Propagate fast-math flags through fpext

Author: mikolaj-pirog

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -3976,7 +3976,10 @@ void SelectionDAGBuilder::visitFPExt(const User &I) {
   SDValue N = getValue(I.getOperand(0));
   EVT DestVT = DAG.getTargetLoweringInfo().getValueType(DAG.getDataLayout(),
                                                         I.getType());
-  setValue(&I, DAG.getNode(ISD::FP_EXTEND, getCurSDLoc(), DestVT, N));
+  SDNodeFlags Flags;
+  if (auto *TruncInst = dyn_cast<FPMathOperator>(&I))
+    Flags.copyFMF(*TruncInst);
+  setValue(&I, DAG.getNode(ISD::FP_EXTEND, getCurSDLoc(), DestVT, N, Flags));
 }
 
 void SelectionDAGBuilder::visitFPToUI(const User &I) {