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      1 //===- FlattenCFGPass.cpp - CFG Flatten Pass ----------------------===//
      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 file implements flattening of CFG.
     11 //
     12 //===----------------------------------------------------------------------===//
     13 
     14 #define DEBUG_TYPE "flattencfg"
     15 #include "llvm/Transforms/Scalar.h"
     16 #include "llvm/Analysis/AliasAnalysis.h"
     17 #include "llvm/Pass.h"
     18 #include "llvm/Support/CFG.h"
     19 #include "llvm/Transforms/Utils/Local.h"
     20 using namespace llvm;
     21 
     22 namespace {
     23 struct FlattenCFGPass : public FunctionPass {
     24   static char ID; // Pass identification, replacement for typeid
     25 public:
     26   FlattenCFGPass() : FunctionPass(ID) {
     27     initializeFlattenCFGPassPass(*PassRegistry::getPassRegistry());
     28   }
     29   bool runOnFunction(Function &F);
     30 
     31   void getAnalysisUsage(AnalysisUsage &AU) const {
     32     AU.addRequired<AliasAnalysis>();
     33   }
     34 
     35 private:
     36   AliasAnalysis *AA;
     37 };
     38 }
     39 
     40 char FlattenCFGPass::ID = 0;
     41 INITIALIZE_PASS_BEGIN(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
     42                       false)
     43 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
     44 INITIALIZE_PASS_END(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
     45                     false)
     46 
     47 // Public interface to the FlattenCFG pass
     48 FunctionPass *llvm::createFlattenCFGPass() { return new FlattenCFGPass(); }
     49 
     50 /// iterativelyFlattenCFG - Call FlattenCFG on all the blocks in the function,
     51 /// iterating until no more changes are made.
     52 static bool iterativelyFlattenCFG(Function &F, AliasAnalysis *AA) {
     53   bool Changed = false;
     54   bool LocalChange = true;
     55   while (LocalChange) {
     56     LocalChange = false;
     57 
     58     // Loop over all of the basic blocks and remove them if they are unneeded...
     59     //
     60     for (Function::iterator BBIt = F.begin(); BBIt != F.end();) {
     61       if (FlattenCFG(BBIt++, AA)) {
     62         LocalChange = true;
     63       }
     64     }
     65     Changed |= LocalChange;
     66   }
     67   return Changed;
     68 }
     69 
     70 bool FlattenCFGPass::runOnFunction(Function &F) {
     71   AA = &getAnalysis<AliasAnalysis>();
     72   bool EverChanged = false;
     73   // iterativelyFlattenCFG can make some blocks dead.
     74   while (iterativelyFlattenCFG(F, AA)) {
     75     removeUnreachableBlocks(F);
     76     EverChanged = true;
     77   }
     78   return EverChanged;
     79 }
     80