[libcxx-commits] [libcxx] [llvm] [clang-tools-extra] [flang] [lld] [compiler-rt] [lldb] [libc] [clang] [Legalizer] Expand fmaximum and fminimum (PR #67301)

[Matt Arsenault via libcxx-commits]

================
@@ -8310,6 +8310,64 @@ SDValue TargetLowering::expandFMINNUM_FMAXNUM(SDNode *Node,
   return SDValue();
 }
 
+SDValue TargetLowering::expandFMINIMUM_FMAXIMUM(SDNode *N,
+                                                SelectionDAG &DAG) const {
+  SDLoc DL(N);
+  SDValue LHS = N->getOperand(0);
+  SDValue RHS = N->getOperand(1);
+  unsigned Opc = N->getOpcode();
+  EVT VT = N->getValueType(0);
+  EVT CCVT = getSetCCResultType(DAG.getDataLayout(), *DAG.getContext(), VT);
+  bool IsMax = Opc == ISD::FMAXIMUM;
+
+  if (VT.isVector() &&
+      isOperationLegalOrCustomOrPromote(Opc, VT.getScalarType()))
+    return SDValue();
+
+  // First, implement comparison not propagating NaN. If no native fmin or fmax
+  // available, use plain select with setcc instead.
+  SDValue MinMax;
+  unsigned CompOpcIeee = IsMax ? ISD::FMAXNUM_IEEE : ISD::FMINNUM_IEEE;
+  unsigned CompOpc = IsMax ? ISD::FMAXNUM : ISD::FMINNUM;
+  if (isOperationLegalOrCustom(CompOpcIeee, VT)) {
+    MinMax = DAG.getNode(CompOpcIeee, DL, VT, LHS, RHS);
+  } else if (isOperationLegalOrCustom(CompOpc, VT)) {
+    MinMax = DAG.getNode(CompOpc, DL, VT, LHS, RHS);
+  } else {
+    // NaN (if exists) will be propagated later, so orderness doesn't matter.
+    SDValue Compare =
+        DAG.getSetCC(DL, CCVT, LHS, RHS, IsMax ? ISD::SETGT : ISD::SETLT);
+    MinMax = DAG.getSelect(DL, VT, Compare, LHS, RHS);
+  }
+
+  // Propagate any NaN of both operands
+  if (!N->getFlags().hasNoNaNs() &&
+      (!DAG.isKnownNeverNaN(LHS) || !DAG.isKnownNeverNaN(RHS))) {
----------------
arsenm wrote:

Should invert the order of these checks. RHS is canonically simpler / cheaper to inspect 

https://github.com/llvm/llvm-project/pull/67301