IntegerRangeAnalysis.cpp

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//===- IntegerRangeAnalysis.cpp - Integer range analysis --------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file defines the dataflow analysis class for integer range inference
// which is used in transformations over the `arith` dialect such as
// branch elimination or signed->unsigned rewriting
//
//===----------------------------------------------------------------------===//
 
#include "mlir/Analysis/DataFlow/IntegerRangeAnalysis.h"
#include "mlir/Analysis/DataFlow/ConstantPropagationAnalysis.h"
#include "mlir/Analysis/DataFlow/SparseAnalysis.h"
#include "mlir/Analysis/DataFlowFramework.h"
#include "mlir/IR/BuiltinAttributes.h"
#include "mlir/IR/Dialect.h"
#include "mlir/IR/OpDefinition.h"
#include "mlir/IR/TypeUtilities.h"
#include "mlir/IR/Value.h"
#include "mlir/Interfaces/ControlFlowInterfaces.h"
#include "mlir/Interfaces/InferIntRangeInterface.h"
#include "mlir/Interfaces/LoopLikeInterface.h"
#include "mlir/Support/LLVM.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include <cassert>
#include <optional>
#include <utility>
 
#define DEBUG_TYPE "int-range-analysis"
 
using namespace mlir;
using namespace mlir::dataflow;
 
void IntegerValueRangeLattice::onUpdate(DataFlowSolver *solver) const {
  Lattice::onUpdate(solver);
 
  // If the integer range can be narrowed to a constant, update the constant
  // value of the SSA value.
  std::optional<APInt> constant = getValue().getValue().getConstantValue();
  auto value = anchor.get<Value>();
  auto *cv = solver->getOrCreateState<Lattice<ConstantValue>>(value);
  if (!constant)
    return solver->propagateIfChanged(
        cv, cv->join(ConstantValue::getUnknownConstant()));
 
  Dialect *dialect;
  if (auto *parent = value.getDefiningOp())
    dialect = parent->getDialect();
  else
    dialect = value.getParentBlock()->getParentOp()->getDialect();
 
  Type type = getElementTypeOrSelf(value);
  solver->propagateIfChanged(
      cv, cv->join(ConstantValue(IntegerAttr::get(type, *constant), dialect)));
}
 
LogicalResult IntegerRangeAnalysis::visitOperation(
    Operation *op, ArrayRef<const IntegerValueRangeLattice *> operands,
    ArrayRef<IntegerValueRangeLattice *> results) {
  auto inferrable = dyn_cast<InferIntRangeInterface>(op);
  if (!inferrable) {
    setAllToEntryStates(results);
    return success();
  }
 
  LLVM_DEBUG(llvm::dbgs() << "Inferring ranges for " << *op << "\n");
  auto argRanges = llvm::map_to_vector(
      operands, [](const IntegerValueRangeLattice *lattice) {
        return lattice->getValue();
      });
 
  auto joinCallback = [&](Value v, const IntegerValueRange &attrs) {
    auto result = dyn_cast<OpResult>(v);
    if (!result)
      return;
    assert(llvm::is_contained(op->getResults(), result));
 
    LLVM_DEBUG(llvm::dbgs() << "Inferred range " << attrs << "\n");
    IntegerValueRangeLattice *lattice = results[result.getResultNumber()];
    IntegerValueRange oldRange = lattice->getValue();
 
    ChangeResult changed = lattice->join(attrs);
 
    // Catch loop results with loop variant bounds and conservatively make
    // them [-inf, inf] so we don't circle around infinitely often (because
    // the dataflow analysis in MLIR doesn't attempt to work out trip counts
    // and often can't).
    bool isYieldedResult = llvm::any_of(v.getUsers(), [](Operation *op) {
      return op->hasTrait<OpTrait::IsTerminator>();
    });
    if (isYieldedResult && !oldRange.isUninitialized() &&
        !(lattice->getValue() == oldRange)) {
      LLVM_DEBUG(llvm::dbgs() << "Loop variant loop result detected\n");
      changed |= lattice->join(IntegerValueRange::getMaxRange(v));
    }
    propagateIfChanged(lattice, changed);
  };
 
  inferrable.inferResultRangesFromOptional(argRanges, joinCallback);
  return success();
}
 
void IntegerRangeAnalysis::visitNonControlFlowArguments(
    Operation *op, const RegionSuccessor &successor,
    ArrayRef<IntegerValueRangeLattice *> argLattices, unsigned firstIndex) {
  if (auto inferrable = dyn_cast<InferIntRangeInterface>(op)) {
    LLVM_DEBUG(llvm::dbgs() << "Inferring ranges for " << *op << "\n");
 
    auto argRanges = llvm::map_to_vector(op->getOperands(), [&](Value value) {
      return getLatticeElementFor(getProgramPointAfter(op), value)->getValue();
    });
 
    auto joinCallback = [&](Value v, const IntegerValueRange &attrs) {
      auto arg = dyn_cast<BlockArgument>(v);
      if (!arg)
        return;
      if (!llvm::is_contained(successor.getSuccessor()->getArguments(), arg))
        return;
 
