[mips] Small code generation improvement for conditional operator (select)

in case the operands are constants and its difference is |1|.
It should be possible in those cases to rematerialize the result using
MIPS's slt and similar instructions.

The small update to some of the tests in cmov.ll, sel1c.ll and sel2c.ll was needed
otherwise the optimization implemented in this patch would have been triggered
(difference between the operands was 1) and that would have changed the semantic
of the tests.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196498 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/Mips/MipsISelLowering.cpp

@@ -559,6 +559,39 @@ static SDValue performSELECTCombine(SDNode *N, SelectionDAG &DAG,
     return DAG.getNode(ISD::SELECT, DL, FalseTy, SetCC, False, True);
   }
 
+  // If both operands are integer constants there's a possibility that we
+  // can do some interesting optimizations.
+  SDValue True = N->getOperand(1);
+  ConstantSDNode *TrueC = dyn_cast<ConstantSDNode>(True);
+
+  if (!TrueC || !True.getValueType().isInteger())
+    return SDValue();
+
+  // We'll also ignore MVT::i64 operands as this optimizations proves
+  // to be ineffective because of the required sign extensions as the result
+  // of a SETCC operator is always MVT::i32 for non-vector types.
+  if (True.getValueType() == MVT::i64)
+    return SDValue();
+
+  int64_t Diff = TrueC->getSExtValue() - FalseC->getSExtValue();
+
+  // 1)  (a < x) ? y : y-1
+  //  slti $reg1, a, x
+  //  addiu $reg2, $reg1, y-1
+  if (Diff == 1)
+    return DAG.getNode(ISD::ADD, DL, SetCC.getValueType(), SetCC, False);
+
+  // 2)  (a < x) ? y-1 : y
+  //  slti $reg1, a, x
+  //  xor $reg1, $reg1, 1
+  //  addiu $reg2, $reg1, y-1
+  if (Diff == -1) {
+    ISD::CondCode CC = cast<CondCodeSDNode>(SetCC.getOperand(2))->get();
+    SetCC = DAG.getSetCC(DL, SetCC.getValueType(), SetCC.getOperand(0),
+                         SetCC.getOperand(1), ISD::getSetCCInverse(CC, true));
+    return DAG.getNode(ISD::ADD, DL, SetCC.getValueType(), SetCC, True);
+  }
+
   // Couldn't optimize.
   return SDValue();
 }