1 //===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
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 pass is used to ensure that functions have at most one return
11 // instruction in them. Additionally, it keeps track of which node is the new
12 // exit node of the CFG. If there are no exit nodes in the CFG, the getExitNode
13 // method will return a null pointer.
14 //
15 //===----------------------------------------------------------------------===//
16
17 #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
18 #include "llvm/IR/BasicBlock.h"
19 #include "llvm/IR/Function.h"
20 #include "llvm/IR/Instructions.h"
21 #include "llvm/IR/Type.h"
22 #include "llvm/Transforms/Utils.h"
23 using namespace llvm;
24
25 char UnifyFunctionExitNodes::ID = 0;
26 INITIALIZE_PASS(UnifyFunctionExitNodes, "mergereturn",
27 "Unify function exit nodes", false, false)
28
createUnifyFunctionExitNodesPass()29 Pass *llvm::createUnifyFunctionExitNodesPass() {
30 return new UnifyFunctionExitNodes();
31 }
32
getAnalysisUsage(AnalysisUsage & AU) const33 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
34 // We preserve the non-critical-edgeness property
35 AU.addPreservedID(BreakCriticalEdgesID);
36 // This is a cluster of orthogonal Transforms
37 AU.addPreservedID(LowerSwitchID);
38 }
39
40 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
41 // BasicBlock, and converting all returns to unconditional branches to this
42 // new basic block. The singular exit node is returned.
43 //
44 // If there are no return stmts in the Function, a null pointer is returned.
45 //
runOnFunction(Function & F)46 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
47 // Loop over all of the blocks in a function, tracking all of the blocks that
48 // return.
49 //
50 std::vector<BasicBlock*> ReturningBlocks;
51 std::vector<BasicBlock*> UnreachableBlocks;
52 for (BasicBlock &I : F)
53 if (isa<ReturnInst>(I.getTerminator()))
54 ReturningBlocks.push_back(&I);
55 else if (isa<UnreachableInst>(I.getTerminator()))
56 UnreachableBlocks.push_back(&I);
57
58 // Then unreachable blocks.
59 if (UnreachableBlocks.empty()) {
60 UnreachableBlock = nullptr;
61 } else if (UnreachableBlocks.size() == 1) {
62 UnreachableBlock = UnreachableBlocks.front();
63 } else {
64 UnreachableBlock = BasicBlock::Create(F.getContext(),
65 "UnifiedUnreachableBlock", &F);
66 new UnreachableInst(F.getContext(), UnreachableBlock);
67
68 for (BasicBlock *BB : UnreachableBlocks) {
69 BB->getInstList().pop_back(); // Remove the unreachable inst.
70 BranchInst::Create(UnreachableBlock, BB);
71 }
72 }
73
74 // Now handle return blocks.
75 if (ReturningBlocks.empty()) {
76 ReturnBlock = nullptr;
77 return false; // No blocks return
78 } else if (ReturningBlocks.size() == 1) {
79 ReturnBlock = ReturningBlocks.front(); // Already has a single return block
80 return false;
81 }
82
83 // Otherwise, we need to insert a new basic block into the function, add a PHI
84 // nodes (if the function returns values), and convert all of the return
85 // instructions into unconditional branches.
86 //
87 BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(),
88 "UnifiedReturnBlock", &F);
89
90 PHINode *PN = nullptr;
91 if (F.getReturnType()->isVoidTy()) {
92 ReturnInst::Create(F.getContext(), nullptr, NewRetBlock);
93 } else {
94 // If the function doesn't return void... add a PHI node to the block...
95 PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
96 "UnifiedRetVal");
97 NewRetBlock->getInstList().push_back(PN);
98 ReturnInst::Create(F.getContext(), PN, NewRetBlock);
99 }
100
101 // Loop over all of the blocks, replacing the return instruction with an
102 // unconditional branch.
103 //
104 for (BasicBlock *BB : ReturningBlocks) {
105 // Add an incoming element to the PHI node for every return instruction that
106 // is merging into this new block...
107 if (PN)
108 PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
109
110 BB->getInstList().pop_back(); // Remove the return insn
111 BranchInst::Create(NewRetBlock, BB);
112 }
113 ReturnBlock = NewRetBlock;
114 return true;
115 }
116