1 //===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This pass is used to ensure that functions have at most one return
10 // instruction in them.  Additionally, it keeps track of which node is the new
11 // exit node of the CFG.  If there are no exit nodes in the CFG, the getExitNode
12 // method will return a null pointer.
13 //
14 //===----------------------------------------------------------------------===//
15 
16 #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
17 #include "llvm/IR/BasicBlock.h"
18 #include "llvm/IR/Function.h"
19 #include "llvm/IR/Instructions.h"
20 #include "llvm/IR/Type.h"
21 #include "llvm/InitializePasses.h"
22 #include "llvm/Transforms/Utils.h"
23 using namespace llvm;
24 
25 char UnifyFunctionExitNodes::ID = 0;
26 
27 UnifyFunctionExitNodes::UnifyFunctionExitNodes() : FunctionPass(ID) {
28   initializeUnifyFunctionExitNodesPass(*PassRegistry::getPassRegistry());
29 }
30 
31 INITIALIZE_PASS(UnifyFunctionExitNodes, "mergereturn",
32                 "Unify function exit nodes", false, false)
33 
34 Pass *llvm::createUnifyFunctionExitNodesPass() {
35   return new UnifyFunctionExitNodes();
36 }
37 
38 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
39   // We preserve the non-critical-edgeness property
40   AU.addPreservedID(BreakCriticalEdgesID);
41   // This is a cluster of orthogonal Transforms
42   AU.addPreservedID(LowerSwitchID);
43 }
44 
45 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
46 // BasicBlock, and converting all returns to unconditional branches to this
47 // new basic block.  The singular exit node is returned.
48 //
49 // If there are no return stmts in the Function, a null pointer is returned.
50 //
51 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
52   // Loop over all of the blocks in a function, tracking all of the blocks that
53   // return.
54   //
55   std::vector<BasicBlock*> ReturningBlocks;
56   std::vector<BasicBlock*> UnreachableBlocks;
57   for (BasicBlock &I : F)
58     if (isa<ReturnInst>(I.getTerminator()))
59       ReturningBlocks.push_back(&I);
60     else if (isa<UnreachableInst>(I.getTerminator()))
61       UnreachableBlocks.push_back(&I);
62 
63   // Then unreachable blocks.
64   if (UnreachableBlocks.empty()) {
65     UnreachableBlock = nullptr;
66   } else if (UnreachableBlocks.size() == 1) {
67     UnreachableBlock = UnreachableBlocks.front();
68   } else {
69     UnreachableBlock = BasicBlock::Create(F.getContext(),
70                                           "UnifiedUnreachableBlock", &F);
71     new UnreachableInst(F.getContext(), UnreachableBlock);
72 
73     for (BasicBlock *BB : UnreachableBlocks) {
74       BB->getInstList().pop_back();  // Remove the unreachable inst.
75       BranchInst::Create(UnreachableBlock, BB);
76     }
77   }
78 
79   // Now handle return blocks.
80   if (ReturningBlocks.empty()) {
81     ReturnBlock = nullptr;
82     return false;                          // No blocks return
83   } else if (ReturningBlocks.size() == 1) {
84     ReturnBlock = ReturningBlocks.front(); // Already has a single return block
85     return false;
86   }
87 
88   // Otherwise, we need to insert a new basic block into the function, add a PHI
89   // nodes (if the function returns values), and convert all of the return
90   // instructions into unconditional branches.
91   //
92   BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(),
93                                                "UnifiedReturnBlock", &F);
94 
95   PHINode *PN = nullptr;
96   if (F.getReturnType()->isVoidTy()) {
97     ReturnInst::Create(F.getContext(), nullptr, NewRetBlock);
98   } else {
99     // If the function doesn't return void... add a PHI node to the block...
100     PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
101                          "UnifiedRetVal");
102     NewRetBlock->getInstList().push_back(PN);
103     ReturnInst::Create(F.getContext(), PN, NewRetBlock);
104   }
105 
106   // Loop over all of the blocks, replacing the return instruction with an
107   // unconditional branch.
108   //
109   for (BasicBlock *BB : ReturningBlocks) {
110     // Add an incoming element to the PHI node for every return instruction that
111     // is merging into this new block...
112     if (PN)
113       PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
114 
115     BB->getInstList().pop_back();  // Remove the return insn
116     BranchInst::Create(NewRetBlock, BB);
117   }
118   ReturnBlock = NewRetBlock;
119   return true;
120 }
121