Index: include/clang/Basic/Builtins.def
===================================================================
--- include/clang/Basic/Builtins.def
+++ include/clang/Basic/Builtins.def
@@ -413,6 +413,9 @@
 BUILTIN(__builtin_popcount  , "iUi"  , "nc")
 BUILTIN(__builtin_popcountl , "iULi" , "nc")
 BUILTIN(__builtin_popcountll, "iULLi", "nc")
+BUILTIN(__builtin_clrsb  , "ii"  , "nc")
+BUILTIN(__builtin_clrsbl , "iLi" , "nc")
+BUILTIN(__builtin_clrsbll, "iLLi", "nc")
 
 // FIXME: These type signatures are not correct for targets with int != 32-bits
 // or with ULL != 64-bits.
Index: lib/CodeGen/CGBuiltin.cpp
===================================================================
--- lib/CodeGen/CGBuiltin.cpp
+++ lib/CodeGen/CGBuiltin.cpp
@@ -1537,6 +1537,33 @@
     return RValue::get(ComplexVal.second);
   }
 
+  case Builtin::BI__builtin_clrsb:
+  case Builtin::BI__builtin_clrsbl:
+  case Builtin::BI__builtin_clrsbll: {
+    // clrsb(x) -> clz(x < 0 ? ~x : x) - 1 or
+    //          -> clz(((x < 0 ? ~x : x) << 1) | 1)
+    Value *ArgValue = EmitScalarExpr(E->getArg(0));
+
+    llvm::Type *ArgType = ArgValue->getType();
+    Value *F = CGM.getIntrinsic(Intrinsic::ctlz, ArgType);
+
+    llvm::Type *ResultType = ConvertType(E->getType());
+    Value *Zero = llvm::Constant::getNullValue(ArgType);
+    Value *IsNeg = Builder.CreateICmpSLT(ArgValue, Zero, "isneg");
+    Value *Inverse = Builder.CreateNot(ArgValue, "not");
+    Value *Tmp = Builder.CreateSelect(IsNeg, Inverse, ArgValue);
+    // Now we need to calculate ctlz(Tmp)-1, but Tmp might be zero. We know
+    // the sign bit is zero, so we can shift it out. Then put a 1 in the LSB.
+    // This removes one leading zero like the subtract does, and replaces it
+    // with a guaranteed one to prevent the value being 0.
+    Value *One = llvm::ConstantInt::get(ArgType, 1);
+    Tmp = Builder.CreateShl(Tmp, One);
+    Tmp = Builder.CreateOr(Tmp, One);
+    Value *Result = Builder.CreateCall(F, {Tmp, Builder.getTrue()});
+    Result = Builder.CreateIntCast(Result, ResultType, /*isSigned*/true,
+                                   "cast");
+    return RValue::get(Result);
+  }
   case Builtin::BI__builtin_ctzs:
   case Builtin::BI__builtin_ctz:
   case Builtin::BI__builtin_ctzl:
Index: test/CodeGen/builtin_clrsb.c
===================================================================
--- /dev/null
+++ test/CodeGen/builtin_clrsb.c
@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 %s -emit-llvm -o - | FileCheck %s
+
+int test__builtin_clrsb(int x) {
+// CHECK-LABEL: test__builtin_clrsb
+// CHECK: [[C:%.*]] = icmp slt i32 [[X:%.*]], 0
+// CHECK: [[INV:%.*]] = xor i32 [[X]], -1
+// CHECK: [[SEL:%.*]] = select i1 [[C]], i32 [[INV]], i32 [[X]]
+// CHECK: [[SHL:%.*]] = shl i32 [[SEL]], 1
+// CHECK: [[OR:%.*]] = or i32 [[SHL]], 1
+// CHECK: call i32 @llvm.ctlz.i32(i32 [[OR]], i1 true)
+  return __builtin_clrsb(x);
+}
+
+int test__builtin_clrsbll(long long x) {
+// CHECK-LABEL: test__builtin_clrsbll
+// CHECK: [[C:%.*]] = icmp slt i64 [[X:%.*]], 0
+// CHECK-NEXT: [[INV:%.*]] = xor i64 [[X]], -1
+// CHECK-NEXT: [[SEL:%.*]] = select i1 [[C]], i64 [[INV]], i64 [[X]]
+// CHECK-NEXT: [[SHL:%.*]] = shl i64 [[SEL]], 1
+// CHECK-NEXT: [[OR:%.*]] = or i64 [[SHL]], 1
+// CHECK-NEXT: [[CTLZ:%.*]] = call i64 @llvm.ctlz.i64(i64 [[OR]], i1 true)
+// CHECK-NEXT: trunc i64 [[CTLZ]] to i32
+  return __builtin_clrsbll(x);
+}