[DAG] Add BuildVectorSDNode::getRepeatedSequence helper to recognise multi-element splat patterns

[RKSimon]

Summary

Replace the X86 specific isSplatZeroExtended helper with a generic BuildVectorSDNode method.

I've just used this to simplify the X86ISD::BROADCASTM lowering so far (and remove isSplatZeroExtended), but we should be able to use this in more places to lower to complex broadcast patterns.

[RKSimon]

Rebase and improve getRepeatedSequence handling of undefs when matching the sequence

[RKSimon]

ping?

[RKSimon]

ping?

[spatel]

I'm assuming we have enough splat codegen tests to be confident that the replacement code behaved the same.
Do you think it's worth creating an SDAG unit test for the new function to further verify?

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

1917

punctuation: change ; to .

[yubing]

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

9830–9831

If we have a BV = [0,1,2,3,0,1,2,3], DemandedElts is 0b11011101, Shouldn't we return a Sequence = [0,undef,2,3]? But it seems if !DemandedElts[I], we will never set SeqOp, so the Sequence[1] will be SDValue().

[RKSimon]

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

9830–9831

We can't create undefs on the fly without access to the SelectionDAG (this method is inside BuildVectorSDNode). If we're not demanding this element at all then it should be safe just to use a null SDValue()

[RKSimon]

Added unit tests inside AArch64SelectionDAGTest

[spatel]

LGTM

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

1927

It would be nice to include an example or two - something similar to what is shown in the unit test.

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

SDValue getSplatValue(const APInt &DemandedElts,
                         BitVector *UndefElements = nullptr) const;
 
   /// Returns the splatted value or a null value if this is not a splat.
   ///
   /// If passed a non-null UndefElements bitvector, it will resize it to match
   /// the vector width and set the bits where elements are undef.
   SDValue getSplatValue(BitVector *UndefElements = nullptr) const;
 
+  /// Find the shortest repeating sequence of values in the build vector.
+  ///
+  /// Currently this must be a power-of-2 build vector.
+  /// The DemandedElts mask indicates the elements that must be present.
+  /// If passed a non-null UndefElements bitvector, it will resize it to match
+  /// the vector width and set the bits where elements are undef.
+  /// If result is false, Sequence will be empty;
+  bool getRepeatedSequence(const APInt &DemandedElts,
+                           SmallVectorImpl<SDValue> &Sequence,
+                           BitVector *UndefElements = nullptr) const;
+
+  /// Find the shortest repeating sequence of values in the build vector.
+  ///
+  /// Currently this must be a power-of-2 build vector.
+  /// If passed a non-null UndefElements bitvector, it will resize it to match
+  /// the vector width and set the bits where elements are undef.
+  /// If result is false, Sequence will be empty;
+  bool getRepeatedSequence(SmallVectorImpl<SDValue> &Sequence,
+                           BitVector *UndefElements = nullptr) const;
+
   /// Returns the demanded splatted constant or null if this is not a constant
   /// splat.
   ///
   /// The DemandedElts mask indicates the elements that must be in the splat.
   /// If passed a non-null UndefElements bitvector, it will resize it to match
   /// the vector width and set the bits where elements are undef.
   ConstantSDNode *
   getConstantSplatNode(const APInt &DemandedElts,

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

SDValue BuildVectorSDNode::getSplatValue(const APInt &DemandedElts,
   return Splatted;
 }
 
 SDValue BuildVectorSDNode::getSplatValue(BitVector *UndefElements) const {
   APInt DemandedElts = APInt::getAllOnesValue(getNumOperands());
   return getSplatValue(DemandedElts, UndefElements);
 }
 
+bool BuildVectorSDNode::getRepeatedSequence(const APInt &DemandedElts,
+                                            SmallVectorImpl<SDValue> &Sequence,
+                                            BitVector *UndefElements) const {
+  unsigned NumOps = getNumOperands();
+  Sequence.clear();
+  if (UndefElements) {
+    UndefElements->clear();
+    UndefElements->resize(NumOps);
+  }
+  assert(NumOps == DemandedElts.getBitWidth() && "Unexpected vector size");
+  if (!DemandedElts || NumOps < 2 || !isPowerOf2_32(NumOps))
+    return false;
+
+  // Set the undefs even if we don't find a sequence (like getSplatValue).
+  if (UndefElements)
+    for (unsigned I = 0; I != NumOps; ++I)
+      if (DemandedElts[I] && getOperand(I).isUndef())
+        (*UndefElements)[I] = true;
+
+  // Iteratively widen the sequence length looking for repetitions.
+  for (unsigned SeqLen = 1; SeqLen < NumOps; SeqLen *= 2) {
+    Sequence.append(SeqLen, SDValue());
+    for (unsigned I = 0; I != NumOps; ++I) {
+      if (!DemandedElts[I])
+        continue;
+      SDValue &SeqOp = Sequence[I % SeqLen];
+      SDValue Op = getOperand(I);
+      if (Op.isUndef()) {
+        if (!SeqOp)
+          SeqOp = Op;
+        continue;
+      }
+      if (SeqOp && !SeqOp.isUndef() && SeqOp != Op) {
+        Sequence.clear();
+        break;
+      }
+      SeqOp = Op;
+    }
+    if (!Sequence.empty())
+      return true;
+  }
+
+  assert(Sequence.empty() && "Failed to empty non-repeating sequence pattern");
+  return false;
+}
+
+bool BuildVectorSDNode::getRepeatedSequence(SmallVectorImpl<SDValue> &Sequence,
+                                            BitVector *UndefElements) const {
+  APInt DemandedElts = APInt::getAllOnesValue(getNumOperands());
+  return getRepeatedSequence(DemandedElts, Sequence, UndefElements);
+}
+
 ConstantSDNode *
 BuildVectorSDNode::getConstantSplatNode(const APInt &DemandedElts,
                                         BitVector *UndefElements) const {
   return dyn_cast_or_null<ConstantSDNode>(
       getSplatValue(DemandedElts, UndefElements));
 }
 
