1 //===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements a simple interprocedural pass which walks the
11 // call-graph, turning invoke instructions into calls, iff the callee cannot
12 // throw an exception. It implements this as a bottom-up traversal of the
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Transforms/IPO.h"
18 #include "llvm/CallGraphSCCPass.h"
19 #include "llvm/Constants.h"
20 #include "llvm/Function.h"
21 #include "llvm/Intrinsics.h"
22 #include "llvm/Instructions.h"
23 #include "llvm/Analysis/CallGraph.h"
24 #include "llvm/ADT/Statistic.h"
25 #include "llvm/Support/CFG.h"
31 Statistic<> NumRemoved("prune-eh", "Number of invokes removed");
32 Statistic<> NumUnreach("prune-eh", "Number of noreturn calls optimized");
34 struct PruneEH : public CallGraphSCCPass {
35 /// DoesNotUnwind - This set contains all of the functions which we have
36 /// determined cannot unwind.
37 std::set<CallGraphNode*> DoesNotUnwind;
39 /// DoesNotReturn - This set contains all of the functions which we have
40 /// determined cannot return normally (but might unwind).
41 std::set<CallGraphNode*> DoesNotReturn;
43 // runOnSCC - Analyze the SCC, performing the transformation if possible.
44 bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
46 bool SimplifyFunction(Function *F);
47 void DeleteBasicBlock(BasicBlock *BB);
49 RegisterOpt<PruneEH> X("prune-eh", "Remove unused exception handling info");
52 ModulePass *llvm::createPruneEHPass() { return new PruneEH(); }
55 bool PruneEH::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
56 CallGraph &CG = getAnalysis<CallGraph>();
57 bool MadeChange = false;
59 // First pass, scan all of the functions in the SCC, simplifying them
60 // according to what we know.
61 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
62 if (Function *F = SCC[i]->getFunction())
63 MadeChange |= SimplifyFunction(F);
65 // Next, check to see if any callees might throw or if there are any external
66 // functions in this SCC: if so, we cannot prune any functions in this SCC.
67 // If this SCC includes the unwind instruction, we KNOW it throws, so
68 // obviously the SCC might throw.
70 bool SCCMightUnwind = false, SCCMightReturn = false;
71 for (unsigned i = 0, e = SCC.size();
72 (!SCCMightUnwind || !SCCMightReturn) && i != e; ++i) {
73 Function *F = SCC[i]->getFunction();
74 if (F == 0 || (F->isExternal() && !F->getIntrinsicID())) {
75 SCCMightUnwind = true;
76 SCCMightReturn = true;
79 SCCMightReturn = true;
81 // Check to see if this function performs an unwind or calls an
82 // unwinding function.
83 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
84 if (isa<UnwindInst>(BB->getTerminator())) { // Uses unwind!
85 SCCMightUnwind = true;
86 } else if (isa<ReturnInst>(BB->getTerminator())) {
87 SCCMightReturn = true;
90 // Invoke instructions don't allow unwinding to continue, so we are
91 // only interested in call instructions.
93 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
94 if (CallInst *CI = dyn_cast<CallInst>(I)) {
95 if (Function *Callee = CI->getCalledFunction()) {
96 CallGraphNode *CalleeNode = CG[Callee];
97 // If the callee is outside our current SCC, or if it is not
98 // known to throw, then we might throw also.
99 if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end()&&
100 !DoesNotUnwind.count(CalleeNode)) {
101 SCCMightUnwind = true;
105 // Indirect call, it might throw.
106 SCCMightUnwind = true;
110 if (SCCMightUnwind && SCCMightReturn) break;
115 // If the SCC doesn't unwind or doesn't throw, note this fact.
117 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
118 DoesNotUnwind.insert(SCC[i]);
120 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
121 DoesNotReturn.insert(SCC[i]);
123 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
124 // Convert any invoke instructions to non-throwing functions in this node
125 // into call instructions with a branch. This makes the exception blocks
127 if (Function *F = SCC[i]->getFunction())
128 MadeChange |= SimplifyFunction(F);
135 // SimplifyFunction - Given information about callees, simplify the specified
136 // function if we have invokes to non-unwinding functions or code after calls to
137 // no-return functions.
138 bool PruneEH::SimplifyFunction(Function *F) {
139 CallGraph &CG = getAnalysis<CallGraph>();
140 bool MadeChange = false;
141 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
142 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
143 if (Function *F = II->getCalledFunction())
144 if (DoesNotUnwind.count(CG[F])) {
145 // Insert a call instruction before the invoke...
146 std::string Name = II->getName(); II->setName("");
147 CallInst *Call = new CallInst(II->getCalledValue(),
148 std::vector<Value*>(II->op_begin()+3,
151 Call->setCallingConv(II->getCallingConv());
153 // Anything that used the value produced by the invoke instruction
154 // now uses the value produced by the call instruction.
155 II->replaceAllUsesWith(Call);
156 BasicBlock *UnwindBlock = II->getUnwindDest();
157 UnwindBlock->removePredecessor(II->getParent());
159 // Insert a branch to the normal destination right before the
161 new BranchInst(II->getNormalDest(), II);
163 // Finally, delete the invoke instruction!
164 BB->getInstList().pop_back();
166 // If the unwind block is now dead, nuke it.
167 if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
168 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
174 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
175 if (CallInst *CI = dyn_cast<CallInst>(I++))
176 if (Function *Callee = CI->getCalledFunction())
177 if (DoesNotReturn.count(CG[Callee]) && !isa<UnreachableInst>(I)) {
178 // This call calls a function that cannot return. Insert an
179 // unreachable instruction after it and simplify the code. Do this
180 // by splitting the BB, adding the unreachable, then deleting the
182 BasicBlock *New = BB->splitBasicBlock(I);
184 // Remove the uncond branch and add an unreachable.
185 BB->getInstList().pop_back();
186 new UnreachableInst(BB);
188 DeleteBasicBlock(New); // Delete the new BB.
198 /// DeleteBasicBlock - remove the specified basic block from the program,
199 /// updating the callgraph to reflect any now-obsolete edges due to calls that
201 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
202 assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
203 CallGraph &CG = getAnalysis<CallGraph>();
205 CallGraphNode *CGN = CG[BB->getParent()];
206 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
208 if (CallInst *CI = dyn_cast<CallInst>(I)) {
209 if (Function *Callee = CI->getCalledFunction())
210 CGN->removeCallEdgeTo(CG[Callee]);
211 } else if (InvokeInst *II = dyn_cast<InvokeInst>(I)) {
212 if (Function *Callee = II->getCalledFunction())
213 CGN->removeCallEdgeTo(CG[Callee]);
216 I->replaceAllUsesWith(UndefValue::get(I->getType()));
219 // Get the list of successors of this block.
220 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
222 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
223 Succs[i]->removePredecessor(BB);
225 BB->eraseFromParent();