[DAGCombiner] [NFC] Improve X div/rem 1 fold

[spatel]

LGTM.

For reference, we do have existing test coverage for these in test/CodeGen/X86/combine-sdiv.ll and combine-udiv.ll.

llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

 static SDValue simplifyDivRem(SDNode *N, SelectionDAG &DAG) {
   SDValue N0 = N->getOperand(0);
   SDValue N1 = N->getOperand(1);
   EVT VT = N->getValueType(0);
   SDLoc DL(N);
 
   unsigned Opc = N->getOpcode();
   bool IsDiv = (ISD::SDIV == Opc) || (ISD::UDIV == Opc);
+  ConstantSDNode *N1C = isConstOrConstSplat(N1);
 
   // X / undef -> undef
   // X % undef -> undef
   // X / 0 -> undef
   // X % 0 -> undef
   // NOTE: This includes vectors where any divisor element is zero/undef.
   if (DAG.isUndef(Opc, {N0, N1}))
     return DAG.getUNDEF(VT);
 
   // undef / X -> 0
   // undef % X -> 0
   if (N0.isUndef())
     return DAG.getConstant(0, DL, VT);
 
   // TODO: 0 / X -> 0
   // TODO: 0 % X -> 0
 
   // X / X -> 1
   // X % X -> 0
   if (N0 == N1)
     return DAG.getConstant(IsDiv ? 1 : 0, DL, VT);
 
-  // TODO: X / 1 -> X
-  // TODO: X % 1 -> 0
+  // X / 1 -> X
+  // X % 1 -> 0
+  if (N1C && N1C->isOne())
+    return IsDiv ? N0 : DAG.getConstant(0, DL, VT);
   // If this is a boolean op (single-bit element type), we can't have
   // division-by-zero or remainder-by-zero, so assume the divisor is 1.
   // Similarly, if we're zero-extending a boolean divisor, then assume it's a 1.
 
   return SDValue();
 }
 
 SDValue DAGCombiner::visitSDIV(SDNode *N) {
SDValue DAGCombiner::visitSDIV(SDNode *N) {
 
   SDLoc DL(N);
 
   // fold (sdiv c1, c2) -> c1/c2
   ConstantSDNode *N0C = isConstOrConstSplat(N0);
   ConstantSDNode *N1C = isConstOrConstSplat(N1);
   if (N0C && N1C && !N0C->isOpaque() && !N1C->isOpaque())
     return DAG.FoldConstantArithmetic(ISD::SDIV, DL, VT, N0C, N1C);
-  // fold (sdiv X, 1) -> X
-  if (N1C && N1C->isOne())
-    return N0;
   // fold (sdiv X, -1) -> 0-X
   if (N1C && N1C->isAllOnesValue())
     return DAG.getNode(ISD::SUB, DL, VT, DAG.getConstant(0, DL, VT), N0);
   // fold (sdiv X, MIN_SIGNED) -> select(X == MIN_SIGNED, 1, 0)
   if (N1C && N1C->getAPIntValue().isMinSignedValue())
     return DAG.getSelect(DL, VT, DAG.getSetCC(DL, CCVT, N0, N1, ISD::SETEQ),
                          DAG.getConstant(1, DL, VT),
                          DAG.getConstant(0, DL, VT));
SDValue DAGCombiner::visitUDIV(SDNode *N) {
 
   // fold (udiv c1, c2) -> c1/c2
   ConstantSDNode *N0C = isConstOrConstSplat(N0);
   ConstantSDNode *N1C = isConstOrConstSplat(N1);
   if (N0C && N1C)
     if (SDValue Folded = DAG.FoldConstantArithmetic(ISD::UDIV, DL, VT,
                                                     N0C, N1C))
       return Folded;
-  // fold (udiv X, 1) -> X
-  if (N1C && N1C->isOne())
-    return N0;
   // fold (udiv X, -1) -> select(X == -1, 1, 0)
   if (N1C && N1C->getAPIntValue().isAllOnesValue())
     return DAG.getSelect(DL, VT, DAG.getSetCC(DL, CCVT, N0, N1, ISD::SETEQ),
                          DAG.getConstant(1, DL, VT),
                          DAG.getConstant(0, DL, VT));
 
   if (SDValue V = simplifyDivRem(N, DAG))
     return V;