[SelectionDAG] try harder to convert funnel shift to rotate

Similar to rL337966 - if the DAGCombiner's rotate matching was 
working as expected, I don't think we'd see any test diffs here.

AArch only goes right, and PPC only goes left. 
x86 has both, so no diffs there.

Differential Revision: https://reviews.llvm.org/D50091

llvm-svn: 339359

1 files changed, 10 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 24e6289fcba..4fb10fadc3c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -5703,14 +5703,21 @@ SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I, unsigned Intrinsic) {
     if (X == Y && isPowerOf2_32(VT.getScalarSizeInBits())) {
       // TODO: This should also be done if the operation is custom, but we have
       // to make sure targets are handling the modulo shift amount as expected.
-      // TODO: If the rotate direction (left or right) corresponding to the
-      // shift is not available, adjust the shift value and invert the
-      // direction.
       auto RotateOpcode = IsFSHL ? ISD::ROTL : ISD::ROTR;
       if (TLI.isOperationLegal(RotateOpcode, VT)) {
         setValue(&I, DAG.getNode(RotateOpcode, sdl, VT, X, Z));
         return nullptr;
       }
+
+      // Some targets only rotate one way. Try the opposite direction.
+      RotateOpcode = IsFSHL ? ISD::ROTR : ISD::ROTL;
+      if (TLI.isOperationLegal(RotateOpcode, VT)) {
+        // Negate the shift amount because it is safe to ignore the high bits.
+        SDValue NegShAmt = DAG.getNode(ISD::SUB, sdl, VT, Zero, Z);
+        setValue(&I, DAG.getNode(RotateOpcode, sdl, VT, X, NegShAmt));
+        return nullptr;
+      }
+
       // fshl (rotl): (X << (Z % BW)) | (X >> ((0 - Z) % BW))
       // fshr (rotr): (X << ((0 - Z) % BW)) | (X >> (Z % BW))
       SDValue NegZ = DAG.getNode(ISD::SUB, sdl, VT, Zero, Z);