X86: add patterns for X86ISD::VSHLV and X86ISD::VSRLV

Replace VSELECT instruction which zeroes their result on exceeding legal SHL/SRL shift amount.

Author: Nekotekina

lib/Target/X86/X86ISelLowering.cpp

@@ -42254,6 +42254,38 @@ static SDValue combineSelect(SDNode *N, SelectionDAG &DAG,
     }
   }
 
+  // Detect pattern for AVX2+ variable shifts (shl, lshr) for inf precision.
+  if (N->getOpcode() == ISD::VSELECT && Cond.getOpcode() == ISD::SETCC &&
+      SupportedVectorVarShift(VT.getSimpleVT(), Subtarget, ISD::SHL)) {
+    ISD::CondCode CC = cast<CondCodeSDNode>(Cond.getOperand(2))->get();
+
+    // Check if one of the arms of the VSELECT is a zero vector. If it's on the
+    // left side invert the predicate to simplify logic below.
+    SDValue Other;
+    if (ISD::isBuildVectorAllZeros(LHS.getNode())) {
+      Other = RHS;
+      CC = ISD::getSetCCInverse(CC, VT.getVectorElementType());
+    } else if (ISD::isBuildVectorAllZeros(RHS.getNode())) {
+      Other = LHS;
+    }
+
+    // Look for the following patterns (>> becomes vsrlv):
+    // y <  32 ? x << y : 0 --> vshlv(x, y)
+    // y <= 31 ? x << y : 0 --> vshlv(x, y)
+    APInt CondRHS;
+    if (Other && Other.getNumOperands() == 2 &&
+        DAG.isEqualTo(Other.getOperand(1), Cond.getOperand(0)) &&
+        (Other.getOpcode() == ISD::SHL || Other.getOpcode() == ISD::SRL) &&
+        ISD::isConstantSplatVector(Cond.getOperand(1).getNode(), CondRHS)) {
+
+      // Replace ISD::SHL or ISD::SHR with appropriate AVX2 vector-vector shift.
+      unsigned op = Other.getOpcode() == ISD::SHL ? X86ISD::VSHLV : X86ISD::VSRLV;
+      if ((CC == ISD::SETULT && CondRHS == VT.getScalarSizeInBits()) ||
+          (CC == ISD::SETULE && CondRHS == VT.getScalarSizeInBits() - 1))
+        return DAG.getNode(op, DL, VT, Other.getOperand(0), Other.getOperand(1));
+    }
+  }
+
   if (SDValue V = combineVSelectWithAllOnesOrZeros(N, DAG, DCI, Subtarget))
     return V;