IntegerRangeAnalysis.h

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//===-IntegerRangeAnalysis.h - 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 declares the dataflow analysis class for integer range inference
// so that it can be used in transformations over the `arith` dialect such as
// branch elimination or signed->unsigned rewriting.
//
// One can also implement InferIntRangeInterface on ops in custom dialects,
// and then use this analysis to propagate ranges with custom semantics.
//
//===----------------------------------------------------------------------===//
 
#ifndef MLIR_ANALYSIS_DATAFLOW_INTEGERANGEANALYSIS_H
#define MLIR_ANALYSIS_DATAFLOW_INTEGERANGEANALYSIS_H
 
#include "mlir/Analysis/DataFlow/SparseAnalysis.h"
#include "mlir/Interfaces/InferIntRangeInterface.h"
 
namespace mlir {
class RewriterBase;
namespace dataflow {
 
/// This lattice element represents the integer value range of an SSA value.
///
/// `join` overrides the base behaviour to apply per-state widening: once
/// the lattice has absorbed enough strictly-increasing merges the range is
/// forced to its max as a sound over-approximation. This is the sole
/// convergence guarantee for `IntegerRangeAnalysis` on loop-carried
/// values; without it, `scf.while` loops with dynamic bounds and nested
/// region ops can keep the solver ratcheting a loop-carried range by +1
/// per worklist visit for up to 2^31 iterations on i32. The budget is
/// sized to be much larger than realistic merge counts on naturally
/// bounded accumulators (e.g. `arith.minsi`/`arith.andi`-clamped iter
/// args) so the analysis still converges to a tight range on those.
///
/// Note that only the `(const AbstractSparseLattice &)` overload is
/// overridden, so the widening fires only at framework merge sites
/// (block-arg / region-successor / callable-arg joins) —
/// transfer-function updates that go through the non-virtual
/// `join(const ValueT &)` overload are unaffected.
class IntegerValueRangeLattice : public Lattice<IntegerValueRange> {
public:
  using Lattice::Lattice;
  // The override below would otherwise hide the inherited
  // `join(const ValueT &)` overload that callers (e.g. transfer functions)
  // rely on for direct-value joins.
  using Lattice::join;
 
  ChangeResult join(const AbstractSparseLattice &rhs) override;
 
private:
  /// Per-state merge-site change counter. Drives the widening budget in
  /// `join`.
  unsigned mergeChangeCount = 0;
};
 
/// Integer range analysis determines the integer value range of SSA values
/// using operations that define `InferIntRangeInterface` and also sets the
/// range of iteration indices of loops with known bounds.
///
/// This analysis depends on DeadCodeAnalysis, and will be a silent no-op
/// if DeadCodeAnalysis is not loaded in the same solver context.
class IntegerRangeAnalysis
    : public SparseForwardDataFlowAnalysis<IntegerValueRangeLattice> {
public:
  using SparseForwardDataFlowAnalysis::SparseForwardDataFlowAnalysis;
 
  /// At an entry point, we cannot reason about integer value ranges.
  void setToEntryState(IntegerValueRangeLattice *lattice) override {
    propagateIfChanged(lattice, lattice->join(IntegerValueRange::getMaxRange(
                                    lattice->getAnchor())));
  }
 
  /// Visit an operation. Invoke the transfer function on each operation that
  /// implements `InferIntRangeInterface`.
  LogicalResult
  visitOperation(Operation *op,
                 ArrayRef<const IntegerValueRangeLattice *> operands,
                 ArrayRef<IntegerValueRangeLattice *> results) override;
 
  /// Visit block arguments or operation results of an operation with region
  /// control-flow for which values are not defined by region control-flow. This
  /// function calls `InferIntRangeInterface` to provide values for block
  /// arguments or tries to reduce the range on loop induction variables with
  /// known bounds.
  void visitNonControlFlowArguments(
      Operation *op, const RegionSuccessor &successor,
      ValueRange nonSuccessorInputs,
      ArrayRef<IntegerValueRangeLattice *> nonSuccessorInputLattices) override;
};
 
/// Succeeds if an op can be converted to its unsigned equivalent without
/// changing its semantics. This is the case when none of its openands or
/// results can be below 0 when analyzed from a signed perspective.
LogicalResult staticallyNonNegative(DataFlowSolver &solver, Operation *op);
 
/// Succeeds when a value is statically non-negative in that it has a lower
/// bound on its value (if it is treated as signed) and that bound is
/// non-negative.
/// Note, the results of this query may not be accurate for `index` if you plan
/// to use a non-64-bit index.
LogicalResult staticallyNonNegative(DataFlowSolver &solver, Value v);
 
LogicalResult maybeReplaceWithConstant(DataFlowSolver &solver,
                                       RewriterBase &rewriter, Value value);
 
} // end namespace dataflow
} // end namespace mlir
 
#endif // MLIR_ANALYSIS_DATAFLOW_INTEGERANGEANALYSIS_H