polly/lib/Support/SCEVValidator.cpp

#include "polly/Support/SCEVValidator.h"

#define DEBUG_TYPE "polly-scev-validator"
#include "llvm/Support/Debug.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/RegionInfo.h"

#include <vector>

using namespace llvm;

namespace SCEVType {
/// @brief The type of a SCEV
///
/// To check for the validity of a SCEV we assign to each SCEV a type. The
/// possible types are INT, PARAM, IV and INVALID. The order of the types is
/// important. The subexpressions of SCEV with a type X can only have a type
/// that is smaller or equal than X.
enum TYPE {
  // An integer value.
  INT,

  // An expression that is constant during the execution of the Scop,
  // but that may depend on parameters unknown at compile time.
  PARAM,

  // An expression that may change during the execution of the SCoP.
  IV,

  // An invalid expression.
  INVALID
};
}

/// @brief The result the validator returns for a SCEV expression.
class ValidatorResult {
  /// @brief The type of the expression
  SCEVType::TYPE Type;

  /// @brief The set of Parameters in the expression.
  std::vector<const SCEV *> Parameters;

public:
  /// @brief The copy constructor
  ValidatorResult(const ValidatorResult &Source) {
    Type = Source.Type;
    Parameters = Source.Parameters;
  }

  /// @brief Construct a result with a certain type and no parameters.
  ValidatorResult(SCEVType::TYPE Type) : Type(Type) {
    assert(Type != SCEVType::PARAM && "Did you forget to pass the parameter");
  }

  /// @brief Construct a result with a certain type and a single parameter.
  ValidatorResult(SCEVType::TYPE Type, const SCEV *Expr) : Type(Type) {
    Parameters.push_back(Expr);
  }

  /// @brief Get the type of the ValidatorResult.
  SCEVType::TYPE getType() { return Type; }

  /// @brief Is the analyzed SCEV constant during the execution of the SCoP.
  bool isConstant() { return Type == SCEVType::INT || Type == SCEVType::PARAM; }

  /// @brief Is the analyzed SCEV valid.
  bool isValid() { return Type != SCEVType::INVALID; }

  /// @brief Is the analyzed SCEV of Type IV.
  bool isIV() { return Type == SCEVType::IV; }

  /// @brief Is the analyzed SCEV of Type INT.
  bool isINT() { return Type == SCEVType::INT; }

  /// @brief Is the analyzed SCEV of Type PARAM.
  bool isPARAM() { return Type == SCEVType::PARAM; }

  /// @brief Get the parameters of this validator result.
  std::vector<const SCEV *> getParameters() { return Parameters; }

  /// @brief Add the parameters of Source to this result.
  void addParamsFrom(class ValidatorResult &Source) {
    Parameters.insert(Parameters.end(), Source.Parameters.begin(),
                      Source.Parameters.end());
  }

  /// @brief Merge a result.
  ///
  /// This means to merge the parameters and to set the Type to the most
  /// specific Type that matches both.
  void merge(class ValidatorResult &ToMerge) {
    Type = std::max(Type, ToMerge.Type);
    addParamsFrom(ToMerge);
  }

  void print(raw_ostream &OS) {
    switch (Type) {
    case SCEVType::INT:
      OS << "SCEVType::INT";
      break;
    case SCEVType::PARAM:
      OS << "SCEVType::PARAM";
      break;
    case SCEVType::IV:
      OS << "SCEVType::IV";
      break;
    case SCEVType::INVALID:
      OS << "SCEVType::INVALID";
      break;
    }
  }
};

raw_ostream &operator<<(raw_ostream &OS, class ValidatorResult &VR) {
  VR.print(OS);
  return OS;
}

