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/StringExtras.h"
24 using namespace llvm;
25 
26 char UnifyFunctionExitNodes::ID = 0;
27 INITIALIZE_PASS(UnifyFunctionExitNodes, "mergereturn",
28                 "Unify function exit nodes", false, false)
29 
30 Pass *llvm::createUnifyFunctionExitNodesPass() {
31   return new UnifyFunctionExitNodes();
32 }
33 
34 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 //
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*> UnwindingBlocks;
54   std::vector<BasicBlock*> UnreachableBlocks;
55   for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
56     if (isa<ReturnInst>(I->getTerminator()))
57       ReturningBlocks.push_back(I);
58     else if (isa<UnwindInst>(I->getTerminator()))
59       UnwindingBlocks.push_back(I);
60     else if (isa<UnreachableInst>(I->getTerminator()))
61       UnreachableBlocks.push_back(I);
62 
63   // Handle unwinding blocks first.
64   if (UnwindingBlocks.empty()) {
65     UnwindBlock = 0;
66   } else if (UnwindingBlocks.size() == 1) {
67     UnwindBlock = UnwindingBlocks.front();
68   } else {
69     UnwindBlock = BasicBlock::Create(F.getContext(), "UnifiedUnwindBlock", &F);
70     new UnwindInst(F.getContext(), UnwindBlock);
71 
72     for (std::vector<BasicBlock*>::iterator I = UnwindingBlocks.begin(),
73            E = UnwindingBlocks.end(); I != E; ++I) {
74       BasicBlock *BB = *I;
75       BB->getInstList().pop_back();  // Remove the unwind insn
76       BranchInst::Create(UnwindBlock, BB);
77     }
78   }
79 
80   // Then unreachable blocks.
81   if (UnreachableBlocks.empty()) {
82     UnreachableBlock = 0;
83   } else if (UnreachableBlocks.size() == 1) {
84     UnreachableBlock = UnreachableBlocks.front();
85   } else {
86     UnreachableBlock = BasicBlock::Create(F.getContext(),
87                                           "UnifiedUnreachableBlock", &F);
88     new UnreachableInst(F.getContext(), 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(F.getContext(),
112                                                "UnifiedReturnBlock", &F);
113 
114   PHINode *PN = 0;
115   if (F.getReturnType()->isVoidTy()) {
116     ReturnInst::Create(F.getContext(), NULL, NewRetBlock);
117   } else {
118     // If the function doesn't return void... add a PHI node to the block...
119     PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
120                          "UnifiedRetVal");
121     NewRetBlock->getInstList().push_back(PN);
122     ReturnInst::Create(F.getContext(), PN, NewRetBlock);
123   }
124 
125   // Loop over all of the blocks, replacing the return instruction with an
126   // unconditional branch.
127   //
128   for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
129          E = ReturningBlocks.end(); I != E; ++I) {
130     BasicBlock *BB = *I;
131 
132     // Add an incoming element to the PHI node for every return instruction that
133     // is merging into this new block...
134     if (PN)
135       PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
136 
137     BB->getInstList().pop_back();  // Remove the return insn
138     BranchInst::Create(NewRetBlock, BB);
139   }
140   ReturnBlock = NewRetBlock;
141   return true;
142 }
143