EdgeProfiling.cpp revision 198090 
1//===- EdgeProfiling.cpp - Insert counters for 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// Note that this implementation is very naive.  We insert a counter for *every*
15// edge in the program, instead of using control flow information to prune the
16// number of counters inserted.
17//
18//===----------------------------------------------------------------------===//
19#define DEBUG_TYPE "insert-edge-profiling"
20#include "ProfilingUtils.h"
21#include "llvm/Module.h"
22#include "llvm/Pass.h"
23#include "llvm/Support/Compiler.h"
24#include "llvm/Support/raw_ostream.h"
25#include "llvm/Transforms/Utils/BasicBlockUtils.h"
26#include "llvm/Transforms/Instrumentation.h"
27#include "llvm/ADT/Statistic.h"
28#include <set>
29using namespace llvm;
30
31STATISTIC(NumEdgesInserted, "The # of edges inserted.");
32
33namespace {
34  class VISIBILITY_HIDDEN EdgeProfiler : public ModulePass {
35    bool runOnModule(Module &M);
36  public:
37    static char ID; // Pass identification, replacement for typeid
38    EdgeProfiler() : ModulePass(&ID) {}
39
40    virtual const char *getPassName() const {
41      return "Edge Profiler";
42    }
43  };
44}
45
46char EdgeProfiler::ID = 0;
47static RegisterPass<EdgeProfiler>
48X("insert-edge-profiling", "Insert instrumentation for edge profiling");
49
50ModulePass *llvm::createEdgeProfilerPass() { return new EdgeProfiler(); }
51
52bool EdgeProfiler::runOnModule(Module &M) {
53  Function *Main = M.getFunction("main");
54  if (Main == 0) {
55    errs() << "WARNING: cannot insert edge profiling into a module"
56           << " with no main function!\n";
57    return false;  // No main, no instrumentation!
58  }
59
60  std::set<BasicBlock*> BlocksToInstrument;
61  unsigned NumEdges = 0;
62  for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
63    if (F->isDeclaration()) continue;
64    // Reserve space for (0,entry) edge.
65    ++NumEdges;
66    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
67      // Keep track of which blocks need to be instrumented.  We don't want to
68      // instrument blocks that are added as the result of breaking critical
69      // edges!
70      BlocksToInstrument.insert(BB);
71      NumEdges += BB->getTerminator()->getNumSuccessors();
72    }
73  }
74
75  const Type *ATy = ArrayType::get(Type::getInt32Ty(M.getContext()), NumEdges);
76  GlobalVariable *Counters =
77    new GlobalVariable(M, ATy, false, GlobalValue::InternalLinkage,
78                       Constant::getNullValue(ATy), "EdgeProfCounters");
79  NumEdgesInserted = NumEdges;
80
81  // Instrument all of the edges...
82  unsigned i = 0;
83  for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
84    if (F->isDeclaration()) continue;
85    // Create counter for (0,entry) edge.
86    IncrementCounterInBlock(&F->getEntryBlock(), i++, Counters);
87    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
88      if (BlocksToInstrument.count(BB)) {  // Don't instrument inserted blocks
89        // Okay, we have to add a counter of each outgoing edge.  If the
90        // outgoing edge is not critical don't split it, just insert the counter
91        // in the source or destination of the edge.
92        TerminatorInst *TI = BB->getTerminator();
93        for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s) {
94          // If the edge is critical, split it.
95          SplitCriticalEdge(TI, s, this);
96
97          // Okay, we are guaranteed that the edge is no longer critical.  If we
98          // only have a single successor, insert the counter in this block,
99          // otherwise insert it in the successor block.
100          if (TI->getNumSuccessors() == 1) {
101            // Insert counter at the start of the block
102            IncrementCounterInBlock(BB, i++, Counters);
103          } else {
104            // Insert counter at the start of the block
105            IncrementCounterInBlock(TI->getSuccessor(s), i++, Counters);
106          }
107        }
108      }
109  }
110
111  // Add the initialization call to main.
112  InsertProfilingInitCall(Main, "llvm_start_edge_profiling", Counters);
113  return true;
114}
115
116