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/CFG.h"
15 #include "llvm/IR/InstrTypes.h"
16 #include "llvm/IR/ValueHandle.h"
17 #include "llvm/InitializePasses.h"
18 #include "llvm/Pass.h"
19 #include "llvm/Transforms/Scalar.h"
20 #include "llvm/Transforms/Utils/Local.h"
21
22 using namespace llvm;
23
24 #define DEBUG_TYPE "flattencfg"
25
26 namespace {
27 struct FlattenCFGPass : public FunctionPass {
28 static char ID; // Pass identification, replacement for typeid
29 public:
FlattenCFGPass__anon7e4d59d30111::FlattenCFGPass30 FlattenCFGPass() : FunctionPass(ID) {
31 initializeFlattenCFGPassPass(*PassRegistry::getPassRegistry());
32 }
33 bool runOnFunction(Function &F) override;
34
getAnalysisUsage__anon7e4d59d30111::FlattenCFGPass35 void getAnalysisUsage(AnalysisUsage &AU) const override {
36 AU.addRequired<AAResultsWrapperPass>();
37 }
38
39 private:
40 AliasAnalysis *AA;
41 };
42 }
43
44 char FlattenCFGPass::ID = 0;
45 INITIALIZE_PASS_BEGIN(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
46 false)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)47 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
48 INITIALIZE_PASS_END(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
49 false)
50
51 // Public interface to the FlattenCFG pass
52 FunctionPass *llvm::createFlattenCFGPass() { return new FlattenCFGPass(); }
53
54 /// iterativelyFlattenCFG - Call FlattenCFG on all the blocks in the function,
55 /// iterating until no more changes are made.
iterativelyFlattenCFG(Function & F,AliasAnalysis * AA)56 static bool iterativelyFlattenCFG(Function &F, AliasAnalysis *AA) {
57 bool Changed = false;
58 bool LocalChange = true;
59
60 // Use block handles instead of iterating over function blocks directly
61 // to avoid using iterators invalidated by erasing blocks.
62 std::vector<WeakVH> Blocks;
63 Blocks.reserve(F.size());
64 for (auto &BB : F)
65 Blocks.push_back(&BB);
66
67 while (LocalChange) {
68 LocalChange = false;
69
70 // Loop over all of the basic blocks and try to flatten them.
71 for (WeakVH &BlockHandle : Blocks) {
72 // Skip blocks erased by FlattenCFG.
73 if (auto *BB = cast_or_null<BasicBlock>(BlockHandle))
74 if (FlattenCFG(BB, AA))
75 LocalChange = true;
76 }
77 Changed |= LocalChange;
78 }
79 return Changed;
80 }
81
runOnFunction(Function & F)82 bool FlattenCFGPass::runOnFunction(Function &F) {
83 AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
84 bool EverChanged = false;
85 // iterativelyFlattenCFG can make some blocks dead.
86 while (iterativelyFlattenCFG(F, AA)) {
87 removeUnreachableBlocks(F);
88 EverChanged = true;
89 }
90 return EverChanged;
91 }
92