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/ADT/StringExtras.h"
19 #include "llvm/IR/BasicBlock.h"
20 #include "llvm/IR/Function.h"
21 #include "llvm/IR/Instructions.h"
22 #include "llvm/IR/Type.h"
23 #include "llvm/Transforms/Scalar.h"
24 using namespace llvm;
25
26 char UnifyFunctionExitNodes::ID = 0;
27 INITIALIZE_PASS(UnifyFunctionExitNodes, "mergereturn",
28 "Unify function exit nodes", false, false)
29
createUnifyFunctionExitNodesPass()30 Pass *llvm::createUnifyFunctionExitNodesPass() {
31 return new UnifyFunctionExitNodes();
32 }
33
getAnalysisUsage(AnalysisUsage & AU) const34 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
35 // We preserve the non-critical-edgeness property
36 AU.addPreservedID(BreakCriticalEdgesID);
37 // This is a cluster of orthogonal Transforms
38 AU.addPreservedID(LowerSwitchID);
39 }
40
41 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
42 // BasicBlock, and converting all returns to unconditional branches to this
43 // new basic block. The singular exit node is returned.
44 //
45 // If there are no return stmts in the Function, a null pointer is returned.
46 //
runOnFunction(Function & F)47 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
48 // Loop over all of the blocks in a function, tracking all of the blocks that
49 // return.
50 //
51 std::vector<BasicBlock*> ReturningBlocks;
52 std::vector<BasicBlock*> UnreachableBlocks;
53 for (BasicBlock &I : F)
54 if (isa<ReturnInst>(I.getTerminator()))
55 ReturningBlocks.push_back(&I);
56 else if (isa<UnreachableInst>(I.getTerminator()))
57 UnreachableBlocks.push_back(&I);
58
59 // Then unreachable blocks.
60 if (UnreachableBlocks.empty()) {
61 UnreachableBlock = nullptr;
62 } else if (UnreachableBlocks.size() == 1) {
63 UnreachableBlock = UnreachableBlocks.front();
64 } else {
65 UnreachableBlock = BasicBlock::Create(F.getContext(),
66 "UnifiedUnreachableBlock", &F);
67 new UnreachableInst(F.getContext(), UnreachableBlock);
68
69 for (BasicBlock *BB : UnreachableBlocks) {
70 BB->getInstList().pop_back(); // Remove the unreachable inst.
71 BranchInst::Create(UnreachableBlock, BB);
72 }
73 }
74
75 // Now handle return blocks.
76 if (ReturningBlocks.empty()) {
77 ReturnBlock = nullptr;
78 return false; // No blocks return
79 } else if (ReturningBlocks.size() == 1) {
80 ReturnBlock = ReturningBlocks.front(); // Already has a single return block
81 return false;
82 }
83
84 // Otherwise, we need to insert a new basic block into the function, add a PHI
85 // nodes (if the function returns values), and convert all of the return
86 // instructions into unconditional branches.
87 //
88 BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(),
89 "UnifiedReturnBlock", &F);
90
91 PHINode *PN = nullptr;
92 if (F.getReturnType()->isVoidTy()) {
93 ReturnInst::Create(F.getContext(), nullptr, NewRetBlock);
94 } else {
95 // If the function doesn't return void... add a PHI node to the block...
96 PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
97 "UnifiedRetVal");
98 NewRetBlock->getInstList().push_back(PN);
99 ReturnInst::Create(F.getContext(), PN, NewRetBlock);
100 }
101
102 // Loop over all of the blocks, replacing the return instruction with an
103 // unconditional branch.
104 //
105 for (BasicBlock *BB : ReturningBlocks) {
106 // Add an incoming element to the PHI node for every return instruction that
107 // is merging into this new block...
108 if (PN)
109 PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
110
111 BB->getInstList().pop_back(); // Remove the return insn
112 BranchInst::Create(NewRetBlock, BB);
113 }
114 ReturnBlock = NewRetBlock;
115 return true;
116 }
117