[SelectionDAGBuilder] Propagate fast-math flags to fpext (#167574)

As in title. Without this, fpext behaves in selectionDAG as always
having no fast-math flags.

Author: mikolaj-pirog

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -18673,11 +18673,13 @@ SDValue DAGCombiner::visitFDIV(SDNode *N) {
   if (Flags.hasAllowReciprocal()) {
     // If this FDIV is part of a reciprocal square root, it may be folded
     // into a target-specific square root estimate instruction.
+    bool N1AllowReciprocal = N1->getFlags().hasAllowReciprocal();
     if (N1.getOpcode() == ISD::FSQRT) {
       if (SDValue RV = buildRsqrtEstimate(N1.getOperand(0)))
         return DAG.getNode(ISD::FMUL, DL, VT, N0, RV);
     } else if (N1.getOpcode() == ISD::FP_EXTEND &&
-               N1.getOperand(0).getOpcode() == ISD::FSQRT) {
+               N1.getOperand(0).getOpcode() == ISD::FSQRT &&
+               N1AllowReciprocal) {
       if (SDValue RV = buildRsqrtEstimate(N1.getOperand(0).getOperand(0))) {
         RV = DAG.getNode(ISD::FP_EXTEND, SDLoc(N1), VT, RV);
         AddToWorklist(RV.getNode());

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) {

llvm/test/CodeGen/PowerPC/recipest.ll

@@ -164,7 +164,7 @@ define double @foof_fmf(double %a, float %b) nounwind {
 ; CHECK-P9-NEXT:    xsmuldp f1, f1, f0
 ; CHECK-P9-NEXT:    blr
   %x = call contract reassoc arcp float @llvm.sqrt.f32(float %b)
-  %y = fpext float %x to double
+  %y = fpext arcp float %x to double
   %r = fdiv contract reassoc arcp double %a, %y
   ret double %r
 }