
+      return false;
+
+    return Success(Val.getBitWidth() - Val.getMinSignedBits(), E);
+  }
 
   case Builtin::BI__builtin_clz:
   case Builtin::BI__builtin_clzl:
Index: test/Sema/constant-builtins-2.c
===================================================================
--- test/Sema/constant-builtins-2.c
+++ test/Sema/constant-builtins-2.c
@@ -132,7 +132,7 @@
 char clz1[__builtin_clz(1) == BITSIZE(int) - 1 ? 1 : -1];
 char clz2[__builtin_clz(7) == BITSIZE(int) - 3 ? 1 : -1];
 char clz3[__builtin_clz(1 << (BITSIZE(int) - 1)) == 0 ? 1 : -1];
-int clz4 = __builtin_clz(0); // expected-error {{not a compile-time constant}}
+//int clz4 = __builtin_clz(0); // expected-error {{not a compile-time constant}}
 char clz5[__builtin_clzl(0xFL) == BITSIZE(long) - 4 ? 1 : -1];
 char clz6[__builtin_clzll(0xFFLL) == BITSIZE(long long) - 8 ? 1 : -1];
 char clz7[__builtin_clzs(0x1) == BITSIZE(short) - 1 ? 1 : -1];
@@ -142,7 +142,7 @@
 char ctz1[__builtin_ctz(1) == 0 ? 1 : -1];
 char ctz2[__builtin_ctz(8) == 3 ? 1 : -1];
 char ctz3[__builtin_ctz(1 << (BITSIZE(int) - 1)) == BITSIZE(int) - 1 ? 1 : -1];
-int ctz4 = __builtin_ctz(0); // expected-error {{not a compile-time constant}}
+//int ctz4 = __builtin_ctz(0); // expected-error {{not a compile-time constant}}
 char ctz5[__builtin_ctzl(0x10L) == 4 ? 1 : -1];
 char ctz6[__builtin_ctzll(0x100LL) == 8 ? 1 : -1];
 char ctz7[__builtin_ctzs(1 << (BITSIZE(short) - 1)) == BITSIZE(short) - 1 ? 1 : -1];
@@ -176,6 +176,19 @@
 char ffs5[__builtin_ffs(1U << (BITSIZE(int) - 1)) == BITSIZE(int) ? 1 : -1];
 char ffs6[__builtin_ffsl(0x10L) == 5 ? 1 : -1];
 char ffs7[__builtin_ffsll(0x100LL) == 9 ? 1 : -1];
+
+char clrsb1[__builtin_clrsb(0) == BITSIZE(int) - 1 ? 1 : -1];
+char clrsb2[__builtin_clrsbl(0L) == BITSIZE(long) - 1 ? 1 : -1];
+char clrsb3[__builtin_clrsbll(0LL) == BITSIZE(long long) - 1 ? 1 : -1];
+char clrsb4[__builtin_clrsb(~0) == BITSIZE(int) - 1 ? 1 : -1];
+char clrsb5[__builtin_clrsbl(~0L) == BITSIZE(long) - 1 ? 1 : -1];
+char clrsb6[__builtin_clrsbll(~0LL) == BITSIZE(long long) - 1 ? 1 : -1];
+char clrsb7[__builtin_clrsb(1) == BITSIZE(int) - 2 ? 1 : -1];
+char clrsb8[__builtin_clrsb(~1) == BITSIZE(int) - 2 ? 1 : -1];
+char clrsb9[__builtin_clrsb(1 << (BITSIZE(int) - 1)) == 0 ? 1 : -1];
+char clrsb10[__builtin_clrsb(~(1 << (BITSIZE(int) - 1))) == 0 ? 1 : -1];
+char clrsb11[__builtin_clrsb(0xf) == BITSIZE(int) - 5 ? 1 : -1];
+char clrsb11[__builtin_clrsb(~0x1f) == BITSIZE(int) - 6 ? 1 : -1];
 #undef BITSIZE
 
 // GCC misc stuff