1 //===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
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 pass is used to ensure that functions have at most one return
10 // instruction in them. Additionally, it keeps track of which node is the new
11 // exit node of the CFG. If there are no exit nodes in the CFG, the getExitNode
12 // method will return a null pointer.
13 //
14 //===----------------------------------------------------------------------===//
15
16 #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
17 #include "llvm/IR/BasicBlock.h"
18 #include "llvm/IR/Function.h"
19 #include "llvm/IR/Instructions.h"
20 #include "llvm/IR/Type.h"
21 #include "llvm/InitializePasses.h"
22 #include "llvm/Transforms/Utils.h"
23 using namespace llvm;
24
25 char UnifyFunctionExitNodes::ID = 0;
26
UnifyFunctionExitNodes()27 UnifyFunctionExitNodes::UnifyFunctionExitNodes() : FunctionPass(ID) {
28 initializeUnifyFunctionExitNodesPass(*PassRegistry::getPassRegistry());
29 }
30
31 INITIALIZE_PASS(UnifyFunctionExitNodes, "mergereturn",
32 "Unify function exit nodes", false, false)
33
createUnifyFunctionExitNodesPass()34 Pass *llvm::createUnifyFunctionExitNodesPass() {
35 return new UnifyFunctionExitNodes();
36 }
37
getAnalysisUsage(AnalysisUsage & AU) const38 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
39 // We preserve the non-critical-edgeness property
40 AU.addPreservedID(BreakCriticalEdgesID);
41 // This is a cluster of orthogonal Transforms
42 AU.addPreservedID(LowerSwitchID);
43 }
44
45 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
46 // BasicBlock, and converting all returns to unconditional branches to this
47 // new basic block. The singular exit node is returned.
48 //
49 // If there are no return stmts in the Function, a null pointer is returned.
50 //
runOnFunction(Function & F)51 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
52 // Loop over all of the blocks in a function, tracking all of the blocks that
53 // return.
54 //
55 std::vector<BasicBlock*> ReturningBlocks;
56 std::vector<BasicBlock*> UnreachableBlocks;
57 for (BasicBlock &I : F)
58 if (isa<ReturnInst>(I.getTerminator()))
59 ReturningBlocks.push_back(&I);
60 else if (isa<UnreachableInst>(I.getTerminator()))
61 UnreachableBlocks.push_back(&I);
62
63 // Then unreachable blocks.
64 if (UnreachableBlocks.empty()) {
65 UnreachableBlock = nullptr;
66 } else if (UnreachableBlocks.size() == 1) {
67 UnreachableBlock = UnreachableBlocks.front();
68 } else {
69 UnreachableBlock = BasicBlock::Create(F.getContext(),
70 "UnifiedUnreachableBlock", &F);
71 new UnreachableInst(F.getContext(), UnreachableBlock);
72
73 for (BasicBlock *BB : UnreachableBlocks) {
74 BB->getInstList().pop_back(); // Remove the unreachable inst.
75 BranchInst::Create(UnreachableBlock, BB);
76 }
77 }
78
79 // Now handle return blocks.
80 if (ReturningBlocks.empty()) {
81 ReturnBlock = nullptr;
82 return false; // No blocks return
83 } else if (ReturningBlocks.size() == 1) {
84 ReturnBlock = ReturningBlocks.front(); // Already has a single return block
85 return false;
86 }
87
88 // Otherwise, we need to insert a new basic block into the function, add a PHI
89 // nodes (if the function returns values), and convert all of the return
90 // instructions into unconditional branches.
91 //
92 BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(),
93 "UnifiedReturnBlock", &F);
94
95 PHINode *PN = nullptr;
96 if (F.getReturnType()->isVoidTy()) {
97 ReturnInst::Create(F.getContext(), nullptr, NewRetBlock);
98 } else {
99 // If the function doesn't return void... add a PHI node to the block...
100 PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
101 "UnifiedRetVal");
102 NewRetBlock->getInstList().push_back(PN);
103 ReturnInst::Create(F.getContext(), PN, NewRetBlock);
104 }
105
106 // Loop over all of the blocks, replacing the return instruction with an
107 // unconditional branch.
108 //
109 for (BasicBlock *BB : ReturningBlocks) {
110 // Add an incoming element to the PHI node for every return instruction that
111 // is merging into this new block...
112 if (PN)
113 PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
114
115 BB->getInstList().pop_back(); // Remove the return insn
116 BranchInst::Create(NewRetBlock, BB);
117 }
118 ReturnBlock = NewRetBlock;
119 return true;
120 }
121