Handle (shl (anyext (shr ...))) in SimpilfyDemandedBits

This is really an extension of the current (shl (shr ...)) -> shl optimization.
The main difference is that certain upper bits must also not be demanded.

The motivating examples are the first two in the testcase, which occur
in llvmpipe output.

llvm-svn: 192783

1 files changed, 25 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 17631c0c9d9..9d6a3b40b91 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -635,6 +635,31 @@ bool TargetLowering::SimplifyDemandedBits(SDValue Op,
                           TLO.DAG.getNode(ISD::ANY_EXTEND, dl, Op.getValueType(),
                                           NarrowShl));
         }
+        // Repeat the SHL optimization above in cases where an extension
+        // intervenes: (shl (anyext (shr x, c1)), c2) to
+        // (shl (anyext x), c2-c1).  This requires that the bottom c1 bits
+        // aren't demanded (as above) and that the shifted upper c1 bits of
+        // x aren't demanded.
+        if (InOp.hasOneUse() &&
+            InnerOp.getOpcode() == ISD::SRL &&
+            InnerOp.hasOneUse() &&
+            isa<ConstantSDNode>(InnerOp.getOperand(1))) {
+          uint64_t InnerShAmt = cast<ConstantSDNode>(InnerOp.getOperand(1))
+            ->getZExtValue();
+          if (InnerShAmt < ShAmt &&
+              InnerShAmt < InnerBits &&
+              NewMask.lshr(InnerBits - InnerShAmt + ShAmt) == 0 &&
+              NewMask.trunc(ShAmt) == 0) {
+            SDValue NewSA =
+              TLO.DAG.getConstant(ShAmt - InnerShAmt,
+                                  Op.getOperand(1).getValueType());
+            EVT VT = Op.getValueType();
+            SDValue NewExt = TLO.DAG.getNode(ISD::ANY_EXTEND, dl, VT,
+                                             InnerOp.getOperand(0));
+            return TLO.CombineTo(Op, TLO.DAG.getNode(ISD::SHL, dl, VT,
+                                                     NewExt, NewSA));
+          }
+        }
       }
 
       KnownZero <<= SA->getZExtValue();