 ConstantSDNode *

llvm/lib/Target/X86/X86ISelLowering.cpp

for (auto *U : N->uses()) {
     if (Opc == ISD::BITCAST) // Ignore bitcasts
       return isFoldableUseOfShuffle(U);
     if (N->hasOneUse())
       return true;
   }
   return false;
 }
 
-// Check if the current node of build vector is a zero extended vector.
-// // If so, return the value extended.
-// // For example: (0,0,0,a,0,0,0,a,0,0,0,a,0,0,0,a) returns a.
-// // NumElt - return the number of zero extended identical values.
-// // EltType - return the type of the value include the zero extend.
-static SDValue isSplatZeroExtended(const BuildVectorSDNode *Op,
-                                   unsigned &NumElt, MVT &EltType) {
-  SDValue ExtValue = Op->getOperand(0);
-  unsigned NumElts = Op->getNumOperands();
-  unsigned Delta = NumElts;
-
-  for (unsigned i = 1; i < NumElts; i++) {
-    if (Op->getOperand(i) == ExtValue) {
-      Delta = i;
-      break;
-    }
-    if (!(Op->getOperand(i).isUndef() || isNullConstant(Op->getOperand(i))))
-      return SDValue();
-  }
-  if (!isPowerOf2_32(Delta) || Delta == 1)
-    return SDValue();
-
-  for (unsigned i = Delta; i < NumElts; i++) {
-    if (i % Delta == 0) {
-      if (Op->getOperand(i) != ExtValue)
-        return SDValue();
-    } else if (!(isNullConstant(Op->getOperand(i)) ||
-                 Op->getOperand(i).isUndef()))
-      return SDValue();
-  }
-  unsigned EltSize = Op->getSimpleValueType(0).getScalarSizeInBits();
-  unsigned ExtVTSize = EltSize * Delta;
-  EltType = MVT::getIntegerVT(ExtVTSize);
-  NumElt = NumElts / Delta;
-  return ExtValue;
-}
-
 /// Attempt to use the vbroadcast instruction to generate a splat value
 /// from a splat BUILD_VECTOR which uses:
 ///  a. A single scalar load, or a constant.
 ///  b. Repeated pattern of constants (e.g. <0,1,0,1> or <0,1,2,3,0,1,2,3>).
 ///
 /// The VBROADCAST node is returned when a pattern is found,
 /// or SDValue() otherwise.
 static SDValue lowerBuildVectorAsBroadcast(BuildVectorSDNode *BVOp,
                                            const X86Subtarget &Subtarget,
                                            SelectionDAG &DAG) {
   // VBROADCAST requires AVX.
   // TODO: Splats could be generated for non-AVX CPUs using SSE
   // instructions, but there's less potential gain for only 128-bit vectors.
   if (!Subtarget.hasAVX())
     return SDValue();
 
   MVT VT = BVOp->getSimpleValueType(0);
+  unsigned NumElts = VT.getVectorNumElements();
   SDLoc dl(BVOp);
 
   assert((VT.is128BitVector() || VT.is256BitVector() || VT.is512BitVector()) &&
          "Unsupported vector type for broadcast.");
 
+  // See if the build vector is a repeating sequence of scalars (inc. splat).
+  SDValue Ld;
   BitVector UndefElements;
-  SDValue Ld = BVOp->getSplatValue(&UndefElements);
+  SmallVector<SDValue, 16> Sequence;
+  if (BVOp->getRepeatedSequence(Sequence, &UndefElements)) {
+    assert((NumElts % Sequence.size()) == 0 && "Sequence doesn't fit.");
+    if (Sequence.size() == 1)
+      Ld = Sequence[0];
+  }
 
