[DAGCombine] Moved X86 rotate_amount % bitwidth == 0 early out to DAG…

…Combiner

Remove common code from custom lowering (code is still safe if somehow a zero value gets used).

llvm-svn: 349028

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -6314,6 +6314,13 @@ SDValue DAGCombiner::visitRotate(SDNode *N) {
   if (isNullOrNullSplat(N1))
     return N0;
 
+  // fold (rot x, c) -> x iff (c % BitSize) == 0
+  if (isPowerOf2_32(Bitsize) && Bitsize > 1) {
+    APInt ModuloMask(N1.getScalarValueSizeInBits(), Bitsize - 1);
+    if (DAG.MaskedValueIsZero(N1, ModuloMask))
+      return N0;
+  }
+
   // fold (rot x, c) -> (rot x, c % BitSize)
   if (ConstantSDNode *Cst = isConstOrConstSplat(N1)) {
     if (Cst->getAPIntValue().uge(Bitsize)) {

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -24793,9 +24793,6 @@ static SDValue LowerRotate(SDValue Op, const X86Subtarget &Subtarget,
     if (auto *BVAmt = dyn_cast<BuildVectorSDNode>(Amt)) {
       if (auto *RotateConst = BVAmt->getConstantSplatNode()) {
         uint64_t RotateAmt = RotateConst->getAPIntValue().urem(EltSizeInBits);
-        if (RotateAmt == 0)
-          return R;
-
         return DAG.getNode(X86ISD::VROTLI, DL, VT, R,
                            DAG.getConstant(RotateAmt, DL, MVT::i8));
       }
@@ -24816,12 +24813,8 @@ static SDValue LowerRotate(SDValue Op, const X86Subtarget &Subtarget,
 
   // Rotate by an uniform constant - expand back to shifts.
   if (auto *BVAmt = dyn_cast<BuildVectorSDNode>(Amt))
-    if (auto *RotateConst = BVAmt->getConstantSplatNode()) {
-      uint64_t RotateAmt = RotateConst->getAPIntValue().urem(EltSizeInBits);
-      if (RotateAmt == 0)
-        return R;
+    if (BVAmt->getConstantSplatNode())
       return SDValue();
-    }
 
   // TODO: ISD::ROT* uses modulo rotate amounts, we need to handle this.