EdgeProfiling.cpp revision 081c34b725980f995be9080eaec24cd3dfaaf065 
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/raw_ostream.h"
24#include "llvm/Transforms/Utils/BasicBlockUtils.h"
25#include "llvm/Transforms/Instrumentation.h"
26#include "llvm/ADT/Statistic.h"
27#include <set>
28using namespace llvm;
29
30STATISTIC(NumEdgesInserted, "The # of edges inserted.");
31
32namespace {
33  class EdgeProfiler : public ModulePass {
34    bool runOnModule(Module &M);
35  public:
36    static char ID; // Pass identification, replacement for typeid
37    EdgeProfiler() : ModulePass(ID) {
38      initializeEdgeProfilerPass(*PassRegistry::getPassRegistry());
39    }
40
41    virtual const char *getPassName() const {
42      return "Edge Profiler";
43    }
44  };
45}
46
47char EdgeProfiler::ID = 0;
48INITIALIZE_PASS(EdgeProfiler, "insert-edge-profiling",
49                "Insert instrumentation for edge profiling", false, false)
50
51ModulePass *llvm::createEdgeProfilerPass() { return new EdgeProfiler(); }
52
53bool EdgeProfiler::runOnModule(Module &M) {
54  Function *Main = M.getFunction("main");
55  if (Main == 0) {
56    errs() << "WARNING: cannot insert edge profiling into a module"
57           << " with no main function!\n";
58    return false;  // No main, no instrumentation!
59  }
60
61  std::set<BasicBlock*> BlocksToInstrument;
62  unsigned NumEdges = 0;
63  for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
64    if (F->isDeclaration()) continue;
65    // Reserve space for (0,entry) edge.
66    ++NumEdges;
67    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
68      // Keep track of which blocks need to be instrumented.  We don't want to
69      // instrument blocks that are added as the result of breaking critical
70      // edges!
71      BlocksToInstrument.insert(BB);
72      NumEdges += BB->getTerminator()->getNumSuccessors();
73    }
74  }
75
76  const Type *ATy = ArrayType::get(Type::getInt32Ty(M.getContext()), NumEdges);
77  GlobalVariable *Counters =
78    new GlobalVariable(M, ATy, false, GlobalValue::InternalLinkage,
79                       Constant::getNullValue(ATy), "EdgeProfCounters");
80  NumEdgesInserted = NumEdges;
81
82  // Instrument all of the edges...
83  unsigned i = 0;
84  for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
85    if (F->isDeclaration()) continue;
86    // Create counter for (0,entry) edge.
87    IncrementCounterInBlock(&F->getEntryBlock(), i++, Counters);
88    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
89      if (BlocksToInstrument.count(BB)) {  // Don't instrument inserted blocks
90        // Okay, we have to add a counter of each outgoing edge.  If the
91        // outgoing edge is not critical don't split it, just insert the counter
92        // in the source or destination of the edge.
93        TerminatorInst *TI = BB->getTerminator();
94        for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s) {
95          // If the edge is critical, split it.
96          SplitCriticalEdge(TI, s, this);
97
98          // Okay, we are guaranteed that the edge is no longer critical.  If we
99          // only have a single successor, insert the counter in this block,
100          // otherwise insert it in the successor block.
101          if (TI->getNumSuccessors() == 1) {
102            // Insert counter at the start of the block
103            IncrementCounterInBlock(BB, i++, Counters);
104          } else {
105            // Insert counter at the start of the block
106            IncrementCounterInBlock(TI->getSuccessor(s), i++, Counters);
107          }
108        }
109      }
110  }
111
112  // Add the initialization call to main.
113  InsertProfilingInitCall(Main, "llvm_start_edge_profiling", Counters);
114  return true;
115}
116
117