include/llvm/CodeGen/MachineBranchProbabilityInfo.h

   1 //=- MachineBranchProbabilityInfo.h - Branch Probability Analysis -*- 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 pass is used to evaluate branch probabilties on machine basic blocks.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #ifndef LLVM_CODEGEN_MACHINEBRANCHPROBABILITYINFO_H
  14 #define LLVM_CODEGEN_MACHINEBRANCHPROBABILITYINFO_H
  15
  16 #include "llvm/CodeGen/MachineBasicBlock.h"
  17 #include "llvm/Pass.h"
  18 #include "llvm/Support/BranchProbability.h"
  19 #include <climits>
  20 #include <numeric>
  21
  22 namespace llvm {
  23
  24 class MachineBranchProbabilityInfo : public ImmutablePass {
  25   virtual void anchor();
  26
  27   // Default weight value. Used when we don't have information about the edge.
  28   // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
  29   // the successors have a weight yet. But it doesn't make sense when providing
  30   // weight to an edge that may have siblings with non-zero weights. This can
  31   // be handled various ways, but it's probably fine for an edge with unknown
  32   // weight to just "inherit" the non-zero weight of an adjacent successor.
  33   static const uint32_t DEFAULT_WEIGHT = 16;
  34
  35 public:
  36   static char ID;
  37
  38   MachineBranchProbabilityInfo() : ImmutablePass(ID) {
  39     PassRegistry &Registry = *PassRegistry::getPassRegistry();
  40     initializeMachineBranchProbabilityInfoPass(Registry);
  41   }
  42
  43   void getAnalysisUsage(AnalysisUsage &AU) const override {
  44     AU.setPreservesAll();
  45   }
  46
  47   // Return edge probability.
  48   BranchProbability getEdgeProbability(const MachineBasicBlock *Src,
  49                                        const MachineBasicBlock *Dst) const;
  50
  51   // Same as above, but using a const_succ_iterator from Src. This is faster
  52   // when the iterator is already available.
  53   BranchProbability
  54   getEdgeProbability(const MachineBasicBlock *Src,
  55                      MachineBasicBlock::const_succ_iterator Dst) const;
  56
  57   // A 'Hot' edge is an edge which probability is >= 80%.
  58   bool isEdgeHot(const MachineBasicBlock *Src,
  59                  const MachineBasicBlock *Dst) const;
  60
  61   // Return a hot successor for the block BB or null if there isn't one.
  62   // NB: This routine's complexity is linear on the number of successors.
  63   MachineBasicBlock *getHotSucc(MachineBasicBlock *MBB) const;
  64
  65   // Print value between 0 (0% probability) and 1 (100% probability),
  66   // however the value is never equal to 0, and can be 1 only iff SRC block
  67   // has only one successor.
  68   raw_ostream &printEdgeProbability(raw_ostream &OS,
  69                                     const MachineBasicBlock *Src,
  70                                     const MachineBasicBlock *Dst) const;
  71 };
  72
  73 }
  74
  75
  76 #endif