llvm/lib/CodeGen/RegAllocEvictionAdvisor.h

   1 //===- RegAllocEvictionAdvisor.h - Interference resolution ------*- 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 #ifndef LLVM_CODEGEN_REGALLOCEVICTIONADVISOR_H
  10 #define LLVM_CODEGEN_REGALLOCEVICTIONADVISOR_H
  11
  12 #include "llvm/ADT/ArrayRef.h"
  13 #include "llvm/ADT/SmallSet.h"
  14 #include "llvm/ADT/StringRef.h"
  15 #include "llvm/CodeGen/Register.h"
  16 #include "llvm/Config/llvm-config.h"
  17 #include "llvm/MC/MCRegister.h"
  18 #include "llvm/Pass.h"
  19
  20 namespace llvm {
  21 class AllocationOrder;
  22 class LiveInterval;
  23 class LiveIntervals;
  24 class LiveRegMatrix;
  25 class MachineFunction;
  26 class MachineRegisterInfo;
  27 class RegisterClassInfo;
  28 class TargetRegisterInfo;
  29 class VirtRegMap;
  30
  31 using SmallVirtRegSet = SmallSet<Register, 16>;
  32
  33 // Live ranges pass through a number of stages as we try to allocate them.
  34 // Some of the stages may also create new live ranges:
  35 //
  36 // - Region splitting.
  37 // - Per-block splitting.
  38 // - Local splitting.
  39 // - Spilling.
  40 //
  41 // Ranges produced by one of the stages skip the previous stages when they are
  42 // dequeued. This improves performance because we can skip interference checks
  43 // that are unlikely to give any results. It also guarantees that the live
  44 // range splitting algorithm terminates, something that is otherwise hard to
  45 // ensure.
  46 enum LiveRangeStage {
  47   /// Newly created live range that has never been queued.
  48   RS_New,
  49
  50   /// Only attempt assignment and eviction. Then requeue as RS_Split.
  51   RS_Assign,
  52
  53   /// Attempt live range splitting if assignment is impossible.
  54   RS_Split,
  55
  56   /// Attempt more aggressive live range splitting that is guaranteed to make
  57   /// progress.  This is used for split products that may not be making
  58   /// progress.
  59   RS_Split2,
  60
  61   /// Live range will be spilled.  No more splitting will be attempted.
  62   RS_Spill,
  63
  64   /// Live range is in memory. Because of other evictions, it might get moved
  65   /// in a register in the end.
  66   RS_Memory,
  67
  68   /// There is nothing more we can do to this live range.  Abort compilation
  69   /// if it can't be assigned.
  70   RS_Done
  71 };
  72
  73 /// Cost of evicting interference - used by default advisor, and the eviction
  74 /// chain heuristic in RegAllocGreedy.
  75 // FIXME: this can be probably made an implementation detail of the default
  76 // advisor, if the eviction chain logic can be refactored.
  77 struct EvictionCost {
  78   unsigned BrokenHints = 0; ///< Total number of broken hints.
  79   float MaxWeight = 0;      ///< Maximum spill weight evicted.
  80
  81   EvictionCost() = default;
  82
  83   bool isMax() const { return BrokenHints == ~0u; }
  84
  85   void setMax() { BrokenHints = ~0u; }
  86
  87   void setBrokenHints(unsigned NHints) { BrokenHints = NHints; }
  88
  89   bool operator<(const EvictionCost &O) const {
  90     return std::tie(BrokenHints, MaxWeight) <
  91            std::tie(O.BrokenHints, O.MaxWeight);
  92   }
  93 };
  94
  95 /// Interface to the eviction advisor, which is responsible for making a
  96 /// decision as to which live ranges should be evicted (if any).
  97 class RAGreedy;
  98 class RegAllocEvictionAdvisor {
  99 public:
 100   RegAllocEvictionAdvisor(const RegAllocEvictionAdvisor &) = delete;
 101   RegAllocEvictionAdvisor(RegAllocEvictionAdvisor &&) = delete;
 102   virtual ~RegAllocEvictionAdvisor() = default;
 103
 104   /// Find a physical register that can be freed by evicting the FixedRegisters,
 105   /// or return NoRegister. The eviction decision is assumed to be correct (i.e.
 106   /// no fixed live ranges are evicted) and profitable.
 107   virtual MCRegister tryFindEvictionCandidate(
 108       const LiveInterval &VirtReg, const AllocationOrder &Order,
 109       uint8_t CostPerUseLimit, const SmallVirtRegSet &FixedRegisters) const = 0;
 110
 111   /// Find out if we can evict the live ranges occupying the given PhysReg,
 112   /// which is a hint (preferred register) for VirtReg.
 113   virtual bool
 114   canEvictHintInterference(const LiveInterval &VirtReg, MCRegister PhysReg,
 115                            const SmallVirtRegSet &FixedRegisters) const = 0;
 116
 117   /// Returns true if the given \p PhysReg is a callee saved register and has
 118   /// not been used for allocation yet.
