[DAG] Handle truncated splat in isBoolConstant

This allows truncated splat / buildvector in isBoolConstant, to allow certain
not instructions to be recognized post-legalization, and allow vselect to
optimize.

An override for x86 avx512 predicated vectors is required to avoid an infinite
recursion from the code that detects zero vectors. From:
```
  // Check if the first operand is all zeros and Cond type is vXi1.
  // If this an avx512 target we can improve the use of zero masking by
  // swapping the operands and inverting the condition.
```

Author: davemgreen

llvm/include/llvm/CodeGen/SelectionDAG.h

@@ -2479,8 +2479,7 @@ class SelectionDAG {
 
   /// Check if a value \op N is a constant using the target's BooleanContent for
   /// its type.
-  LLVM_ABI std::optional<bool>
-  isBoolConstant(SDValue N, bool AllowTruncation = false) const;
+  LLVM_ABI std::optional<bool> isBoolConstant(SDValue N) const;
 
   /// Set CallSiteInfo to be associated with Node.
   void addCallSiteInfo(const SDNode *Node, CallSiteInfo &&CallInfo) {

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -4375,6 +4375,8 @@ class LLVM_ABI TargetLowering : public TargetLoweringBase {
            Op.getOpcode() == ISD::SPLAT_VECTOR_PARTS;
   }
 
+  virtual bool isTargetCanonicalSelect(SDNode *N) const { return false; }
+
   struct DAGCombinerInfo {
     void *DC;  // The DAG Combiner object.
     CombineLevel Level;

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -12967,8 +12967,9 @@ SDValue DAGCombiner::visitVSELECT(SDNode *N) {
     return V;
 
   // vselect (not Cond), N1, N2 -> vselect Cond, N2, N1
-  if (SDValue F = extractBooleanFlip(N0, DAG, TLI, false))
-    return DAG.getSelect(DL, VT, F, N2, N1);
+  if (!TLI.isTargetCanonicalSelect(N))
+    if (SDValue F = extractBooleanFlip(N0, DAG, TLI, false))
+      return DAG.getSelect(DL, VT, F, N2, N1);
 
   // select (sext m), (add X, C), X --> (add X, (and C, (sext m))))
   if (N1.getOpcode() == ISD::ADD && N1.getOperand(0) == N2 && N1->hasOneUse() &&

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -10349,7 +10349,7 @@ SDValue SelectionDAG::simplifySelect(SDValue Cond, SDValue T, SDValue F) {
 
   // select true, T, F --> T
   // select false, T, F --> F
-  if (auto C = isBoolConstant(Cond, /*AllowTruncation=*/true))
+  if (auto C = isBoolConstant(Cond))
     return *C ? T : F;
 
   // select ?, T, T --> T
@@ -13562,13 +13562,14 @@ bool SelectionDAG::isConstantFPBuildVectorOrConstantFP(SDValue N) const {
   return false;
 }
 
-std::optional<bool> SelectionDAG::isBoolConstant(SDValue N,
-                                                 bool AllowTruncation) const {
-  ConstantSDNode *Const = isConstOrConstSplat(N, false, AllowTruncation);
+std::optional<bool> SelectionDAG::isBoolConstant(SDValue N) const {
+  ConstantSDNode *Const =
+      isConstOrConstSplat(N, false, /*AllowTruncation=*/true);
   if (!Const)
     return std::nullopt;
 
-  const APInt &CVal = Const->getAPIntValue();
+  EVT VT = N->getValueType(0);
+  const APInt CVal = Const->getAPIntValue().trunc(VT.getScalarSizeInBits());
   switch (TLI->getBooleanContents(N.getValueType())) {
   case TargetLowering::ZeroOrOneBooleanContent:
     if (CVal.isOne())

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -4975,6 +4975,15 @@ X86TargetLowering::getTargetConstantFromLoad(LoadSDNode *LD) const {
   return getTargetConstantFromNode(LD);
 }
 
+bool X86TargetLowering::isTargetCanonicalSelect(SDNode *N) const {
+  SDValue Cond = N->getOperand(0);
+  SDValue RHS = N->getOperand(2);
+  EVT CondVT = Cond.getValueType();
+  return N->getOpcode() == ISD::VSELECT && Subtarget.hasAVX512() &&
+         CondVT.getVectorElementType() == MVT::i1 &&
+         ISD::isBuildVectorAllZeros(RHS.getNode());
+}
+
 // Extract raw constant bits from constant pools.
 static bool getTargetConstantBitsFromNode(SDValue Op, unsigned EltSizeInBits,
                                           APInt &UndefElts,

llvm/lib/Target/X86/X86ISelLowering.h

@@ -1356,6 +1356,8 @@ namespace llvm {
              TargetLowering::isTargetCanonicalConstantNode(Op);
     }
 
+    bool isTargetCanonicalSelect(SDNode *N) const override;
+
     const Constant *getTargetConstantFromLoad(LoadSDNode *LD) const override;
 
     SDValue unwrapAddress(SDValue N) const override;