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.addPreserved("mem2reg");
39 AU.addPreservedID(LowerSwitchID);
40 }
41
42 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
43 // BasicBlock, and converting all returns to unconditional branches to this
44 // new basic block. The singular exit node is returned.
45 //
46 // If there are no return stmts in the Function, a null pointer is returned.
47 //
runOnFunction(Function & F)48 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
49 // Loop over all of the blocks in a function, tracking all of the blocks that
50 // return.
51 //
52 std::vector<BasicBlock*> ReturningBlocks;
53 std::vector<BasicBlock*> UnreachableBlocks;
54 for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
55 if (isa<ReturnInst>(I->getTerminator()))
56 ReturningBlocks.push_back(I);
57 else if (isa<UnreachableInst>(I->getTerminator()))
58 UnreachableBlocks.push_back(I);
59
60 // Then unreachable blocks.
61 if (UnreachableBlocks.empty()) {
62 UnreachableBlock = 0;
63 } else if (UnreachableBlocks.size() == 1) {
64 UnreachableBlock = UnreachableBlocks.front();
65 } else {
66 UnreachableBlock = BasicBlock::Create(F.getContext(),
67 "UnifiedUnreachableBlock", &F);
68 new UnreachableInst(F.getContext(), UnreachableBlock);
69
70 for (std::vector<BasicBlock*>::iterator I = UnreachableBlocks.begin(),
71 E = UnreachableBlocks.end(); I != E; ++I) {
72 BasicBlock *BB = *I;
73 BB->getInstList().pop_back(); // Remove the unreachable inst.
74 BranchInst::Create(UnreachableBlock, BB);
75 }
76 }
77
78 // Now handle return blocks.
79 if (ReturningBlocks.empty()) {
80 ReturnBlock = 0;
81 return false; // No blocks return
82 } else if (ReturningBlocks.size() == 1) {
83 ReturnBlock = ReturningBlocks.front(); // Already has a single return block
84 return false;
85 }
86
87 // Otherwise, we need to insert a new basic block into the function, add a PHI
88 // nodes (if the function returns values), and convert all of the return
89 // instructions into unconditional branches.
90 //
91 BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(),
92 "UnifiedReturnBlock", &F);
93
94 PHINode *PN = 0;
95 if (F.getReturnType()->isVoidTy()) {
96 ReturnInst::Create(F.getContext(), NULL, NewRetBlock);
97 } else {
98 // If the function doesn't return void... add a PHI node to the block...
99 PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
100 "UnifiedRetVal");
101 NewRetBlock->getInstList().push_back(PN);
102 ReturnInst::Create(F.getContext(), PN, NewRetBlock);
103 }
104
105 // Loop over all of the blocks, replacing the return instruction with an
106 // unconditional branch.
107 //
108 for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
109 E = ReturningBlocks.end(); I != E; ++I) {
110 BasicBlock *BB = *I;
111
112 // Add an incoming element to the PHI node for every return instruction that
113 // is merging into this new block...
114 if (PN)
115 PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
116
117 BB->getInstList().pop_back(); // Remove the return insn
118 BranchInst::Create(NewRetBlock, BB);
119 }
120 ReturnBlock = NewRetBlock;
121 return true;
122 }
123