EdgeProfiling.cpp revision 4ae20278018af959091a3dcd990dc2f78bf632d0 
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
20#include "ProfilingUtils.h"
21#include "llvm/Constants.h"
22#include "llvm/DerivedTypes.h"
23#include "llvm/LLVMContext.h"
24#include "llvm/Module.h"
25#include "llvm/Pass.h"
26#include "llvm/Support/Compiler.h"
27#include "llvm/Support/Streams.h"
28#include "llvm/Transforms/Utils/BasicBlockUtils.h"
29#include "llvm/Transforms/Instrumentation.h"
30#include <set>
31using namespace llvm;
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}
41
42char EdgeProfiler::ID = 0;
43static RegisterPass<EdgeProfiler>
44X("insert-edge-profiling", "Insert instrumentation for edge profiling");
45
46ModulePass *llvm::createEdgeProfilerPass() { return new EdgeProfiler(); }
47
48bool EdgeProfiler::runOnModule(Module &M) {
49  Function *Main = M.getFunction("main");
50  if (Main == 0) {
51    cerr << "WARNING: cannot insert edge profiling into a module"
52         << " with no main function!\n";
53    return false;  // No main, no instrumentation!
54  }
55
56  std::set<BasicBlock*> BlocksToInstrument;
57  unsigned NumEdges = 0;
58  for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
59    if (F->isDeclaration()) continue;
60    // Reserve space for (0,entry) edge.
61    ++NumEdges;
62    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
63      // Keep track of which blocks need to be instrumented.  We don't want to
64      // instrument blocks that are added as the result of breaking critical
65      // edges!
66      BlocksToInstrument.insert(BB);
67      NumEdges += BB->getTerminator()->getNumSuccessors();
68    }
69  }
70
71  const Type *ATy = ArrayType::get(Type::Int32Ty, NumEdges);
72  GlobalVariable *Counters =
73    new GlobalVariable(M, ATy, false, GlobalValue::InternalLinkage,
74                       Constant::getNullValue(ATy), "EdgeProfCounters");
75
76  // Instrument all of the edges...
77  unsigned i = 0;
78  for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
79    if (F->isDeclaration()) continue;
80    // Create counter for (0,entry) edge.
81    IncrementCounterInBlock(&F->getEntryBlock(), i++, Counters);
82    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
83      if (BlocksToInstrument.count(BB)) {  // Don't instrument inserted blocks
84        // Okay, we have to add a counter of each outgoing edge.  If the
85        // outgoing edge is not critical don't split it, just insert the counter
86        // in the source or destination of the edge.
87        TerminatorInst *TI = BB->getTerminator();
88        for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s) {
89          // If the edge is critical, split it.
90          SplitCriticalEdge(TI, s, this);
91
92          // Okay, we are guaranteed that the edge is no longer critical.  If we
93          // only have a single successor, insert the counter in this block,
94          // otherwise insert it in the successor block.
95          if (TI->getNumSuccessors() == 1) {
96            // Insert counter at the start of the block
97            IncrementCounterInBlock(BB, i++, Counters);
98          } else {
99            // Insert counter at the start of the block
100            IncrementCounterInBlock(TI->getSuccessor(s), i++, Counters);
101          }
102        }
103      }
104  }
105
106  // Add the initialization call to main.
107  InsertProfilingInitCall(Main, "llvm_start_edge_profiling", Counters);
108  return true;
109}
110
111