OptimalEdgeProfiling.cpp revision 39859438715fc8f9ff16d7cec6cf2a9cb2ac0803 
1//===- OptimalEdgeProfiling.cpp - Insert counters for opt. edge profiling -===//
2//
3//                      The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass instruments the specified program with counters for edge profiling.
11// Edge profiling can give a reasonable approximation of the hot paths through a
12// program, and is used for a wide variety of program transformations.
13//
14//===----------------------------------------------------------------------===//
15#define DEBUG_TYPE "insert-optimal-edge-profiling"
16#include "ProfilingUtils.h"
17#include "llvm/Module.h"
18#include "llvm/Pass.h"
19#include "llvm/Analysis/Passes.h"
20#include "llvm/Support/Compiler.h"
21#include "llvm/Support/raw_ostream.h"
22#include "llvm/Support/Debug.h"
23#include "llvm/Transforms/Utils/BasicBlockUtils.h"
24#include "llvm/Transforms/Instrumentation.h"
25#include "llvm/ADT/DenseSet.h"
26#include "llvm/ADT/Statistic.h"
27#include "MaximumSpanningTree.h"
28#include <set>
29using namespace llvm;
30
31STATISTIC(NumEdgesInserted, "The # of edges inserted.");
32
33namespace {
34  class VISIBILITY_HIDDEN OptimalEdgeProfiler : public ModulePass {
35    bool runOnModule(Module &M);
36  public:
37    static char ID; // Pass identification, replacement for typeid
38    OptimalEdgeProfiler() : ModulePass(&ID) {}
39
40    void getAnalysisUsage(AnalysisUsage &AU) const {
41      AU.addRequiredID(ProfileEstimatorPassID);
42      AU.addRequired<ProfileInfo>();
43    }
44
45    virtual const char *getPassName() const {
46      return "Optimal Edge Profiler";
47    }
48  };
49}
50
51char OptimalEdgeProfiler::ID = 0;
52static RegisterPass<OptimalEdgeProfiler>
53X("insert-optimal-edge-profiling",
54  "Insert optimal instrumentation for edge profiling");
55
56ModulePass *llvm::createOptimalEdgeProfilerPass() {
57  return new OptimalEdgeProfiler();
58}
59
60inline static void printEdgeCounter(ProfileInfo::Edge e,
61                                    BasicBlock* b,
62                                    unsigned i) {
63  DEBUG(errs() << "--Edge Counter for " << (e) << " in " \
64               << ((b)?(b)->getNameStr():"0") << " (# " << (i) << ")\n");
65}
66
67bool OptimalEdgeProfiler::runOnModule(Module &M) {
68  Function *Main = M.getFunction("main");
69  if (Main == 0) {
70    errs() << "WARNING: cannot insert edge profiling into a module"
71           << " with no main function!\n";
72    return false;  // No main, no instrumentation!
73  }
74
75  // NumEdges counts all the edges that may be instrumented. Later on its
76  // decided which edges to actually instrument, to achieve optimal profiling.
77  // For the entry block a virtual edge (0,entry) is reserved, for each block
78  // with no successors an edge (BB,0) is reserved. These edges are necessary
79  // to calculate a truly optimal maximum spanning tree and thus an optimal
80  // instrumentation.
81  unsigned NumEdges = 0;
82
83  for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
84    if (F->isDeclaration()) continue;
85    // Reserve space for (0,entry) edge.
86    ++NumEdges;
87    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
88      // Keep track of which blocks need to be instrumented.  We don't want to
89      // instrument blocks that are added as the result of breaking critical
90      // edges!
91      if (BB->getTerminator()->getNumSuccessors() == 0) {
92        // Reserve space for (BB,0) edge.
93        ++NumEdges;
94      } else {
95        NumEdges += BB->getTerminator()->getNumSuccessors();
96      }
97    }
98  }
99
100  // In the profiling output a counter for each edge is reserved, but only few
101  // are used. This is done to be able to read back in the profile without
102  // calulating the maximum spanning tree again, instead each edge counter that
103  // is not used is initialised with -1 to signal that this edge counter has to
104  // be calculated from other edge counters on reading the profile info back
105  // in.
106
107  const Type *Int32 = Type::getInt32Ty(M.getContext());
108  const ArrayType *ATy = ArrayType::get(Int32, NumEdges);
109  GlobalVariable *Counters =
110    new GlobalVariable(M, ATy, false, GlobalValue::InternalLinkage,
111                       Constant::getNullValue(ATy), "OptEdgeProfCounters");
112  NumEdgesInserted = 0;
113
114  std::vector<Constant*> Initializer(NumEdges);
115  Constant* zeroc = ConstantInt::get(Int32, 0);
116  Constant* minusonec = ConstantInt::get(Int32, ProfileInfo::MissingValue);
117
118  // Instrument all of the edges not in MST...
