1 //=- MachineBranchProbabilityInfo.h - Branch Probability Analysis -*- C++ -*-=//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass is used to evaluate branch probabilties on machine basic blocks.
12 //===----------------------------------------------------------------------===//
14 #ifndef LLVM_CODEGEN_MACHINEBRANCHPROBABILITYINFO_H
15 #define LLVM_CODEGEN_MACHINEBRANCHPROBABILITYINFO_H
17 #include "llvm/CodeGen/MachineBasicBlock.h"
18 #include "llvm/Pass.h"
19 #include "llvm/Support/BranchProbability.h"
24 class MachineBranchProbabilityInfo : public ImmutablePass {
25 virtual void anchor();
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;
38 MachineBranchProbabilityInfo() : ImmutablePass(ID) {
39 PassRegistry &Registry = *PassRegistry::getPassRegistry();
40 initializeMachineBranchProbabilityInfoPass(Registry);
43 void getAnalysisUsage(AnalysisUsage &AU) const override {
47 // Return edge weight. If we don't have any informations about it - return
49 uint32_t getEdgeWeight(const MachineBasicBlock *Src,
50 const MachineBasicBlock *Dst) const;
52 // Same thing, but using a const_succ_iterator from Src. This is faster when
53 // the iterator is already available.
54 uint32_t getEdgeWeight(const MachineBasicBlock *Src,
55 MachineBasicBlock::const_succ_iterator Dst) const;
57 // Get sum of the block successors' weights, potentially scaling them to fit
58 // within 32-bits. If scaling is required, sets Scale based on the necessary
59 // adjustment. Any edge weights used with the sum should be divided by Scale.
60 uint32_t getSumForBlock(const MachineBasicBlock *MBB, uint32_t &Scale) const;
62 // Get sum of the block successors' weights, and force normalizing the
63 // successors' weights of MBB so that their sum fit within 32-bits.
64 uint32_t getSumForBlock(MachineBasicBlock *MBB) const;
66 // A 'Hot' edge is an edge which probability is >= 80%.
67 bool isEdgeHot(const MachineBasicBlock *Src,
68 const MachineBasicBlock *Dst) const;
70 // Return a hot successor for the block BB or null if there isn't one.
71 // NB: This routine's complexity is linear on the number of successors.
72 MachineBasicBlock *getHotSucc(MachineBasicBlock *MBB) const;
74 // Return a probability as a fraction between 0 (0% probability) and
75 // 1 (100% probability), however the value is never equal to 0, and can be 1
76 // only iff SRC block has only one successor.
77 // NB: This routine's complexity is linear on the number of successors of
78 // Src. Querying sequentially for each successor's probability is a quadratic
80 BranchProbability getEdgeProbability(const MachineBasicBlock *Src,
81 const MachineBasicBlock *Dst) const;
83 // Print value between 0 (0% probability) and 1 (100% probability),
84 // however the value is never equal to 0, and can be 1 only iff SRC block
85 // has only one successor.
86 raw_ostream &printEdgeProbability(raw_ostream &OS,
87 const MachineBasicBlock *Src,
88 const MachineBasicBlock *Dst) const;
90 // Normalize a list of weights by scaling them down so that the sum of them
91 // doesn't exceed UINT32_MAX. Return the scale.
92 template <class WeightList>
93 static uint32_t normalizeEdgeWeights(WeightList &Weights);
96 template <class WeightList>
98 MachineBranchProbabilityInfo::normalizeEdgeWeights(WeightList &Weights) {
99 assert(Weights.size() < UINT32_MAX && "Too many weights in the list!");
100 // First we compute the sum with 64-bits of precision.
101 uint64_t Sum = std::accumulate(Weights.begin(), Weights.end(), uint64_t(0));
103 // If the computed sum fits in 32-bits, we're done.
104 if (Sum <= UINT32_MAX)
107 // Otherwise, compute the scale necessary to cause the weights to fit, and
108 // re-sum with that scale applied.
109 assert((Sum / UINT32_MAX) < UINT32_MAX &&
110 "The sum of weights exceeds UINT32_MAX^2!");
111 uint32_t Scale = (Sum / UINT32_MAX) + 1;
112 for (auto &W : Weights)