/// Check if a SCEV is valid in a SCoP.
struct SCEVValidator :
    public SCEVVisitor<SCEVValidator, class ValidatorResult> {
private:
  const Region *R;
  ScalarEvolution &SE;
  const Value *BaseAddress;

public:
  SCEVValidator(const Region *R, ScalarEvolution &SE, const Value *BaseAddress)
      : R(R), SE(SE), BaseAddress(BaseAddress) {}

  class ValidatorResult visitConstant(const SCEVConstant *Constant) {
    return ValidatorResult(SCEVType::INT);
  }

  class ValidatorResult visitTruncateExpr(const SCEVTruncateExpr *Expr) {
    ValidatorResult Op = visit(Expr->getOperand());

    switch (Op.getType()) {
    case SCEVType::INT:
    case SCEVType::PARAM:
      // We currently do not represent a truncate expression as an affine
      // expression. If it is constant during Scop execution, we treat it as a
      // parameter.
      return ValidatorResult(SCEVType::PARAM, Expr);
    case SCEVType::IV:
      DEBUG(dbgs() << "INVALID: Truncation of SCEVType::IV expression");
      return ValidatorResult(SCEVType::INVALID);
    case SCEVType::INVALID:
      return Op;
    }

    llvm_unreachable("Unknown SCEVType");
  }

  class ValidatorResult visitZeroExtendExpr(const SCEVZeroExtendExpr *Expr) {
    ValidatorResult Op = visit(Expr->getOperand());

    switch (Op.getType()) {
    case SCEVType::INT:
    case SCEVType::PARAM:
      // We currently do not represent a truncate expression as an affine
      // expression. If it is constant during Scop execution, we treat it as a
      // parameter.
      return ValidatorResult(SCEVType::PARAM, Expr);
    case SCEVType::IV:
      DEBUG(dbgs() << "INVALID: ZeroExtend of SCEVType::IV expression");
      return ValidatorResult(SCEVType::INVALID);
    case SCEVType::INVALID:
      return Op;
    }

    llvm_unreachable("Unknown SCEVType");
  }

  class ValidatorResult visitSignExtendExpr(const SCEVSignExtendExpr *Expr) {
    // We currently allow only signed SCEV expressions. In the case of a
    // signed value, a sign extend is a noop.
    //
    // TODO: Reconsider this when we add support for unsigned values.
    return visit(Expr->getOperand());
  }

  class ValidatorResult visitAddExpr(const SCEVAddExpr *Expr) {
    ValidatorResult Return(SCEVType::INT);

    for (int i = 0, e = Expr->getNumOperands(); i < e; ++i) {
      ValidatorResult Op = visit(Expr->getOperand(i));
      Return.merge(Op);

      // Early exit.
      if (!Return.isValid())
        break;
    }

    // TODO: Check for NSW and NUW.
    return Return;
  }

  class ValidatorResult visitMulExpr(const SCEVMulExpr *Expr) {
    ValidatorResult Return(SCEVType::INT);

    for (int i = 0, e = Expr->getNumOperands(); i < e; ++i) {
      ValidatorResult Op = visit(Expr->getOperand(i));

      if (Op.isINT())
        continue;

      if ((Op.isIV() || Op.isPARAM()) && !Return.isINT()) {
        DEBUG(dbgs() << "INVALID: More than one non-int operand in MulExpr\n"
                     << "\tExpr: " << *Expr << "\n"
                     << "\tPrevious expression type: " << Return << "\n"
                     << "\tNext operand (" << Op
                     << "): " << *Expr->getOperand(i) << "\n");

        return ValidatorResult(SCEVType::INVALID);
      }

      Return.merge(Op);
    }

    // TODO: Check for NSW and NUW.
    return Return;
  }

  class ValidatorResult visitUDivExpr(const SCEVUDivExpr *Expr) {
    ValidatorResult LHS = visit(Expr->getLHS());
    ValidatorResult RHS = visit(Expr->getRHS());

    // We currently do not represent an unsigned division as an affine
    // expression. If the division is constant during Scop execution we treat it
    // as a parameter, otherwise we bail out.
    if (LHS.isConstant() && RHS.isConstant())
      return ValidatorResult(SCEVType::PARAM, Expr);

    DEBUG(dbgs() << "INVALID: unsigned division of non-constant expressions");
    return ValidatorResult(SCEVType::INVALID);
  }

  class ValidatorResult visitAddRecExpr(const SCEVAddRecExpr *Expr) {
    if (!Expr->isAffine()) {
      DEBUG(dbgs() << "INVALID: AddRec is not affine");
      return ValidatorResult(SCEVType::INVALID);
    }

