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EdgeProfiling.cpp

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00001 //===- EdgeProfiling.cpp - Insert counters for edge profiling -------------===//
00002 // 
00003 //                      The LLVM Compiler Infrastructure
00004 //
00005 // This file was developed by the LLVM research group and is distributed under
00006 // the University of Illinois Open Source License. See LICENSE.TXT for details.
00007 // 
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This pass instruments the specified program with counters for edge profiling.
00011 // Edge profiling can give a reasonable approximation of the hot paths through a
00012 // program, and is used for a wide variety of program transformations.
00013 //
00014 // Note that this implementation is very naive.  We insert a counter for *every*
00015 // edge in the program, instead of using control flow information to prune the
00016 // number of counters inserted.
00017 //
00018 //===----------------------------------------------------------------------===//
00019 
00020 #include "llvm/Constants.h"
00021 #include "llvm/DerivedTypes.h"
00022 #include "llvm/Module.h"
00023 #include "llvm/Pass.h"
00024 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
00025 #include "ProfilingUtils.h"
00026 #include <iostream>
00027 #include <set>
00028 using namespace llvm;
00029 
00030 namespace {
00031   class EdgeProfiler : public ModulePass {
00032     bool runOnModule(Module &M);
00033   };
00034 
00035   RegisterOpt<EdgeProfiler> X("insert-edge-profiling",
00036                               "Insert instrumentation for edge profiling");
00037 }
00038 
00039 bool EdgeProfiler::runOnModule(Module &M) {
00040   Function *Main = M.getMainFunction();
00041   if (Main == 0) {
00042     std::cerr << "WARNING: cannot insert edge profiling into a module"
00043               << " with no main function!\n";
00044     return false;  // No main, no instrumentation!
00045   }
00046 
00047   std::set<BasicBlock*> BlocksToInstrument;
00048   unsigned NumEdges = 0;
00049   for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F)
00050     for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
00051       // Keep track of which blocks need to be instrumented.  We don't want to
00052       // instrument blocks that are added as the result of breaking critical
00053       // edges!
00054       BlocksToInstrument.insert(BB);
00055       NumEdges += BB->getTerminator()->getNumSuccessors();
00056     }
00057 
00058   const Type *ATy = ArrayType::get(Type::UIntTy, NumEdges);
00059   GlobalVariable *Counters =
00060     new GlobalVariable(ATy, false, GlobalValue::InternalLinkage,
00061                        Constant::getNullValue(ATy), "EdgeProfCounters", &M);
00062 
00063   // Instrument all of the edges...
00064   unsigned i = 0;
00065   for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F)
00066     for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
00067       if (BlocksToInstrument.count(BB)) {  // Don't instrument inserted blocks
00068         // Okay, we have to add a counter of each outgoing edge.  If the
00069         // outgoing edge is not critical don't split it, just insert the counter
00070         // in the source or destination of the edge.
00071         TerminatorInst *TI = BB->getTerminator();
00072         for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s) {
00073           // If the edge is critical, split it.
00074           SplitCriticalEdge(TI, s, this);
00075 
00076           // Okay, we are guaranteed that the edge is no longer critical.  If we
00077           // only have a single successor, insert the counter in this block,
00078           // otherwise insert it in the successor block.
00079           if (TI->getNumSuccessors() == 0) {
00080             // Insert counter at the start of the block
00081             IncrementCounterInBlock(BB, i++, Counters);
00082           } else {
00083             // Insert counter at the start of the block
00084             IncrementCounterInBlock(TI->getSuccessor(s), i++, Counters);
00085           }
00086         }
00087       }
00088 
00089   // Add the initialization call to main.
00090   InsertProfilingInitCall(Main, "llvm_start_edge_profiling", Counters);
00091   return true;
00092 }
00093