 119   bool isUnusedCalleeSavedReg(MCRegister PhysReg) const;
 120
 121 protected:
 122   RegAllocEvictionAdvisor(const MachineFunction &MF, const RAGreedy &RA);
 123
 124   bool canReassign(const LiveInterval &VirtReg, MCRegister FromReg) const;
 125
 126   // Get the upper limit of elements in the given Order we need to analize.
 127   // TODO: is this heuristic,  we could consider learning it.
 128   std::optional<unsigned> getOrderLimit(const LiveInterval &VirtReg,
 129                                         const AllocationOrder &Order,
 130                                         unsigned CostPerUseLimit) const;
 131
 132   // Determine if it's worth trying to allocate this reg, given the
 133   // CostPerUseLimit
 134   // TODO: this is a heuristic component we could consider learning, too.
 135   bool canAllocatePhysReg(unsigned CostPerUseLimit, MCRegister PhysReg) const;
 136
 137   const MachineFunction &MF;
 138   const RAGreedy &RA;
 139   LiveRegMatrix *const Matrix;
 140   LiveIntervals *const LIS;
 141   VirtRegMap *const VRM;
 142   MachineRegisterInfo *const MRI;
 143   const TargetRegisterInfo *const TRI;
 144   const RegisterClassInfo &RegClassInfo;
 145   const ArrayRef<uint8_t> RegCosts;
 146
 147   /// Run or not the local reassignment heuristic. This information is
 148   /// obtained from the TargetSubtargetInfo.
 149   const bool EnableLocalReassign;
 150 };
 151
 152 /// ImmutableAnalysis abstraction for fetching the Eviction Advisor. We model it
 153 /// as an analysis to decouple the user from the implementation insofar as
 154 /// dependencies on other analyses goes. The motivation for it being an
 155 /// immutable pass is twofold:
 156 /// - in the ML implementation case, the evaluator is stateless but (especially
 157 /// in the development mode) expensive to set up. With an immutable pass, we set
 158 /// it up once.
 159 /// - in the 'development' mode ML case, we want to capture the training log
 160 /// during allocation (this is a log of features encountered and decisions
 161 /// made), and then measure a score, potentially a few steps after allocation
 162 /// completes. So we need the properties of an immutable pass to keep the logger
 163 /// state around until we can make that measurement.
 164 ///
 165 /// Because we need to offer additional services in 'development' mode, the
 166 /// implementations of this analysis need to implement RTTI support.
 167 class RegAllocEvictionAdvisorAnalysis : public ImmutablePass {
 168 public:
 169   enum class AdvisorMode : int { Default, Release, Development };
 170
 171   RegAllocEvictionAdvisorAnalysis(AdvisorMode Mode)
 172       : ImmutablePass(ID), Mode(Mode){};
 173   static char ID;
 174
 175   /// Get an advisor for the given context (i.e. machine function, etc)
 176   virtual std::unique_ptr<RegAllocEvictionAdvisor>
 177   getAdvisor(const MachineFunction &MF, const RAGreedy &RA) = 0;
 178   AdvisorMode getAdvisorMode() const { return Mode; }
 179   virtual void logRewardIfNeeded(const MachineFunction &MF,
 180                                  llvm::function_ref<float()> GetReward){};
 181
 182 protected:
 183   // This analysis preserves everything, and subclasses may have additional
 184   // requirements.
 185   void getAnalysisUsage(AnalysisUsage &AU) const override {
 186     AU.setPreservesAll();
 187   }
 188
 189 private:
 190   StringRef getPassName() const override;
 191   const AdvisorMode Mode;
 192 };
 193
 194 /// Specialization for the API used by the analysis infrastructure to create
 195 /// an instance of the eviction advisor.
 196 template <> Pass *callDefaultCtor<RegAllocEvictionAdvisorAnalysis>();
 197
 198 RegAllocEvictionAdvisorAnalysis *createReleaseModeAdvisor();
 199
 200 RegAllocEvictionAdvisorAnalysis *createDevelopmentModeAdvisor();
 201
 202 // TODO: move to RegAllocEvictionAdvisor.cpp when we move implementation
 203 // out of RegAllocGreedy.cpp
 204 class DefaultEvictionAdvisor : public RegAllocEvictionAdvisor {
 205 public:
 206   DefaultEvictionAdvisor(const MachineFunction &MF, const RAGreedy &RA)
 207       : RegAllocEvictionAdvisor(MF, RA) {}
 208
 209 private:
 210   MCRegister tryFindEvictionCandidate(const LiveInterval &,
 211                                       const AllocationOrder &, uint8_t,
 212                                       const SmallVirtRegSet &) const override;
 213   bool canEvictHintInterference(const LiveInterval &, MCRegister,
 214                                 const SmallVirtRegSet &) const override;
 215   bool canEvictInterferenceBasedOnCost(const LiveInterval &, MCRegister, bool,
 216                                        EvictionCost &,
 217                                        const SmallVirtRegSet &) const;
 218   bool shouldEvict(const LiveInterval &A, bool, const LiveInterval &B,
 219                    bool) const;
 220 };
 221 } // namespace llvm
 222
 223 #endif // LLVM_CODEGEN_REGALLOCEVICTIONADVISOR_H