   // Attempt to use VBROADCASTM
   // From this pattern:
   // a. t0 = (zext_i64 (bitcast_i8 v2i1 X))
   // b. t1 = (build_vector t0 t0)
   //
   // Create (VBROADCASTM v2i1 X)
-  if (Subtarget.hasCDI()) {
-    MVT EltType = VT.getScalarType();
-    unsigned NumElts = VT.getVectorNumElements();
-    SDValue BOperand;
-    SDValue ZeroExtended = isSplatZeroExtended(BVOp, NumElts, EltType);
-    if ((ZeroExtended && ZeroExtended.getOpcode() == ISD::BITCAST) ||
-        (ZeroExtended && ZeroExtended.getOpcode() == ISD::ZERO_EXTEND &&
-         ZeroExtended.getOperand(0).getOpcode() == ISD::BITCAST) ||
-        (Ld && Ld.getOpcode() == ISD::ZERO_EXTEND &&
-         Ld.getOperand(0).getOpcode() == ISD::BITCAST)) {
-      if (ZeroExtended && ZeroExtended.getOpcode() == ISD::BITCAST)
-        BOperand = ZeroExtended.getOperand(0);
-      else if (ZeroExtended)
-        BOperand = ZeroExtended.getOperand(0).getOperand(0);
-      else
-        BOperand = Ld.getOperand(0).getOperand(0);
+  if (!Sequence.empty() && Subtarget.hasCDI()) {
+    // If not a splat, are the upper sequence values zeroable?
+    unsigned SeqLen = Sequence.size();
+    bool UpperZeroOrUndef =
+        SeqLen == 1 ||
+        llvm::all_of(makeArrayRef(Sequence).drop_front(), [](SDValue V) {
+          return V.isUndef() || isNullConstant(V);
+        });
+    SDValue Op0 = Sequence[0];
+    if (UpperZeroOrUndef && ((Op0.getOpcode() == ISD::BITCAST) ||
+                             (Op0.getOpcode() == ISD::ZERO_EXTEND &&
+                              Op0.getOperand(0).getOpcode() == ISD::BITCAST))) {
+      SDValue BOperand = Op0.getOpcode() == ISD::BITCAST
+                             ? Op0.getOperand(0)
+                             : Op0.getOperand(0).getOperand(0);
       MVT MaskVT = BOperand.getSimpleValueType();
+      MVT EltType = MVT::getIntegerVT(VT.getScalarSizeInBits() * SeqLen);
       if ((EltType == MVT::i64 && MaskVT == MVT::v8i1) ||  // for broadcastmb2q
           (EltType == MVT::i32 && MaskVT == MVT::v16i1)) { // for broadcastmw2d
-        MVT BcstVT = MVT::getVectorVT(EltType, NumElts);
+        MVT BcstVT = MVT::getVectorVT(EltType, NumElts / SeqLen);
         if (!VT.is512BitVector() && !Subtarget.hasVLX()) {
           unsigned Scale = 512 / VT.getSizeInBits();
-          BcstVT = MVT::getVectorVT(EltType, NumElts * Scale);
+          BcstVT = MVT::getVectorVT(EltType, Scale * (NumElts / SeqLen));
         }
         SDValue Bcst = DAG.getNode(X86ISD::VBROADCASTM, dl, BcstVT, BOperand);
         if (BcstVT.getSizeInBits() != VT.getSizeInBits())
           Bcst = extractSubVector(Bcst, 0, DAG, dl, VT.getSizeInBits());
         return DAG.getBitcast(VT, Bcst);
       }
     }
   }
 
-  unsigned NumElts = VT.getVectorNumElements();
   unsigned NumUndefElts = UndefElements.count();
   if (!Ld || (NumElts - NumUndefElts) <= 1) {
     APInt SplatValue, Undef;
     unsigned SplatBitSize;
     bool HasUndef;
     // Check if this is a repeated constant pattern suitable for broadcasting.
     if (BVOp->isConstantSplat(SplatValue, Undef, SplatBitSize, HasUndef) &&
         SplatBitSize > VT.getScalarSizeInBits() &&
if (!Ld || (NumElts - NumUndefElts) <= 1) {
     if (!(UndefElements[0] || (ScalarSize != 32 && ScalarSize != 64)))
       return SDValue();
   }
 
   bool ConstSplatVal =
       (Ld.getOpcode() == ISD::Constant || Ld.getOpcode() == ISD::ConstantFP);
   bool IsLoad = ISD::isNormalLoad(Ld.getNode());
 
+  // TODO: Handle broadcasts of non-constant sequences.
+
   // Make sure that all of the users of a non-constant load are from the
   // BUILD_VECTOR node.
   // FIXME: Is the use count needed for non-constant, non-load case?
   if (!ConstSplatVal && !IsLoad && !BVOp->isOnlyUserOf(Ld.getNode()))
     return SDValue();
 
   unsigned ScalarSize = Ld.getValueSizeInBits();
   bool IsGE256 = (VT.getSizeInBits() >= 256);