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/Transforms/Scalar.h"
19 #include "llvm/BasicBlock.h"
20 #include "llvm/Function.h"
21 #include "llvm/Instructions.h"
22 #include "llvm/Type.h"
23 #include "llvm/ADT/SmallVector.h"
24 #include "llvm/ADT/StringExtras.h"
25 using namespace llvm;
26 
27 char UnifyFunctionExitNodes::ID = 0;
28 static RegisterPass<UnifyFunctionExitNodes>
29 X("mergereturn", "Unify function exit nodes");
30 
31 Pass *llvm::createUnifyFunctionExitNodesPass() {
32   return new UnifyFunctionExitNodes();
33 }
34 
35 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
36   // We preserve the non-critical-edgeness property
37   AU.addPreservedID(BreakCriticalEdgesID);
38   // This is a cluster of orthogonal Transforms
39   AU.addPreservedID(PromoteMemoryToRegisterID);
40   AU.addPreservedID(LowerSwitchID);
41 }
42 
43 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
44 // BasicBlock, and converting all returns to unconditional branches to this
45 // new basic block.  The singular exit node is returned.
46 //
47 // If there are no return stmts in the Function, a null pointer is returned.
48 //
49 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
50   // Loop over all of the blocks in a function, tracking all of the blocks that
51   // return.
52   //
53   std::vector<BasicBlock*> ReturningBlocks;
54   std::vector<BasicBlock*> UnwindingBlocks;
55   std::vector<BasicBlock*> UnreachableBlocks;
56   for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
57     if (isa<ReturnInst>(I->getTerminator()))
58       ReturningBlocks.push_back(I);
59     else if (isa<UnwindInst>(I->getTerminator()))
60       UnwindingBlocks.push_back(I);
61     else if (isa<UnreachableInst>(I->getTerminator()))
62       UnreachableBlocks.push_back(I);
63 
64   // Handle unwinding blocks first.
65   if (UnwindingBlocks.empty()) {
66     UnwindBlock = 0;
67   } else if (UnwindingBlocks.size() == 1) {
68     UnwindBlock = UnwindingBlocks.front();
69   } else {
70     UnwindBlock = BasicBlock::Create("UnifiedUnwindBlock", &F);
71     new UnwindInst(UnwindBlock);
72 
73     for (std::vector<BasicBlock*>::iterator I = UnwindingBlocks.begin(),
74            E = UnwindingBlocks.end(); I != E; ++I) {
75       BasicBlock *BB = *I;
76       BB->getInstList().pop_back();  // Remove the unwind insn
77       BranchInst::Create(UnwindBlock, BB);
78     }
79   }
80 
81   // Then unreachable blocks.
82   if (UnreachableBlocks.empty()) {
83     UnreachableBlock = 0;
84   } else if (UnreachableBlocks.size() == 1) {
85     UnreachableBlock = UnreachableBlocks.front();
86   } else {
87     UnreachableBlock = BasicBlock::Create("UnifiedUnreachableBlock", &F);
88     new UnreachableInst(UnreachableBlock);
89 
90     for (std::vector<BasicBlock*>::iterator I = UnreachableBlocks.begin(),
91            E = UnreachableBlocks.end(); I != E; ++I) {
92       BasicBlock *BB = *I;
93       BB->getInstList().pop_back();  // Remove the unreachable inst.
94       BranchInst::Create(UnreachableBlock, BB);
95     }
96   }
97 
98   // Now handle return blocks.
99   if (ReturningBlocks.empty()) {
100     ReturnBlock = 0;
101     return false;                          // No blocks return
102   } else if (ReturningBlocks.size() == 1) {
103     ReturnBlock = ReturningBlocks.front(); // Already has a single return block
104     return false;
105   }
106 
107   // Otherwise, we need to insert a new basic block into the function, add a PHI
108   // nodes (if the function returns values), and convert all of the return
109   // instructions into unconditional branches.
110   //
111   BasicBlock *NewRetBlock = BasicBlock::Create("UnifiedReturnBlock", &F);
112 
113   PHINode *PN = 0;
114   if (F.getReturnType() == Type::VoidTy) {
115     ReturnInst::Create(NULL, NewRetBlock);
116   } else {
117     // If the function doesn't return void... add a PHI node to the block...
118     PN = PHINode::Create(F.getReturnType(), "UnifiedRetVal");
119     NewRetBlock->getInstList().push_back(PN);
120     ReturnInst::Create(PN, NewRetBlock);
121   }
122 
123   // Loop over all of the blocks, replacing the return instruction with an
124   // unconditional branch.
125   //
126   for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
127          E = ReturningBlocks.end(); I != E; ++I) {
128     BasicBlock *BB = *I;
129 
130     // Add an incoming element to the PHI node for every return instruction that
131     // is merging into this new block...
132     if (PN)
133       PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
134 
135     BB->getInstList().pop_back();  // Remove the return insn
136     BranchInst::Create(NewRetBlock, BB);
137   }
138   ReturnBlock = NewRetBlock;
139   return true;
140 }
141