include/llvm/Analysis/InlineCost.h

   1 //===- InlineCost.h - Cost analysis for inliner -----------------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements heuristics for inlining decisions.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #ifndef LLVM_ANALYSIS_INLINECOST_H
  14 #define LLVM_ANALYSIS_INLINECOST_H
  15
  16 #include "llvm/Analysis/AssumptionCache.h"
  17 #include "llvm/Analysis/CallGraphSCCPass.h"
  18 #include "llvm/Analysis/OptimizationRemarkEmitter.h"
  19 #include <cassert>
  20 #include <climits>
  21
  22 namespace llvm {
  23 class AssumptionCacheTracker;
  24 class BlockFrequencyInfo;
  25 class CallBase;
  26 class DataLayout;
  27 class Function;
  28 class ProfileSummaryInfo;
  29 class TargetTransformInfo;
  30
  31 namespace InlineConstants {
  32 // Various thresholds used by inline cost analysis.
  33 /// Use when optsize (-Os) is specified.
  34 const int OptSizeThreshold = 50;
  35
  36 /// Use when minsize (-Oz) is specified.
  37 const int OptMinSizeThreshold = 5;
  38
  39 /// Use when -O3 is specified.
  40 const int OptAggressiveThreshold = 250;
  41
  42 // Various magic constants used to adjust heuristics.
  43 const int InstrCost = 5;
  44 const int IndirectCallThreshold = 100;
  45 const int CallPenalty = 25;
  46 const int LastCallToStaticBonus = 15000;
  47 const int ColdccPenalty = 2000;
  48 /// Do not inline functions which allocate this many bytes on the stack
  49 /// when the caller is recursive.
  50 const unsigned TotalAllocaSizeRecursiveCaller = 1024;
  51 }
  52
  53 /// Represents the cost of inlining a function.
  54 ///
  55 /// This supports special values for functions which should "always" or
  56 /// "never" be inlined. Otherwise, the cost represents a unitless amount;
  57 /// smaller values increase the likelihood of the function being inlined.
  58 ///
  59 /// Objects of this type also provide the adjusted threshold for inlining
  60 /// based on the information available for a particular callsite. They can be
  61 /// directly tested to determine if inlining should occur given the cost and
  62 /// threshold for this cost metric.
  63 class InlineCost {
  64   enum SentinelValues {
  65     AlwaysInlineCost = INT_MIN,
  66     NeverInlineCost = INT_MAX
  67   };
  68
  69   /// The estimated cost of inlining this callsite.
  70   int Cost;
  71
  72   /// The adjusted threshold against which this cost was computed.
  73   int Threshold;
  74
  75   /// Must be set for Always and Never instances.
  76   const char *Reason = nullptr;
  77
  78   // Trivial constructor, interesting logic in the factory functions below.
  79   InlineCost(int Cost, int Threshold, const char *Reason = nullptr)
  80       : Cost(Cost), Threshold(Threshold), Reason(Reason) {
  81     assert((isVariable() || Reason) &&
  82            "Reason must be provided for Never or Always");
  83   }
  84
  85 public:
  86   static InlineCost get(int Cost, int Threshold) {
  87     assert(Cost > AlwaysInlineCost && "Cost crosses sentinel value");
  88     assert(Cost < NeverInlineCost && "Cost crosses sentinel value");
  89     return InlineCost(Cost, Threshold);
  90   }
  91   static InlineCost getAlways(const char *Reason) {
  92     return InlineCost(AlwaysInlineCost, 0, Reason);
  93   }
  94   static InlineCost getNever(const char *Reason) {
  95     return InlineCost(NeverInlineCost, 0, Reason);
  96   }
  97
  98   /// Test whether the inline cost is low enough for inlining.
  99   explicit operator bool() const {
 100     return Cost < Threshold;
 101   }
 102
 103   bool isAlways() const { return Cost == AlwaysInlineCost; }
 104   bool isNever() const { return Cost == NeverInlineCost; }
 105   bool isVariable() const { return !isAlways() && !isNever(); }
 106
 107   /// Get the inline cost estimate.
 108   /// It is an error to call this on an "always" or "never" InlineCost.
 109   int getCost() const {
 110     assert(isVariable() && "Invalid access of InlineCost");
 111     return Cost;
 112   }
 113
 114   /// Get the threshold against which the cost was computed
 115   int getThreshold() const {
 116     assert(isVariable() && "Invalid access of InlineCost");
 117     return Threshold;
 118   }
 119
 120   /// Get the reason of Always or Never.
 121   const char *getReason() const {
 122     assert((Reason || isVariable()) &&
 123            "InlineCost reason must be set for Always or Never");
 124     return Reason;
 125   }
 126
 127   /// Get the cost delta from the threshold for inlining.
 128   /// Only valid if the cost is of the variable kind. Returns a negative
 129   /// value if the cost is too high to inline.
 130   int getCostDelta() const { return Threshold - getCost(); }
 131 };
 132
 133 /// InlineResult is basically true or false. For false results the message
 134 /// describes a reason why it is decided not to inline.
