[X86] Add DAG combine to turn (trunc (srl (mul ext, ext), 16) into PMULHW/PMULHUW.

Ultimately I want to use this to remove the intrinsics for these instructions.

llvm-svn: 330520

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -36102,6 +36102,59 @@ static SDValue detectAddSubSatPattern(SDValue In, EVT VT, SelectionDAG &DAG,
                           AddSubSatBuilder);
 }
 
+// Try to form a MULHU or MULHS node by looking for
+// (trunc (srl (mul ext, ext), 16))
+// TODO: This is X86 specific because we want to be able to handle wide types
+// before type legalization. But we can only do it if the vector will be
+// legalized via widening/splitting. Type legalization can't handle promotion
+// of a MULHU/MULHS. There isn't a way to convey this to the generic DAG
+// combiner.
+static SDValue combinePMULH(SDValue Src, EVT VT, const SDLoc &DL,
+                            SelectionDAG &DAG, const X86Subtarget &Subtarget) {
+  // First instruction should be a right shift of a multiply.
+  if (Src.getOpcode() != ISD::SRL ||
+      Src.getOperand(0).getOpcode() != ISD::MUL)
+    return SDValue();
+
+  if (!Subtarget.hasSSE2())
+    return SDValue();
+
+  // Only handle vXi16 types that are at least 128-bits.
+  if (!VT.isVector() || VT.getVectorElementType() != MVT::i16 ||
+      VT.getVectorNumElements() < 8)
+    return SDValue();
+
+  // Input type should be vXi32.
+  EVT InVT = Src.getValueType();
+  if (InVT.getVectorElementType() != MVT::i32)
+    return SDValue();
+
+  // Need a shift by 16.
+  APInt ShiftAmt;
+  if (!ISD::isConstantSplatVector(Src.getOperand(1).getNode(), ShiftAmt) ||
+      ShiftAmt != 16)
+    return SDValue();
+
+  SDValue LHS = Src.getOperand(0).getOperand(0);
+  SDValue RHS = Src.getOperand(0).getOperand(1);
+
+  unsigned ExtOpc = LHS.getOpcode();
+  if ((ExtOpc != ISD::SIGN_EXTEND && ExtOpc != ISD::ZERO_EXTEND) ||
+      RHS.getOpcode() != ExtOpc)
+    return SDValue();
+
+  // Peek through the extends.
+  LHS = LHS.getOperand(0);
+  RHS = RHS.getOperand(0);
+
+  // Ensure the input types match.
+  if (LHS.getValueType() != VT || RHS.getValueType() != VT)
+    return SDValue();
+
+  unsigned Opc = ExtOpc == ISD::SIGN_EXTEND ? ISD::MULHS : ISD::MULHU;
+  return DAG.getNode(Opc, DL, VT, LHS, RHS);
+}
+
 static SDValue combineTruncate(SDNode *N, SelectionDAG &DAG,
                                const X86Subtarget &Subtarget) {
   EVT VT = N->getValueType(0);
@@ -36124,6 +36177,10 @@ static SDValue combineTruncate(SDNode *N, SelectionDAG &DAG,
   if (SDValue Val = combineTruncateWithSat(Src, VT, DL, DAG, Subtarget))
     return Val;
 
+  // Try to combine PMULHUW/PMULHW for vXi16.
+  if (SDValue V = combinePMULH(Src, VT, DL, DAG, Subtarget))
+    return V;
+
   // The bitcast source is a direct mmx result.
   // Detect bitcasts between i32 to x86mmx
   if (Src.getOpcode() == ISD::BITCAST && VT == MVT::i32) {