[ARM] Flatten identity shuffles through vqdmulh nodes

[dmgreen]

Summary

Given a shuffle(vqdmulh(shuffle, shuffle), we can flatter the shuffles out if they become an identity mask. This can come up during lane interleaving, when we do that better.

[samtebbs]

llvm/lib/Target/ARM/ARMISelLowering.cpp

13974

Small typo, regognize -> recognise/recognize

[dmgreen]

Fix typo.

[samtebbs]

Nice 👍🏻

llvm/lib/Target/ARM/ARMISelLowering.cpp

while (X.getValueType() != VT && X->getOpcode() == ISD::BITCAST)
       X = X->getOperand(0);
     if (X.getValueType() == VT)
       return X;
   }
 
   return SDValue();
 }
 
+// When lowering complex nodes that we regognize, like VQDMULH and MULH, we
+// can end up with shuffle(binop(shuffle, shuffle)), that can be simplified to
+// binop as the shuffles cancel out.
+static SDValue FlattenVectorShuffle(ShuffleVectorSDNode *N, SelectionDAG &DAG) {
+  EVT VT = N->getValueType(0);
+  if (!N->getOperand(1).isUndef() || N->getOperand(0).getValueType() != VT)
+    return SDValue();
+  SDValue Op = N->getOperand(0);
+
+  // Looking for binary operators that will have been folded from
+  // truncates/extends.
+  switch (Op.getOpcode()) {
+  case ARMISD::VQDMULH:
+  case ISD::MULHS:
+  case ISD::MULHU:
+    break;
+  default:
+    return SDValue();
+  }
+
+  ShuffleVectorSDNode *Op0 = dyn_cast<ShuffleVectorSDNode>(Op.getOperand(0));
+  ShuffleVectorSDNode *Op1 = dyn_cast<ShuffleVectorSDNode>(Op.getOperand(1));
+  if (!Op0 || !Op1 || !Op0->getOperand(1).isUndef() ||
+      !Op1->getOperand(1).isUndef() || Op0->getMask() != Op1->getMask() ||
+      Op0->getOperand(0).getValueType() != VT)
+    return SDValue();
+
+  // Check the mask turns into an identity shuffle.
+  ArrayRef<int> NMask = N->getMask();
+  ArrayRef<int> OpMask = Op0->getMask();
+  for (int i = 0, e = NMask.size(); i != e; i++) {
+    if (NMask[i] > 0 && OpMask[NMask[i]] > 0 && OpMask[NMask[i]] != i)
+      return SDValue();
+  }
+
+  return DAG.getNode(Op.getOpcode(), SDLoc(Op), Op.getValueType(),
+                     Op0->getOperand(0), Op1->getOperand(0));
+}
+
 /// PerformVECTOR_SHUFFLECombine - Target-specific dag combine xforms for
 /// ISD::VECTOR_SHUFFLE.
 static SDValue PerformVECTOR_SHUFFLECombine(SDNode *N, SelectionDAG &DAG) {
+  if (SDValue R = FlattenVectorShuffle(cast<ShuffleVectorSDNode>(N), DAG))
+    return R;
+
   // The LLVM shufflevector instruction does not require the shuffle mask
   // length to match the operand vector length, but ISD::VECTOR_SHUFFLE does
   // have that requirement.  When translating to ISD::VECTOR_SHUFFLE, if the
   // operands do not match the mask length, they are extended by concatenating
   // them with undef vectors.  That is probably the right thing for other
   // targets, but for NEON it is better to concatenate two double-register
   // size vector operands into a single quad-register size vector.  Do that
   // transformation here:

llvm/test/CodeGen/Thumb2/mve-vqdmulh.ll

entry:
   %l9 = select <8 x i1> %l8, <8 x i22> %l7, <8 x i22> <i22 32767, i22 32767, i22 32767, i22 32767, i22 32767, i22 32767, i22 32767, i22 32767>
   %l10 = trunc <8 x i22> %l9 to <8 x i16>
   ret <8 x i16> %l10
 }
 
 define arm_aapcs_vfpcc <8 x i16> @vqdmulh_i16_interleaved(<8 x i16> %s0, <8 x i16> %s1) {
 ; CHECK-LABEL: vqdmulh_i16_interleaved:
 ; CHECK:       @ %bb.0: @ %entry
-; CHECK-NEXT:    vmov.u16 r0, q0[0]
-; CHECK-NEXT:    vmov.16 q2[0], r0
-; CHECK-NEXT:    vmov.u16 r0, q0[2]
-; CHECK-NEXT:    vmov.16 q2[1], r0
-; CHECK-NEXT:    vmov.u16 r0, q0[4]
-; CHECK-NEXT:    vmov.16 q2[2], r0
-; CHECK-NEXT:    vmov.u16 r0, q0[6]
-; CHECK-NEXT:    vmov.16 q2[3], r0
-; CHECK-NEXT:    vmov.u16 r0, q0[1]
-; CHECK-NEXT:    vmov.16 q2[4], r0
-; CHECK-NEXT:    vmov.u16 r0, q0[3]
-; CHECK-NEXT:    vmov.16 q2[5], r0
-; CHECK-NEXT:    vmov.u16 r0, q0[5]
-; CHECK-NEXT:    vmov.16 q2[6], r0
-; CHECK-NEXT:    vmov.u16 r0, q0[7]
-; CHECK-NEXT:    vmov.16 q2[7], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[0]
-; CHECK-NEXT:    vmov.16 q0[0], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[2]
-; CHECK-NEXT:    vmov.16 q0[1], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[4]
-; CHECK-NEXT:    vmov.16 q0[2], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[6]
-; CHECK-NEXT:    vmov.16 q0[3], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[1]
-; CHECK-NEXT:    vmov.16 q0[4], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[3]
-; CHECK-NEXT:    vmov.16 q0[5], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[5]
-; CHECK-NEXT:    vmov.16 q0[6], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[7]
-; CHECK-NEXT:    vmov.16 q0[7], r0
-; CHECK-NEXT:    vqdmulh.s16 q1, q0, q2
-; CHECK-NEXT:    vmov.u16 r0, q1[0]
-; CHECK-NEXT:    vmov.16 q0[0], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[4]
-; CHECK-NEXT:    vmov.16 q0[1], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[1]
-; CHECK-NEXT:    vmov.16 q0[2], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[5]
-; CHECK-NEXT:    vmov.16 q0[3], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[2]
-; CHECK-NEXT:    vmov.16 q0[4], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[6]
-; CHECK-NEXT:    vmov.16 q0[5], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[3]
-; CHECK-NEXT:    vmov.16 q0[6], r0
-; CHECK-NEXT:    vmov.u16 r0, q1[7]
-; CHECK-NEXT:    vmov.16 q0[7], r0
+; CHECK-NEXT:    vqdmulh.s16 q0, q1, q0
 ; CHECK-NEXT:    bx lr
 entry:
   %0 = shufflevector <8 x i16> %s0, <8 x i16> undef, <8 x i32> <i32 0, i32 2, i32 4, i32 6, i32 1, i32 3, i32 5, i32 7>
   %1 = sext <8 x i16> %0 to <8 x i32>
   %l2 = sext <8 x i16> %s0 to <8 x i32>
   %2 = shufflevector <8 x i16> %s1, <8 x i16> undef, <8 x i32> <i32 0, i32 2, i32 4, i32 6, i32 1, i32 3, i32 5, i32 7>
   %3 = sext <8 x i16> %2 to <8 x i32>
   %l5 = sext <8 x i16> %s1 to <8 x i32>