1 //===- FlattenCFGPass.cpp - CFG Flatten Pass ----------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements flattening of CFG.
10 //
11 //===----------------------------------------------------------------------===//
12
13 #include "llvm/Analysis/AliasAnalysis.h"
14 #include "llvm/IR/PassManager.h"
15 #include "llvm/IR/ValueHandle.h"
16 #include "llvm/InitializePasses.h"
17 #include "llvm/Pass.h"
18 #include "llvm/Transforms/Scalar.h"
19 #include "llvm/Transforms/Scalar/FlattenCFG.h"
20 #include "llvm/Transforms/Utils/Local.h"
21
22 using namespace llvm;
23
24 #define DEBUG_TYPE "flattencfg"
25
26 namespace {
27 struct FlattenCFGLegacyPass : public FunctionPass {
28 static char ID; // Pass identification, replacement for typeid
29 public:
FlattenCFGLegacyPass__anonb51340500111::FlattenCFGLegacyPass30 FlattenCFGLegacyPass() : FunctionPass(ID) {
31 initializeFlattenCFGLegacyPassPass(*PassRegistry::getPassRegistry());
32 }
33 bool runOnFunction(Function &F) override;
34
getAnalysisUsage__anonb51340500111::FlattenCFGLegacyPass35 void getAnalysisUsage(AnalysisUsage &AU) const override {
36 AU.addRequired<AAResultsWrapperPass>();
37 }
38
39 private:
40 AliasAnalysis *AA;
41 };
42
43 /// iterativelyFlattenCFG - Call FlattenCFG on all the blocks in the function,
44 /// iterating until no more changes are made.
iterativelyFlattenCFG(Function & F,AliasAnalysis * AA)45 bool iterativelyFlattenCFG(Function &F, AliasAnalysis *AA) {
46 bool Changed = false;
47 bool LocalChange = true;
48
49 // Use block handles instead of iterating over function blocks directly
50 // to avoid using iterators invalidated by erasing blocks.
51 std::vector<WeakVH> Blocks;
52 Blocks.reserve(F.size());
53 for (auto &BB : F)
54 Blocks.push_back(&BB);
55
56 while (LocalChange) {
57 LocalChange = false;
58
59 // Loop over all of the basic blocks and try to flatten them.
60 for (WeakVH &BlockHandle : Blocks) {
61 // Skip blocks erased by FlattenCFG.
62 if (auto *BB = cast_or_null<BasicBlock>(BlockHandle))
63 if (FlattenCFG(BB, AA))
64 LocalChange = true;
65 }
66 Changed |= LocalChange;
67 }
68 return Changed;
69 }
70 } // namespace
71
72 char FlattenCFGLegacyPass::ID = 0;
73
74 INITIALIZE_PASS_BEGIN(FlattenCFGLegacyPass, "flattencfg", "Flatten the CFG",
75 false, false)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)76 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
77 INITIALIZE_PASS_END(FlattenCFGLegacyPass, "flattencfg", "Flatten the CFG",
78 false, false)
79
80 // Public interface to the FlattenCFG pass
81 FunctionPass *llvm::createFlattenCFGPass() {
82 return new FlattenCFGLegacyPass();
83 }
84
runOnFunction(Function & F)85 bool FlattenCFGLegacyPass::runOnFunction(Function &F) {
86 AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
87 bool EverChanged = false;
88 // iterativelyFlattenCFG can make some blocks dead.
89 while (iterativelyFlattenCFG(F, AA)) {
90 removeUnreachableBlocks(F);
91 EverChanged = true;
92 }
93 return EverChanged;
94 }
95
run(Function & F,FunctionAnalysisManager & AM)96 PreservedAnalyses FlattenCFGPass::run(Function &F,
97 FunctionAnalysisManager &AM) {
98 bool EverChanged = false;
99 AliasAnalysis *AA = &AM.getResult<AAManager>(F);
100 // iterativelyFlattenCFG can make some blocks dead.
101 while (iterativelyFlattenCFG(F, AA)) {
102 removeUnreachableBlocks(F);
103 EverChanged = true;
104 }
105 return EverChanged ? PreservedAnalyses::none() : PreservedAnalyses::all();
106 }
107