[DAGCombiner] visitRotate patch to optimize pair of ROTR/ROTL instructions into one with combined shift operand.

For two ROTR operations with shifts C1, C2; combined shift operand will be (C1 + C2) % bitsize.

Differential revision: https://reviews.llvm.org/D12833

llvm-svn: 307179

1 files changed, 19 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index e40788eea1d..6fb5e3996e2 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -5279,6 +5279,25 @@ SDValue DAGCombiner::visitRotate(SDNode *N) {
     if (SDValue NewOp1 = distributeTruncateThroughAnd(N1.getNode()))
       return DAG.getNode(N->getOpcode(), dl, VT, N0, NewOp1);
   }
+
+  unsigned NextOp = N0.getOpcode();
+  // fold (rot* (rot* x, c2), c1) -> (rot* x, c1 +- c2 % bitsize)
+  if (NextOp == ISD::ROTL || NextOp == ISD::ROTR)
+    if (SDNode *C1 = DAG.isConstantIntBuildVectorOrConstantInt(N1))
+      if (SDNode *C2 =
+          DAG.isConstantIntBuildVectorOrConstantInt(N0.getOperand(1))) {
+        bool SameSide = (N->getOpcode() == NextOp);
+        unsigned CombineOp = SameSide ? ISD::ADD : ISD::SUB;
+        if (SDValue CombinedShift =
+            DAG.FoldConstantArithmetic(CombineOp, dl, VT, C1, C2)) {
+          unsigned Bitsize = VT.getScalarSizeInBits();
+          SDValue BitsizeC = DAG.getConstant(Bitsize, dl, VT);
+          SDValue CombinedShiftNorm = DAG.FoldConstantArithmetic(
+            ISD::SREM, dl, VT, CombinedShift.getNode(), BitsizeC.getNode());
+          return DAG.getNode(
+            N->getOpcode(), dl, VT, N0->getOperand(0), CombinedShiftNorm);
+        }
+      }
   return SDValue();
 }