      LLVM_DEBUG(llvm::dbgs() << "Inferred range " << attrs << "\n");
      IntegerValueRangeLattice *lattice = argLattices[arg.getArgNumber()];
      IntegerValueRange oldRange = lattice->getValue();
 
      ChangeResult changed = lattice->join(attrs);
 
      // Catch loop results with loop variant bounds and conservatively make
      // them [-inf, inf] so we don't circle around infinitely often (because
      // the dataflow analysis in MLIR doesn't attempt to work out trip counts
      // and often can't).
      bool isYieldedValue = llvm::any_of(v.getUsers(), [](Operation *op) {
        return op->hasTrait<OpTrait::IsTerminator>();
      });
      if (isYieldedValue && !oldRange.isUninitialized() &&
          !(lattice->getValue() == oldRange)) {
        LLVM_DEBUG(llvm::dbgs() << "Loop variant loop result detected\n");
        changed |= lattice->join(IntegerValueRange::getMaxRange(v));
      }
      propagateIfChanged(lattice, changed);
    };
 
    inferrable.inferResultRangesFromOptional(argRanges, joinCallback);
    return;
  }
 
  /// Given the results of getConstant{Lower,Upper}Bound() or getConstantStep()
  /// on a LoopLikeInterface return the lower/upper bound for that result if
  /// possible.
  auto getLoopBoundFromFold = [&](std::optional<OpFoldResult> loopBound,
                                  Type boundType, bool getUpper) {
    unsigned int width = ConstantIntRanges::getStorageBitwidth(boundType);
    if (loopBound.has_value()) {
      if (loopBound->is<Attribute>()) {
        if (auto bound =
                dyn_cast_or_null<IntegerAttr>(loopBound->get<Attribute>()))
          return bound.getValue();
      } else if (auto value = llvm::dyn_cast_if_present<Value>(*loopBound)) {
        const IntegerValueRangeLattice *lattice =
            getLatticeElementFor(getProgramPointAfter(op), value);
        if (lattice != nullptr && !lattice->getValue().isUninitialized())
          return getUpper ? lattice->getValue().getValue().smax()
                          : lattice->getValue().getValue().smin();
      }
    }
    // Given the results of getConstant{Lower,Upper}Bound()
    // or getConstantStep() on a LoopLikeInterface return the lower/upper
    // bound
    return getUpper ? APInt::getSignedMaxValue(width)
                    : APInt::getSignedMinValue(width);
  };
 
  // Infer bounds for loop arguments that have static bounds
  if (auto loop = dyn_cast<LoopLikeOpInterface>(op)) {
    std::optional<Value> iv = loop.getSingleInductionVar();
    if (!iv) {
      return SparseForwardDataFlowAnalysis ::visitNonControlFlowArguments(
          op, successor, argLattices, firstIndex);
    }
    std::optional<OpFoldResult> lowerBound = loop.getSingleLowerBound();
    std::optional<OpFoldResult> upperBound = loop.getSingleUpperBound();
    std::optional<OpFoldResult> step = loop.getSingleStep();
    APInt min = getLoopBoundFromFold(lowerBound, iv->getType(),
                                     /*getUpper=*/false);
    APInt max = getLoopBoundFromFold(upperBound, iv->getType(),
                                     /*getUpper=*/true);
    // Assume positivity for uniscoverable steps by way of getUpper = true.
    APInt stepVal =
        getLoopBoundFromFold(step, iv->getType(), /*getUpper=*/true);
 
    if (stepVal.isNegative()) {
      std::swap(min, max);
    } else {
      // Correct the upper bound by subtracting 1 so that it becomes a <=
      // bound, because loops do not generally include their upper bound.
      max -= 1;
    }
 
    // If we infer the lower bound to be larger than the upper bound, the
    // resulting range is meaningless and should not be used in further
    // inferences.
    if (max.sge(min)) {
      IntegerValueRangeLattice *ivEntry = getLatticeElement(*iv);
      auto ivRange = ConstantIntRanges::fromSigned(min, max);
      propagateIfChanged(ivEntry, ivEntry->join(IntegerValueRange{ivRange}));
    }
    return;
  }
 
  return SparseForwardDataFlowAnalysis::visitNonControlFlowArguments(
      op, successor, argLattices, firstIndex);
}