[ARM] Port shouldBeAdjustedToZero to ARM

Author: AZero13

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -4839,14 +4839,44 @@ static bool isFloatingPointZero(SDValue Op) {
   return false;
 }
 
+// TODO: This is copied from AArch64TargetLowering.cpp, which only has
+// ands, subs, and adds affecting flags. In ARM, we have more than that, so this
+// should be expanded to cover all the cases where we can adjust the condition
+// code to zero.
+static bool shouldBeAdjustedToZero(SDValue LHS, APInt C, ISD::CondCode &CC) {
+  // Special case where we can use pl or mi instead of lt or ge.
+  // Any instruction that sets flags can be optimized to use mi or pl
+  // this way.
+  if (C.isAllOnes() && (CC == ISD::SETLE || CC == ISD::SETGT)) {
+    CC = (CC == ISD::SETLE) ? ISD::SETLT : ISD::SETGE;
+    return true;
+  }
+
+  // TODO: Cover all cases where a comparison with 0 would be profitable...
+  if (LHS.getOpcode() != ISD::AND)
+    return false;
+
+  if (C.isOne() && (CC == ISD::SETLT || CC == ISD::SETGE)) {
+    CC = (CC == ISD::SETLT) ? ISD::SETLE : ISD::SETGT;
+    return true;
+  }
+
+  return false;
+}
+
 /// Returns appropriate ARM CMP (cmp) and corresponding condition code for
 /// the given operands.
 SDValue ARMTargetLowering::getARMCmp(SDValue LHS, SDValue RHS, ISD::CondCode CC,
                                      SDValue &ARMcc, SelectionDAG &DAG,
                                      const SDLoc &dl) const {
   if (ConstantSDNode *RHSC = dyn_cast<ConstantSDNode>(RHS.getNode())) {
-    unsigned C = RHSC->getZExtValue();
-    if (!isLegalICmpImmediate((int32_t)C)) {
+    APInt CInt = RHSC->getAPIntValue();
+    unsigned C = CInt.getZExtValue();
+    if (shouldBeAdjustedToZero(LHS, CInt, CC)) {
+      // Adjust the constant to zero.
+      // CC has already been adjusted.
+      RHS = DAG.getConstant(0, dl, VT);
+    } else if (!isLegalICmpImmediate((int32_t)C)) {
       // Constant does not fit, try adjusting it by one.
       switch (CC) {
       default: break;