llvm/lib/CodeGen/BranchFolding.h

   1 //===- BranchFolding.h - Fold machine code branch instructions --*- 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_LIB_CODEGEN_BRANCHFOLDING_H
  10 #define LLVM_LIB_CODEGEN_BRANCHFOLDING_H
  11
  12 #include "llvm/ADT/DenseMap.h"
  13 #include "llvm/ADT/SmallPtrSet.h"
  14 #include "llvm/CodeGen/LivePhysRegs.h"
  15 #include "llvm/CodeGen/MachineBasicBlock.h"
  16 #include "llvm/Support/Compiler.h"
  17 #include <vector>
  18
  19 namespace llvm {
  20
  21 class BasicBlock;
  22 class MachineBranchProbabilityInfo;
  23 class MachineFunction;
  24 class MachineLoopInfo;
  25 class MachineRegisterInfo;
  26 class MBFIWrapper;
  27 class ProfileSummaryInfo;
  28 class TargetInstrInfo;
  29 class TargetRegisterInfo;
  30
  31   class LLVM_LIBRARY_VISIBILITY BranchFolder {
  32   public:
  33     explicit BranchFolder(bool DefaultEnableTailMerge, bool CommonHoist,
  34                           MBFIWrapper &FreqInfo,
  35                           const MachineBranchProbabilityInfo &ProbInfo,
  36                           ProfileSummaryInfo *PSI,
  37                           // Min tail length to merge. Defaults to commandline
  38                           // flag. Ignored for optsize.
  39                           unsigned MinTailLength = 0);
  40
  41     /// Perhaps branch folding, tail merging and other CFG optimizations on the
  42     /// given function.  Block placement changes the layout and may create new
  43     /// tail merging opportunities.
  44     bool OptimizeFunction(MachineFunction &MF, const TargetInstrInfo *tii,
  45                           const TargetRegisterInfo *tri,
  46                           MachineLoopInfo *mli = nullptr,
  47                           bool AfterPlacement = false);
  48
  49   private:
  50     class MergePotentialsElt {
  51       unsigned Hash;
  52       MachineBasicBlock *Block;
  53
  54     public:
  55       MergePotentialsElt(unsigned h, MachineBasicBlock *b)
  56         : Hash(h), Block(b) {}
  57
  58       unsigned getHash() const { return Hash; }
  59       MachineBasicBlock *getBlock() const { return Block; }
  60
  61       void setBlock(MachineBasicBlock *MBB) {
  62         Block = MBB;
  63       }
  64
  65       bool operator<(const MergePotentialsElt &) const;
  66     };
  67
  68     using MPIterator = std::vector<MergePotentialsElt>::iterator;
  69
  70     std::vector<MergePotentialsElt> MergePotentials;
  71     SmallPtrSet<const MachineBasicBlock*, 2> TriedMerging;
  72     DenseMap<const MachineBasicBlock *, int> EHScopeMembership;
  73
  74     class SameTailElt {
  75       MPIterator MPIter;
  76       MachineBasicBlock::iterator TailStartPos;
  77
  78     public:
  79       SameTailElt(MPIterator mp, MachineBasicBlock::iterator tsp)
  80         : MPIter(mp), TailStartPos(tsp) {}
  81
  82       MPIterator getMPIter() const {
  83         return MPIter;
  84       }
  85
  86       MergePotentialsElt &getMergePotentialsElt() const {
  87         return *getMPIter();
  88       }
  89
  90       MachineBasicBlock::iterator getTailStartPos() const {
  91         return TailStartPos;
  92       }
  93
  94       unsigned getHash() const {
  95         return getMergePotentialsElt().getHash();
  96       }
  97
  98       MachineBasicBlock *getBlock() const {
  99         return getMergePotentialsElt().getBlock();
 100       }
 101
 102       bool tailIsWholeBlock() const {
 103         return TailStartPos == getBlock()->begin();
 104       }
 105
 106       void setBlock(MachineBasicBlock *MBB) {
 107         getMergePotentialsElt().setBlock(MBB);
 108       }
 109
 110       void setTailStartPos(MachineBasicBlock::iterator Pos) {
 111         TailStartPos = Pos;
 112       }
 113     };
 114     std::vector<SameTailElt> SameTails;
 115
 116     bool AfterBlockPlacement = false;
 117     bool EnableTailMerge = false;
 118     bool EnableHoistCommonCode = false;
 119     bool UpdateLiveIns = false;
 120     unsigned MinCommonTailLength;
 121     const TargetInstrInfo *TII = nullptr;
 122     const MachineRegisterInfo *MRI = nullptr;
 123     const TargetRegisterInfo *TRI = nullptr;
 124     MachineLoopInfo *MLI = nullptr;
 125     LivePhysRegs LiveRegs;
 126
 127   private:
 128     MBFIWrapper &MBBFreqInfo;
 129     const MachineBranchProbabilityInfo &MBPI;
 130     ProfileSummaryInfo *PSI;
 131
 132     bool TailMergeBlocks(MachineFunction &MF);
 133     bool TryTailMergeBlocks(MachineBasicBlock* SuccBB,
 134                        MachineBasicBlock* PredBB,
 135                        unsigned MinCommonTailLength);
 136     void setCommonTailEdgeWeights(MachineBasicBlock &TailMBB);
 137
 138     /// Delete the instruction OldInst and everything after it, replacing it
 139     /// with an unconditional branch to NewDest.
