AffineAnalysis.h

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//===- AffineAnalysis.h - analyses for affine structures --------*- 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 header file defines prototypes for methods that perform analysis
// involving affine structures (AffineExprStorage, AffineMap, IntegerSet, etc.)
// and other IR structures that in turn use these.
//
//===----------------------------------------------------------------------===//
 
#ifndef MLIR_DIALECT_AFFINE_ANALYSIS_AFFINEANALYSIS_H
#define MLIR_DIALECT_AFFINE_ANALYSIS_AFFINEANALYSIS_H
 
#include "mlir/Analysis/Presburger/IntegerRelation.h"
#include "mlir/Dialect/Arith/IR/Arith.h"
#include "mlir/IR/Value.h"
#include "llvm/ADT/SmallVector.h"
#include <optional>
 
namespace mlir {
class Operation;
 
namespace affine {
class AffineApplyOp;
class AffineForOp;
class AffineValueMap;
class FlatAffineRelation;
class FlatAffineValueConstraints;
 
/// A description of a (parallelizable) reduction in an affine loop.
struct LoopReduction {
  /// Reduction kind.
  arith::AtomicRMWKind kind;
 
  /// Position of the iteration argument that acts as accumulator.
  unsigned iterArgPosition;
 
  /// The value being reduced.
  Value value;
};
 
/// Populate `supportedReductions` with descriptors of the supported reductions.
void getSupportedReductions(
    AffineForOp forOp, SmallVectorImpl<LoopReduction> &supportedReductions);
 
/// Returns true if `forOp' is a parallel loop. If `parallelReductions` is
/// provided, populates it with descriptors of the parallelizable reductions and
/// treats them as not preventing parallelization.
bool isLoopParallel(
    AffineForOp forOp,
    SmallVectorImpl<LoopReduction> *parallelReductions = nullptr);
 
/// Returns true if `forOp' doesn't have memory dependences preventing
/// parallelization. Memrefs that are allocated inside `forOp` do not impact its
/// dependences and parallelism. This function does not check iter_args (for
/// values other than memref types) and should be used only as a building block
/// for complete parallelism-checking functions.
bool isLoopMemoryParallel(AffineForOp forOp);
 
/// Returns in `affineApplyOps`, the sequence of those AffineApplyOp
/// Operations that are reachable via a search starting from `operands` and
/// ending at those operands that are not the result of an AffineApplyOp.
void getReachableAffineApplyOps(ArrayRef<Value> operands,
                                SmallVectorImpl<Operation *> &affineApplyOps);
 
/// Builds a system of constraints with dimensional variables corresponding to
/// the loop IVs of the forOps and AffineIfOp's operands appearing in
/// that order. Bounds of the loop are used to add appropriate inequalities.
/// Constraints from the index sets of AffineIfOp are also added. Any symbols
/// founds in the bound operands are added as symbols in the system. Returns
/// failure for the yet unimplemented cases. `ops` accepts both AffineForOp and
/// AffineIfOp.
//  TODO: handle non-unit strides.
LogicalResult getIndexSet(MutableArrayRef<Operation *> ops,
                          FlatAffineValueConstraints *domain);
 
/// Encapsulates a memref load or store access information.
struct MemRefAccess {
  Value memref;
  Operation *opInst = nullptr;
  SmallVector<Value, 4> indices;
 
  /// Constructs a MemRefAccess from an affine read/write operation.
  explicit MemRefAccess(Operation *memOp);
 
  MemRefAccess() = default;
 
  // Returns the rank of the memref associated with this access.
  unsigned getRank() const;
  // Returns true if this access is of a store op.
  bool isStore() const;
 
