diff --git a/llvm/lib/Transforms/Scalar/SCCP.cpp b/llvm/lib/Transforms/Scalar/SCCP.cpp
index 59cdee652dc7..907ece8fcce9 100644
--- a/llvm/lib/Transforms/Scalar/SCCP.cpp
+++ b/llvm/lib/Transforms/Scalar/SCCP.cpp
@@ -1452,6 +1452,8 @@ bool SCCPSolver::ResolvedUndefsIn(Function &F) {
         // After a zero extend, we know the top part is zero.  SExt doesn't have
         // to be handled here, because we don't know whether the top part is 1's
         // or 0's.
+      case Instruction::SIToFP:  // some FP values are not possible, just use 0.
+      case Instruction::UIToFP:  // some FP values are not possible, just use 0.
         markForcedConstant(I, Constant::getNullValue(ITy));
         return true;
       case Instruction::Mul:
diff --git a/llvm/test/Transforms/SCCP/undef-resolve.ll b/llvm/test/Transforms/SCCP/undef-resolve.ll
index d2f3d0206d7a..bed561c8e4f2 100644
--- a/llvm/test/Transforms/SCCP/undef-resolve.ll
+++ b/llvm/test/Transforms/SCCP/undef-resolve.ll
@@ -1,8 +1,19 @@
 ; RUN: opt %s -sccp -S | FileCheck %s
+
+
+; PR6940
+define double @test1() {
+  %t = sitofp i32 undef to double
+  ret double %t
+; CHECK: @test1
+; CHECK: ret double 0.0
+}
+
+
 ; rdar://7832370
 ; Check that lots of stuff doesn't get turned into undef.
-
-define i32 @main() nounwind readnone ssp {
+define i32 @test2() nounwind readnone ssp {
+; CHECK: @test2
 init:
   br label %control.outer.outer