 140     void replaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
 141                                  MachineBasicBlock &NewDest);
 142
 143     /// Given a machine basic block and an iterator into it, split the MBB so
 144     /// that the part before the iterator falls into the part starting at the
 145     /// iterator.  This returns the new MBB.
 146     MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB,
 147                                   MachineBasicBlock::iterator BBI1,
 148                                   const BasicBlock *BB);
 149
 150     /// Look through all the blocks in MergePotentials that have hash CurHash
 151     /// (guaranteed to match the last element).  Build the vector SameTails of
 152     /// all those that have the (same) largest number of instructions in common
 153     /// of any pair of these blocks.  SameTails entries contain an iterator into
 154     /// MergePotentials (from which the MachineBasicBlock can be found) and a
 155     /// MachineBasicBlock::iterator into that MBB indicating the instruction
 156     /// where the matching code sequence begins.  Order of elements in SameTails
 157     /// is the reverse of the order in which those blocks appear in
 158     /// MergePotentials (where they are not necessarily consecutive).
 159     unsigned ComputeSameTails(unsigned CurHash, unsigned minCommonTailLength,
 160                               MachineBasicBlock *SuccBB,
 161                               MachineBasicBlock *PredBB);
 162
 163     /// Remove all blocks with hash CurHash from MergePotentials, restoring
 164     /// branches at ends of blocks as appropriate.
 165     void RemoveBlocksWithHash(unsigned CurHash, MachineBasicBlock* SuccBB,
 166                                                 MachineBasicBlock* PredBB);
 167
 168     /// None of the blocks to be tail-merged consist only of the common tail.
 169     /// Create a block that does by splitting one.
 170     bool CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
 171                                    MachineBasicBlock *SuccBB,
 172                                    unsigned maxCommonTailLength,
 173                                    unsigned &commonTailIndex);
 174
 175     /// Create merged DebugLocs of identical instructions across SameTails and
 176     /// assign it to the instruction in common tail; merge MMOs and undef flags.
 177     void mergeCommonTails(unsigned commonTailIndex);
 178
 179     bool OptimizeBranches(MachineFunction &MF);
 180
 181     /// Analyze and optimize control flow related to the specified block. This
 182     /// is never called on the entry block.
 183     bool OptimizeBlock(MachineBasicBlock *MBB);
 184
 185     /// Remove the specified dead machine basic block from the function,
 186     /// updating the CFG.
 187     void RemoveDeadBlock(MachineBasicBlock *MBB);
 188
 189     /// Hoist common instruction sequences at the start of basic blocks to their
 190     /// common predecessor.
 191     bool HoistCommonCode(MachineFunction &MF);
 192
 193     /// If the successors of MBB has common instruction sequence at the start of
 194     /// the function, move the instructions before MBB terminator if it's legal.
 195     bool HoistCommonCodeInSuccs(MachineBasicBlock *MBB);
 196   };
 197
 198 } // end namespace llvm
 199
 200 #endif // LLVM_LIB_CODEGEN_BRANCHFOLDING_H