- Move isLane0KnownActive before looking through reinterpret_cast

 - Add comment to changes in canRemovePTestInstr

Author: kmclaughlin-arm

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -27544,12 +27544,12 @@ static SDValue performPTestFirstCombine(SDNode *N,
   auto Mask = N->getOperand(0);
   auto Pred = N->getOperand(1);
 
-  if (Pred->getOpcode() == AArch64ISD::REINTERPRET_CAST)
-    Pred = Pred->getOperand(0);
-
   if (!isLane0KnownActive(Mask))
     return SDValue();
 
+  if (Pred->getOpcode() == AArch64ISD::REINTERPRET_CAST)
+    Pred = Pred->getOperand(0);
+
   if (Pred->getOpcode() == ISD::CONCAT_VECTORS) {
     Pred = Pred->getOperand(0);
     Pred = DAG.getNode(AArch64ISD::REINTERPRET_CAST, DL, MVT::nxv16i1, Pred);

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

@@ -1503,6 +1503,9 @@ AArch64InstrInfo::canRemovePTestInstr(MachineInstr *PTest, MachineInstr *Mask,
             getElementSizeForOpcode(PredOpcode))
       return PredOpcode;
 
+    // For PTEST_FIRST(PTRUE_ALL, WHILE), the PTEST_FIRST is redundant since
+    // WHILEcc performs an implicit PTEST with an all active mask, setting
+    // the N flag as the PTEST_FIRST would.
     if (PTest->getOpcode() == AArch64::PTEST_PP_FIRST &&
         isPTrueOpcode(MaskOpcode) && Mask->getOperand(1).getImm() == 31)
       return PredOpcode;