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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 #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"
19 using namespace llvm;
20 
21 #define DEBUG_TYPE "flattencfg"
22 
23 namespace {
24 struct FlattenCFGPass : public FunctionPass {
25   static char ID; // Pass identification, replacement for typeid
26 public:
FlattenCFGPass__anonaeaae8360111::FlattenCFGPass27   FlattenCFGPass() : FunctionPass(ID) {
28     initializeFlattenCFGPassPass(*PassRegistry::getPassRegistry());
29   }
30   bool runOnFunction(Function &F) override;
31 
getAnalysisUsage__anonaeaae8360111::FlattenCFGPass32   void getAnalysisUsage(AnalysisUsage &AU) const override {
33     AU.addRequired<AAResultsWrapperPass>();
34   }
35 
36 private:
37   AliasAnalysis *AA;
38 };
39 }
40 
41 char FlattenCFGPass::ID = 0;
42 INITIALIZE_PASS_BEGIN(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
43                       false)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)44 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
45 INITIALIZE_PASS_END(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
46                     false)
47 
48 // Public interface to the FlattenCFG pass
49 FunctionPass *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.
iterativelyFlattenCFG(Function & F,AliasAnalysis * AA)53 static 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 
runOnFunction(Function & F)71 bool FlattenCFGPass::runOnFunction(Function &F) {
72   AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
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