InlineCost.h

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//===- InlineCost.h - Cost analysis for inliner -----------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements heuristics for inlining decisions.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_ANALYSIS_INLINECOST_H
#define LLVM_ANALYSIS_INLINECOST_H

#include "llvm/Analysis/CallGraphSCCPass.h"
#include <cassert>
#include <climits>

namespace llvm {
class AssumptionTracker;
class CallSite;
class DataLayout;
class Function;
class TargetTransformInfo;

namespace InlineConstants {
  // Various magic constants used to adjust heuristics.
  const int InstrCost = 5;
  const int IndirectCallThreshold = 100;
  const int CallPenalty = 25;
  const int LastCallToStaticBonus = -15000;
  const int ColdccPenalty = 2000;
  const int NoreturnPenalty = 10000;
  /// Do not inline functions which allocate this many bytes on the stack
  /// when the caller is recursive.
  const unsigned TotalAllocaSizeRecursiveCaller = 1024;
}

/// \brief Represents the cost of inlining a function.
///
/// This supports special values for functions which should "always" or
/// "never" be inlined. Otherwise, the cost represents a unitless amount;
/// smaller values increase the likelihood of the function being inlined.
///
/// Objects of this type also provide the adjusted threshold for inlining
/// based on the information available for a particular callsite. They can be
/// directly tested to determine if inlining should occur given the cost and
/// threshold for this cost metric.
class InlineCost {
  enum SentinelValues {
    AlwaysInlineCost = INT_MIN,
    NeverInlineCost = INT_MAX
  };

  /// \brief The estimated cost of inlining this callsite.
  const int Cost;

  /// \brief The adjusted threshold against which this cost was computed.
  const int Threshold;

  // Trivial constructor, interesting logic in the factory functions below.
  InlineCost(int Cost, int Threshold) : Cost(Cost), Threshold(Threshold) {}

public:
  static InlineCost get(int Cost, int Threshold) {
    assert(Cost > AlwaysInlineCost && "Cost crosses sentinel value");
    assert(Cost < NeverInlineCost && "Cost crosses sentinel value");
    return InlineCost(Cost, Threshold);
  }
  static InlineCost getAlways() {
    return InlineCost(AlwaysInlineCost, 0);
  }
  static InlineCost getNever() {
    return InlineCost(NeverInlineCost, 0);
  }

  /// \brief Test whether the inline cost is low enough for inlining.
  LLVM_EXPLICIT operator bool() const {
    return Cost < Threshold;
  }

  bool isAlways() const { return Cost == AlwaysInlineCost; }
  bool isNever() const { return Cost == NeverInlineCost; }
  bool isVariable() const { return !isAlways() && !isNever(); }

  /// \brief Get the inline cost estimate.
  /// It is an error to call this on an "always" or "never" InlineCost.
  int getCost() const {
    assert(isVariable() && "Invalid access of InlineCost");
    return Cost;
  }

  /// \brief Get the cost delta from the threshold for inlining.
  /// Only valid if the cost is of the variable kind. Returns a negative
  /// value if the cost is too high to inline.
  int getCostDelta() const { return Threshold - getCost(); }
};

/// \brief Cost analyzer used by inliner.
class InlineCostAnalysis : public CallGraphSCCPass {
  const TargetTransformInfo *TTI;
  AssumptionTracker *AT;

public:
  static char ID;

  InlineCostAnalysis();
  ~InlineCostAnalysis();

  // Pass interface implementation.
  void getAnalysisUsage(AnalysisUsage &AU) const override;
  bool runOnSCC(CallGraphSCC &SCC) override;

  /// \brief Get an InlineCost object representing the cost of inlining this
  /// callsite.
  ///
  /// Note that threshold is passed into this function. Only costs below the
  /// threshold are computed with any accuracy. The threshold can be used to
  /// bound the computation necessary to determine whether the cost is
  /// sufficiently low to warrant inlining.
  ///
  /// Also note that calling this function *dynamically* computes the cost of
  /// inlining the callsite. It is an expensive, heavyweight call.
  InlineCost getInlineCost(CallSite CS, int Threshold);

  /// \brief Get an InlineCost with the callee explicitly specified.
  /// This allows you to calculate the cost of inlining a function via a
  /// pointer. This behaves exactly as the version with no explicit callee
  /// parameter in all other respects.
  //
  //  Note: This is used by out-of-tree passes, please do not remove without
  //  adding a replacement API.
  InlineCost getInlineCost(CallSite CS, Function *Callee, int Threshold);

  /// \brief Minimal filter to detect invalid constructs for inlining.
  bool isInlineViable(Function &Callee);
};

}

#endif