Index: llvm/trunk/lib/Analysis/ConstantFolding.cpp
===================================================================
--- llvm/trunk/lib/Analysis/ConstantFolding.cpp
+++ llvm/trunk/lib/Analysis/ConstantFolding.cpp
@@ -1520,8 +1520,14 @@
                  (Ty->isHalfTy() || Ty->isFloatTy() || Ty->isDoubleTy())) {
           if (V >= -0.0)
             return ConstantFoldFP(sqrt, V, Ty);
-          else // Undefined
-            return Constant::getNullValue(Ty);
+          else {
+            // Unlike the sqrt definitions in C/C++, POSIX, and IEEE-754 - which
+            // all guarantee or favor returning NaN - the square root of a
+            // negative number is not defined for the LLVM sqrt intrinsic.
+            // This is because the intrinsic should only be emitted in place of
+            // libm's sqrt function when using "no-nans-fp-math".
+            return UndefValue::get(Ty);
+          }
         }
         break;
       case 's':
Index: llvm/trunk/test/Transforms/InstCombine/constant-fold-math.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/constant-fold-math.ll
+++ llvm/trunk/test/Transforms/InstCombine/constant-fold-math.ll
@@ -7,6 +7,7 @@
 declare double @llvm.fma.f64(double, double, double) #0
 declare double @llvm.fmuladd.f64(double, double, double) #0
 
+declare double @llvm.sqrt.f64(double) #0
 
 
 ; CHECK-LABEL: @constant_fold_fma_f32
@@ -44,4 +45,12 @@
   ret double %x
 }
 
+; The sqrt intrinsic is undefined for negative inputs besides -0.0.
+; CHECK-LABEL: @bad_sqrt
+; CHECK-NEXT: ret double undef
+define double @bad_sqrt() {
+  %x = call double @llvm.sqrt.f64(double -2.000000e+00)
+  ret double %x
+}
+
 attributes #0 = { nounwind readnone }