 135 struct InlineResult {
 136   const char *message = nullptr;
 137   InlineResult(bool result, const char *message = nullptr)
 138       : message(result ? nullptr : (message ? message : "cost > threshold")) {}
 139   InlineResult(const char *message = nullptr) : message(message) {}
 140   operator bool() const { return !message; }
 141   operator const char *() const { return message; }
 142 };
 143
 144 /// Thresholds to tune inline cost analysis. The inline cost analysis decides
 145 /// the condition to apply a threshold and applies it. Otherwise,
 146 /// DefaultThreshold is used. If a threshold is Optional, it is applied only
 147 /// when it has a valid value. Typically, users of inline cost analysis
 148 /// obtain an InlineParams object through one of the \c getInlineParams methods
 149 /// and pass it to \c getInlineCost. Some specialized versions of inliner
 150 /// (such as the pre-inliner) might have custom logic to compute \c InlineParams
 151 /// object.
 152
 153 struct InlineParams {
 154   /// The default threshold to start with for a callee.
 155   int DefaultThreshold;
 156
 157   /// Threshold to use for callees with inline hint.
 158   Optional<int> HintThreshold;
 159
 160   /// Threshold to use for cold callees.
 161   Optional<int> ColdThreshold;
 162
 163   /// Threshold to use when the caller is optimized for size.
 164   Optional<int> OptSizeThreshold;
 165
 166   /// Threshold to use when the caller is optimized for minsize.
 167   Optional<int> OptMinSizeThreshold;
 168
 169   /// Threshold to use when the callsite is considered hot.
 170   Optional<int> HotCallSiteThreshold;
 171
 172   /// Threshold to use when the callsite is considered hot relative to function
 173   /// entry.
 174   Optional<int> LocallyHotCallSiteThreshold;
 175
 176   /// Threshold to use when the callsite is considered cold.
 177   Optional<int> ColdCallSiteThreshold;
 178
 179   /// Compute inline cost even when the cost has exceeded the threshold.
 180   Optional<bool> ComputeFullInlineCost;
 181 };
 182
 183 /// Generate the parameters to tune the inline cost analysis based only on the
 184 /// commandline options.
 185 InlineParams getInlineParams();
 186
 187 /// Generate the parameters to tune the inline cost analysis based on command
 188 /// line options. If -inline-threshold option is not explicitly passed,
 189 /// \p Threshold is used as the default threshold.
 190 InlineParams getInlineParams(int Threshold);
 191
 192 /// Generate the parameters to tune the inline cost analysis based on command
 193 /// line options. If -inline-threshold option is not explicitly passed,
 194 /// the default threshold is computed from \p OptLevel and \p SizeOptLevel.
 195 /// An \p OptLevel value above 3 is considered an aggressive optimization mode.
 196 /// \p SizeOptLevel of 1 corresponds to the -Os flag and 2 corresponds to
 197 /// the -Oz flag.
 198 InlineParams getInlineParams(unsigned OptLevel, unsigned SizeOptLevel);
 199
 200 /// Return the cost associated with a callsite, including parameter passing
 201 /// and the call/return instruction.
 202 int getCallsiteCost(CallBase &Call, const DataLayout &DL);
 203
 204 /// Get an InlineCost object representing the cost of inlining this
 205 /// callsite.
 206 ///
 207 /// Note that a default threshold is passed into this function. This threshold
 208 /// could be modified based on callsite's properties and only costs below this
 209 /// new threshold are computed with any accuracy. The new threshold can be
 210 /// used to bound the computation necessary to determine whether the cost is
 211 /// sufficiently low to warrant inlining.
 212 ///
 213 /// Also note that calling this function *dynamically* computes the cost of
 214 /// inlining the callsite. It is an expensive, heavyweight call.
 215 InlineCost getInlineCost(
 216     CallBase &Call, const InlineParams &Params, TargetTransformInfo &CalleeTTI,
 217     std::function<AssumptionCache &(Function &)> &GetAssumptionCache,
 218     Optional<function_ref<BlockFrequencyInfo &(Function &)>> GetBFI,
 219     ProfileSummaryInfo *PSI, OptimizationRemarkEmitter *ORE = nullptr);
 220
 221 /// Get an InlineCost with the callee explicitly specified.
 222 /// This allows you to calculate the cost of inlining a function via a
 223 /// pointer. This behaves exactly as the version with no explicit callee
 224 /// parameter in all other respects.
 225 //
 226 InlineCost
 227 getInlineCost(CallBase &Call, Function *Callee, const InlineParams &Params,
 228               TargetTransformInfo &CalleeTTI,
 229               std::function<AssumptionCache &(Function &)> &GetAssumptionCache,
 230               Optional<function_ref<BlockFrequencyInfo &(Function &)>> GetBFI,
 231               ProfileSummaryInfo *PSI, OptimizationRemarkEmitter *ORE);
 232
 233 /// Minimal filter to detect invalid constructs for inlining.
 234 InlineResult isInlineViable(Function &Callee);
 235 }
 236
 237 #endif