[InstCombine] fold icmp of select with constants and invertible op

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -4332,6 +4332,98 @@ Instruction *InstCombinerImpl::foldICmpInstWithConstantNotInt(ICmpInst &I) {
   return nullptr;
 }
 
+/// If the APInt C has the same invertible function with Operator RefOp in Pred,
+/// return the operands of the function corresponding to each input. Otherwise,
+/// return std::nullopt. This is equivalent to saying that Op1 pred Op2 is true
+/// exactly when the specified pair of RefOp pred C is true.
+/// alive2: https://alive2.llvm.org/ce/z/4jniEb
+static std::optional<std::pair<Value *, Value *>>
+getInvertibleOperandsWithPredicte(const Operator *RefOp, const APInt C,
+                                  CmpInst::Predicate Pred) {
+  APInt Op1C;
+  // for BinaryOperator just handle RefOp with constant Operand(1)
+  if (isa<BinaryOperator>(RefOp)) {
+    if (isa<ConstantInt>(RefOp->getOperand(1)))
+      Op1C = cast<ConstantInt>(RefOp->getOperand(1))->getValue();
+    else
+      return std::nullopt;
+  }
+
+  auto getOperands = [&](APInt A) -> auto {
+    return std::make_pair(RefOp->getOperand(0),
+                          ConstantInt::get(RefOp->getOperand(0)->getType(), A));
+  };
+  switch (RefOp->getOpcode()) {
+  default:
+    break;
+  case Instruction::Or:
+    if (cast<PossiblyDisjointInst>(RefOp)->isDisjoint() && ((C & Op1C) == Op1C))
+      return getOperands(C ^ Op1C);
+    break;
+  case Instruction::Add: {
+    // TODO: add/sub could support nsw/nuw for scmp/ucmp
+    if (CmpInst::isEquality(Pred))
+      return getOperands(C - Op1C);
+    break;
+  }
+  case Instruction::Xor: {
+    if (CmpInst::isEquality(Pred))
+      return getOperands(C ^ Op1C);
+    break;
+  }
+  case Instruction::Sub: {
+    if (CmpInst::isEquality(Pred))
+      return getOperands(C + Op1C);
+    break;
+  }
+  // alive2: https://alive2.llvm.org/ce/z/WPQznV
+  case Instruction::Shl: {
+    // Z = shl nsw X, Y <=> X = ashr exact Z, Y
+    // Z = shl nuw X, Y <=> X = lshr exact Z, Y
+    if (C.ashr(Op1C).shl(Op1C) == C) {
+      auto *OBO1 = cast<OverflowingBinaryOperator>(RefOp);
+      if (OBO1->hasNoSignedWrap())
+        return getOperands(C.ashr(Op1C));
+      else if (OBO1->hasNoUnsignedWrap() && !ICmpInst::isSigned(Pred))
+        return getOperands(C.lshr(Op1C));
+    }
+    break;
+  }
+  case Instruction::AShr: {
+    // Z = ashr exact X, Y <=> X = shl nsw Z, Y
+    auto *PEO1 = cast<PossiblyExactOperator>(RefOp);
+    if (PEO1->isExact() && C.shl(Op1C).ashr(Op1C) == C)
+      return getOperands(C.shl(Op1C));
+    break;
+  }
+  case Instruction::LShr: {
+    // Z = lshr exact X, Y <=> X = shl nuw Z, Y
+    auto *PEO1 = cast<PossiblyExactOperator>(RefOp);
+    if (PEO1->isExact() && C.shl(Op1C).lshr(Op1C) == C &&
+        !ICmpInst::isSigned(Pred))
+      return getOperands(C.shl(Op1C));
+    break;
+  }
+  case Instruction::SExt: {
+    unsigned NumBits = RefOp->getType()->getScalarSizeInBits();
+    unsigned NumBitsOp0 =
+        RefOp->getOperand(0)->getType()->getScalarSizeInBits();
+    if (C.trunc(NumBitsOp0).sext(NumBits) == C)
+      return getOperands(C.trunc(NumBitsOp0));
+    break;
+  }
+  case Instruction::ZExt: {
+    unsigned NumBits = RefOp->getType()->getScalarSizeInBits();
+    unsigned NumBitsOp0 =
+        RefOp->getOperand(0)->getType()->getScalarSizeInBits();
+    if (C.trunc(NumBitsOp0).zext(NumBits) == C && !ICmpInst::isSigned(Pred))
+      return getOperands(C.trunc(NumBitsOp0));
+    break;
+  }
+  }
+  return std::nullopt;
+}
+
 Instruction *InstCombinerImpl::foldSelectICmp(CmpPredicate Pred, SelectInst *SI,
                                               Value *RHS, const ICmpInst &I) {
   // Try to fold the comparison into the select arms, which will cause the
@@ -4391,6 +4483,24 @@ Instruction *InstCombinerImpl::foldSelectICmp(CmpPredicate Pred, SelectInst *SI,
     return SelectInst::Create(SI->getOperand(0), Op1, Op2);
   }
 
+  // fold select with constants and invertible op
+  Value *Cond;
+  const APInt *C1, *C2;
+  auto *RHSOp = dyn_cast<Operator>(RHS);
+  if (RHSOp &&
+      match(SI, m_OneUse(m_Select(m_Value(Cond), m_APInt(C1), m_APInt(C2))))) {
+    if (auto Values0 = getInvertibleOperandsWithPredicte(RHSOp, *C1, Pred)) {
+      if (auto Values1 = getInvertibleOperandsWithPredicte(RHSOp, *C2, Pred)) {
+        assert(Values0->first == Values1->first &&
+               "Invertible Operand0 mismatch");
+        auto *NewSI = Builder.CreateSelect(Cond, Values0->second,
+                                           Values1->second, SI->getName());
+        return ICmpInst::Create(Instruction::ICmp, I.getPredicate(), NewSI,
+                                Values0->first, I.getName());
+      }
+    }
+  }
+
   return nullptr;
 }