119  unsigned i = 0;
120  for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
121    if (F->isDeclaration()) continue;
122    DEBUG(errs()<<"Working on "<<F->getNameStr()<<"\n");
123
124    // Calculate a Maximum Spanning Tree with the edge weights determined by
125    // ProfileEstimator. ProfileEstimator also assign weights to the virtual
126    // edges (0,entry) and (BB,0) (for blocks with no successors) and this
127    // edges also participate in the maximum spanning tree calculation.
128    // The third parameter of MaximumSpanningTree() has the effect that not the
129    // actual MST is returned but the edges _not_ in the MST.
130
131    ProfileInfo::EdgeWeights ECs =
132      getAnalysisID<ProfileInfo>(ProfileEstimatorPassID, *F).getEdgeWeights(F);
133    std::vector<ProfileInfo::EdgeWeight> EdgeVector(ECs.begin(), ECs.end());
134    MaximumSpanningTree MST = MaximumSpanningTree(EdgeVector);
135
136    // Check if (0,entry) not in the MST. If not, instrument edge
137    // (IncrementCounterInBlock()) and set the counter initially to zero, if
138    // the edge is in the MST the counter is initialised to -1.
139
140    BasicBlock *entry = &(F->getEntryBlock());
141    ProfileInfo::Edge edge = ProfileInfo::getEdge(0,entry);
142    if (!std::binary_search(MST.begin(), MST.end(), edge)) {
143      printEdgeCounter(edge,entry,i);
144      IncrementCounterInBlock(entry, i, Counters); NumEdgesInserted++;
145      Initializer[i++] = (zeroc);
146    } else{
147      Initializer[i++] = (minusonec);
148    }
149
150    // InsertedBlocks contains all blocks that were inserted for splitting an
151    // edge, this blocks do not have to be instrumented.
152    DenseSet<BasicBlock*> InsertedBlocks;
153    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
154      // Check if block was not inserted and thus does not have to be
155      // instrumented.
156      if (InsertedBlocks.count(BB)) continue;
157
158      // Okay, we have to add a counter of each outgoing edge not in MST. If
159      // the outgoing edge is not critical don't split it, just insert the
160      // counter in the source or destination of the edge. Also, if the block
161      // has no successors, the virtual edge (BB,0) is processed.
162      TerminatorInst *TI = BB->getTerminator();
163      if (TI->getNumSuccessors() == 0) {
164        ProfileInfo::Edge edge = ProfileInfo::getEdge(BB,0);
165        if (!std::binary_search(MST.begin(), MST.end(), edge)) {
166          printEdgeCounter(edge,BB,i);
167          IncrementCounterInBlock(BB, i, Counters); NumEdgesInserted++;
168          Initializer[i++] = (zeroc);
169        } else{
170          Initializer[i++] = (minusonec);
171        }
172      }
173      for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s) {
174        BasicBlock *Succ = TI->getSuccessor(s);
175        ProfileInfo::Edge edge = ProfileInfo::getEdge(BB,Succ);
176        if (!std::binary_search(MST.begin(), MST.end(), edge)) {
177
178          // If the edge is critical, split it.
179          bool wasInserted = SplitCriticalEdge(TI, s, this);
180          Succ = TI->getSuccessor(s);
181          if (wasInserted)
182            InsertedBlocks.insert(Succ);
183
184          // Okay, we are guaranteed that the edge is no longer critical.  If
185          // we only have a single successor, insert the counter in this block,
186          // otherwise insert it in the successor block.
187          if (TI->getNumSuccessors() == 1) {
188            // Insert counter at the start of the block
189            printEdgeCounter(edge,BB,i);
190            IncrementCounterInBlock(BB, i, Counters); NumEdgesInserted++;
191          } else {
192            // Insert counter at the start of the block
193            printEdgeCounter(edge,Succ,i);
194            IncrementCounterInBlock(Succ, i, Counters); NumEdgesInserted++;
195          }
196          Initializer[i++] = (zeroc);
197        } else {
198          Initializer[i++] = (minusonec);
199        }
200      }
201    }
202  }
203
204  // Check if the number of edges counted at first was the number of edges we
205  // considered for instrumentation.
206  assert(i==NumEdges && "the number of edges in counting array is wrong");
207
208  // Assing the now completely defined initialiser to the array.
209  Constant *init = ConstantArray::get(ATy, Initializer);
210  Counters->setInitializer(init);
211
212  // Add the initialization call to main.
213  InsertProfilingInitCall(Main, "llvm_start_opt_edge_profiling", Counters);
214  return true;
215}
216
217