FlattenCFGPass.cpp revision 263508 
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"
20using namespace llvm;
21
22namespace {
23struct FlattenCFGPass : public FunctionPass {
24  static char ID; // Pass identification, replacement for typeid
25public:
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
35private:
36  AliasAnalysis *AA;
37};
38}
39
40char FlattenCFGPass::ID = 0;
41INITIALIZE_PASS_BEGIN(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
42                      false)
43INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
44INITIALIZE_PASS_END(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
45                    false)
46
47// Public interface to the FlattenCFG pass
48FunctionPass *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.
52static 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
70bool 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