lib/Analysis/LoopAnalysis.cpp

//===- LoopAnalysis.cpp - Misc loop analysis routines //-------------------===//
//
// Copyright 2019 The MLIR Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// =============================================================================
//
// This file implements miscellaneous loop analysis routines.
//
//===----------------------------------------------------------------------===//

#include "mlir/Analysis/LoopAnalysis.h"

#include "mlir/Analysis/AffineAnalysis.h"
#include "mlir/IR/AffineExpr.h"
#include "mlir/IR/AffineMap.h"
#include "mlir/IR/Statements.h"
#include "mlir/Support/MathExtras.h"

using namespace mlir;

/// Returns the trip count of the loop as an affine expression if the latter is
/// expressible as an affine expression, and nullptr otherwise. The trip count
/// expression is simplified before returning.
AffineExpr mlir::getTripCountExpr(const ForStmt &forStmt) {
  // upper_bound - lower_bound + 1
  int64_t loopSpan;

  int64_t step = forStmt.getStep();
  auto *context = forStmt.getContext();

  if (forStmt.hasConstantBounds()) {
    int64_t lb = forStmt.getConstantLowerBound();
    int64_t ub = forStmt.getConstantUpperBound();
    loopSpan = ub - lb + 1;
  } else {
    auto *lbMap = forStmt.getLowerBoundMap();
    auto *ubMap = forStmt.getUpperBoundMap();
    // TODO(bondhugula): handle max/min of multiple expressions.
    if (lbMap->getNumResults() != 1 || ubMap->getNumResults() != 1)
      return nullptr;

    // TODO(bondhugula): handle bounds with different operands.
    // Bounds have different operands, unhandled for now.
    if (!forStmt.matchingBoundOperandList())
      return nullptr;

    // ub_expr - lb_expr + 1
    AffineExpr lbExpr(lbMap->getResult(0));
    AffineExpr ubExpr(ubMap->getResult(0));
    auto loopSpanExpr = simplifyAffineExpr(
        ubExpr - lbExpr + 1, std::max(lbMap->getNumDims(), ubMap->getNumDims()),
        std::max(lbMap->getNumSymbols(), ubMap->getNumSymbols()));
    auto cExpr = loopSpanExpr.dyn_cast<AffineConstantExpr>();
    if (!cExpr)
      return loopSpanExpr.ceilDiv(step);
    loopSpan = cExpr.getValue();
  }

  // 0 iteration loops.
  if (loopSpan < 0)
    return 0;

  return getAffineConstantExpr(static_cast<uint64_t>(ceilDiv(loopSpan, step)),
                               context);
}

/// Returns the trip count of the loop if it's a constant, None otherwise. This
/// method uses affine expression analysis (in turn using getTripCount) and is
/// able to determine constant trip count in non-trivial cases.
llvm::Optional<uint64_t> mlir::getConstantTripCount(const ForStmt &forStmt) {
  auto tripCountExpr = getTripCountExpr(forStmt);

  if (!tripCountExpr)
    return None;

  if (auto constExpr = tripCountExpr.dyn_cast<AffineConstantExpr>())
    return constExpr.getValue();

  return None;
}

/// Returns the greatest known integral divisor of the trip count. Affine
/// expression analysis is used (indirectly through getTripCount), and
/// this method is thus able to determine non-trivial divisors.
uint64_t mlir::getLargestDivisorOfTripCount(const ForStmt &forStmt) {
  auto tripCountExpr = getTripCountExpr(forStmt);

  if (!tripCountExpr)
    return 1;

  if (auto constExpr = tripCountExpr.dyn_cast<AffineConstantExpr>()) {
    uint64_t tripCount = constExpr.getValue();

    // 0 iteration loops (greatest divisor is 2^64 - 1).
    if (tripCount == 0)
      return ULONG_MAX;

    // The greatest divisor is the trip count.
    return tripCount;
  }

  // Trip count is not a known constant; return its largest known divisor.
  return tripCountExpr.getLargestKnownDivisor();
}