    ValidatorResult Start = visit(Expr->getStart());
    ValidatorResult Recurrence = visit(Expr->getStepRecurrence(SE));

    if (!Start.isValid())
      return Start;

    if (!Recurrence.isValid())
      return Recurrence;

    if (R->contains(Expr->getLoop())) {
      if (Recurrence.isINT()) {
        ValidatorResult Result(SCEVType::IV);
        Result.addParamsFrom(Start);
        return Result;
      }

      DEBUG(dbgs() << "INVALID: AddRec within scop has non-int"
                      "recurrence part");
      return ValidatorResult(SCEVType::INVALID);
    }

    assert(Start.isConstant() && Recurrence.isConstant() &&
           "Expected 'Start' and 'Recurrence' to be constant");
    return ValidatorResult(SCEVType::PARAM, Expr);
  }

  class ValidatorResult visitSMaxExpr(const SCEVSMaxExpr *Expr) {
    ValidatorResult Return(SCEVType::INT, Expr);

    for (int i = 0, e = Expr->getNumOperands(); i < e; ++i) {
      ValidatorResult Op = visit(Expr->getOperand(i));

      if (!Op.isValid())
        return Op;

      Return.merge(Op);
    }

    return Return;
  }

  class ValidatorResult visitUMaxExpr(const SCEVUMaxExpr *Expr) {
    // We do not support unsigned operations. If 'Expr' is constant during Scop
    // execution we treat this as a parameter, otherwise we bail out.
    for (int i = 0, e = Expr->getNumOperands(); i < e; ++i) {
      ValidatorResult Op = visit(Expr->getOperand(i));

      if (!Op.isConstant()) {
        DEBUG(dbgs() << "INVALID: UMaxExpr has a non-constant operand");
        return ValidatorResult(SCEVType::INVALID);
      }
    }

    return ValidatorResult(SCEVType::PARAM, Expr);
  }

  ValidatorResult visitUnknown(const SCEVUnknown *Expr) {
    Value *V = Expr->getValue();

    // We currently only support integer types. It may be useful to support
    // pointer types, e.g. to support code like:
    //
    //   if (A)
    //     A[i] = 1;
    //
    // See test/CodeGen/20120316-InvalidCast.ll
    if (!Expr->getType()->isIntegerTy()) {
      DEBUG(dbgs() << "INVALID: UnknownExpr is not an integer type");
      return ValidatorResult(SCEVType::INVALID);
    }

    if (isa<UndefValue>(V)) {
      DEBUG(dbgs() << "INVALID: UnknownExpr references an undef value");
      return ValidatorResult(SCEVType::INVALID);
    }

    if (Instruction *I = dyn_cast<Instruction>(Expr->getValue()))
      if (R->contains(I)) {
        DEBUG(dbgs() << "INVALID: UnknownExpr references an instruction "
                        "within the region\n");
        return ValidatorResult(SCEVType::INVALID);
      }

    if (BaseAddress == V) {
      DEBUG(dbgs() << "INVALID: UnknownExpr references BaseAddress\n");
      return ValidatorResult(SCEVType::INVALID);
    }

    return ValidatorResult(SCEVType::PARAM, Expr);
  }
};

namespace polly {
bool isAffineExpr(const Region *R, const SCEV *Expr, ScalarEvolution &SE,
                  const Value *BaseAddress) {
  if (isa<SCEVCouldNotCompute>(Expr))
    return false;

  SCEVValidator Validator(R, SE, BaseAddress);
  DEBUG(
    dbgs() << "\n";
    dbgs() << "Expr: " << *Expr << "\n";
    dbgs() << "Region: " << R->getNameStr() << "\n";
    dbgs() << " -> "
  );

  ValidatorResult Result = Validator.visit(Expr);

  DEBUG(
    if (Result.isValid())
      dbgs() << "VALID\n";
    dbgs() << "\n";
  );

  return Result.isValid();
}

std::vector<const SCEV *>
getParamsInAffineExpr(const Region *R, const SCEV *Expr, ScalarEvolution &SE,
                      const Value *BaseAddress) {
  if (isa<SCEVCouldNotCompute>(Expr))
    return std::vector<const SCEV *>();

  SCEVValidator Validator(R, SE, BaseAddress);
  ValidatorResult Result = Validator.visit(Expr);

  return Result.getParameters();
}
}