  /// Creates an access relation for the access. An access relation maps
  /// elements of an iteration domain to the element(s) of an array domain
  /// accessed by that iteration of the associated statement through some array
  /// reference. For example, given the MLIR code:
  ///
  /// affine.for %i0 = 0 to 10 {
  ///   affine.for %i1 = 0 to 10 {
  ///     %a = affine.load %arr[%i0 + %i1, %i0 + 2 * %i1] : memref<100x100xf32>
  ///   }
  /// }
  ///
  /// The access relation, assuming that the memory locations for %arr are
  /// represented as %m0, %m1 would be:
  ///
  ///   (%i0, %i1) -> (%m0, %m1)
  ///   %m0 = %i0 + %i1
  ///   %m1 = %i0 + 2 * %i1
  ///   0  <= %i0 < 10
  ///   0  <= %i1 < 10
  ///
  /// Returns failure for yet unimplemented/unsupported cases (see docs of
  /// mlir::getIndexSet and mlir::getRelationFromMap for these cases).
  LogicalResult getAccessRelation(presburger::IntegerRelation &accessRel) const;
 
  /// Populates 'accessMap' with composition of AffineApplyOps reachable from
  /// 'indices'.
  void getAccessMap(AffineValueMap *accessMap) const;
 
  /// Equal if both affine accesses can be proved to be equivalent at compile
  /// time (considering the memrefs, their respective affine access maps  and
  /// operands). The equality of access functions + operands is checked by
  /// subtracting fully composed value maps, and then simplifying the difference
  /// using the expression flattener. This does not account for aliasing of
  /// memrefs.
  bool operator==(const MemRefAccess &rhs) const;
  bool operator!=(const MemRefAccess &rhs) const { return !(*this == rhs); }
 
  explicit operator bool() const { return !!memref; }
};
 
// DependenceComponent contains state about the direction of a dependence as an
// interval [lb, ub] for an AffineForOp.
// Distance vectors components are represented by the interval [lb, ub] with
// lb == ub.
// Direction vectors components are represented by the interval [lb, ub] with
// lb < ub. Note that ub/lb == None means unbounded.
struct DependenceComponent {
  // The AffineForOp Operation associated with this dependence component.
  Operation *op = nullptr;
  // The lower bound of the dependence distance.
  std::optional<int64_t> lb;
  // The upper bound of the dependence distance (inclusive).
  std::optional<int64_t> ub;
  DependenceComponent() : lb(std::nullopt), ub(std::nullopt) {}
};
 
/// Checks whether two accesses to the same memref access the same element.
/// Each access is specified using the MemRefAccess structure, which contains
/// the operation, indices and memref associated with the access. Returns
/// 'NoDependence' if it can be determined conclusively that the accesses do not
/// access the same memref element. If 'allowRAR' is true, will consider
/// read-after-read dependences (typically used by applications trying to
/// optimize input reuse).
// TODO: Wrap 'dependenceConstraints' and 'dependenceComponents' into a single
// struct.
// TODO: Make 'dependenceConstraints' optional arg.
struct DependenceResult {
  enum ResultEnum {
    HasDependence, // A dependence exists between 'srcAccess' and 'dstAccess'.
    NoDependence,  // No dependence exists between 'srcAccess' and 'dstAccess'.
    Failure,       // Dependence check failed due to unsupported cases.
  } value;
  DependenceResult(ResultEnum v) : value(v) {}
};
 
DependenceResult checkMemrefAccessDependence(
    const MemRefAccess &srcAccess, const MemRefAccess &dstAccess,
    unsigned loopDepth,
    FlatAffineValueConstraints *dependenceConstraints = nullptr,
    SmallVector<DependenceComponent, 2> *dependenceComponents = nullptr,
    bool allowRAR = false);
 
/// Utility function that returns true if the provided DependenceResult
/// corresponds to a dependence result.
inline bool hasDependence(DependenceResult result) {
  return result.value == DependenceResult::HasDependence;
}
 
/// Returns true if the provided DependenceResult corresponds to the absence of
/// a dependence.
inline bool noDependence(DependenceResult result) {
  return result.value == DependenceResult::NoDependence;
}
 
/// Returns in 'depCompsVec', dependence components for dependences between all
/// load and store ops in loop nest rooted at 'forOp', at loop depths in range
/// [1, maxLoopDepth].
void getDependenceComponents(
    AffineForOp forOp, unsigned maxLoopDepth,
    std::vector<SmallVector<DependenceComponent, 2>> *depCompsVec);
 
} // namespace affine
} // namespace mlir
 
#endif // MLIR_DIALECT_AFFINE_ANALYSIS_AFFINEANALYSIS_H