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#include "llvm/Transforms/Scalar.h"
15#include "llvm/Analysis/AliasAnalysis.h"
16#include "llvm/IR/CFG.h"
17#include "llvm/Pass.h"
18#include "llvm/Transforms/Utils/Local.h"
19using namespace llvm;
20
21#define DEBUG_TYPE "flattencfg"
22
23namespace {
24struct FlattenCFGPass : public FunctionPass {
25  static char ID; // Pass identification, replacement for typeid
26public:
27  FlattenCFGPass() : FunctionPass(ID) {
28    initializeFlattenCFGPassPass(*PassRegistry::getPassRegistry());
29  }
30  bool runOnFunction(Function &F) override;
31
32  void getAnalysisUsage(AnalysisUsage &AU) const override {
33    AU.addRequired<AliasAnalysis>();
34  }
35
36private:
37  AliasAnalysis *AA;
38};
39}
40
41char FlattenCFGPass::ID = 0;
42INITIALIZE_PASS_BEGIN(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
43                      false)
44INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
45INITIALIZE_PASS_END(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
46                    false)
47
48// Public interface to the FlattenCFG pass
49FunctionPass *llvm::createFlattenCFGPass() { return new FlattenCFGPass(); }
50
51/// iterativelyFlattenCFG - Call FlattenCFG on all the blocks in the function,
52/// iterating until no more changes are made.
53static bool iterativelyFlattenCFG(Function &F, AliasAnalysis *AA) {
54  bool Changed = false;
55  bool LocalChange = true;
56  while (LocalChange) {
57    LocalChange = false;
58
59    // Loop over all of the basic blocks and remove them if they are unneeded...
60    //
61    for (Function::iterator BBIt = F.begin(); BBIt != F.end();) {
62      if (FlattenCFG(BBIt++, AA)) {
63        LocalChange = true;
64      }
65    }
66    Changed |= LocalChange;
67  }
68  return Changed;
69}
70
71bool FlattenCFGPass::runOnFunction(Function &F) {
72  AA = &getAnalysis<AliasAnalysis>();
73  bool EverChanged = false;
74  // iterativelyFlattenCFG can make some blocks dead.
75  while (iterativelyFlattenCFG(F, AA)) {
76    removeUnreachableBlocks(F);
77    EverChanged = true;
78  }
79  return EverChanged;
80}
81