diff --git a/llvm/lib/Transforms/Scalar/Reassociate.cpp b/llvm/lib/Transforms/Scalar/Reassociate.cpp
index 72fbcd1677e8..1c06ffce17e8 100644
--- a/llvm/lib/Transforms/Scalar/Reassociate.cpp
+++ b/llvm/lib/Transforms/Scalar/Reassociate.cpp
@@ -268,11 +268,7 @@ static BinaryOperator *CreateNeg(Value *S1, const Twine &Name,
 static BinaryOperator *LowerNegateToMultiply(Instruction *Neg) {
   assert((isa<UnaryOperator>(Neg) || isa<BinaryOperator>(Neg)) &&
          "Expected a Negate!");
-  // It's not safe to lower a unary FNeg into a FMul by -1.0. However,
-  // we can only reach this function with fast flags set, so it's
-  // safe to do with nnan.
-  assert((!isa<FPMathOperator>(Neg) || Neg->isFast()) &&
-          "Expecting FastMathFlags!");
+  // FIXME: It's not safe to lower a unary FNeg into a FMul by -1.0.
   unsigned OpNo = isa<BinaryOperator>(Neg) ? 1 : 0;
   Type *Ty = Neg->getType();
   Constant *NegOne = Ty->isIntOrIntVectorTy() ?
diff --git a/llvm/test/Transforms/Reassociate/pr42349.ll b/llvm/test/Transforms/Reassociate/pr42349.ll
new file mode 100644
index 000000000000..ce008c3d1cb3
--- /dev/null
+++ b/llvm/test/Transforms/Reassociate/pr42349.ll
@@ -0,0 +1,18 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -reassociate -S | FileCheck %s
+
+define  float @wibble(float %tmp6) #0 {
+; CHECK-LABEL: @wibble(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    [[TMP7:%.*]] = fmul float [[TMP6:%.*]], -1.000000e+00
+; CHECK-NEXT:    [[TMP0:%.*]] = fsub float -0.000000e+00, 0.000000e+00
+; CHECK-NEXT:    [[TMP9:%.*]] = fmul fast float [[TMP6]], 0xFFF0000000000000
+; CHECK-NEXT:    ret float [[TMP9]]
+;
+bb:
+  %tmp7 = fsub float -0.000000e+00, %tmp6
+  %tmp9 = fmul fast float %tmp7, 0x7FF0000000000000
+  ret float %tmp9
+}
+
+attributes #0 = { "use-soft-float"="false" }