1 //===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // 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, and marking functions 'nounwind' if they cannot throw.
13 // It implements this as a bottom-up traversal of the call-graph.
15 //===----------------------------------------------------------------------===//
17 #define DEBUG_TYPE "prune-eh"
18 #include "llvm/Transforms/IPO.h"
19 #include "llvm/CallGraphSCCPass.h"
20 #include "llvm/Constants.h"
21 #include "llvm/Function.h"
22 #include "llvm/Instructions.h"
23 #include "llvm/Analysis/CallGraph.h"
24 #include "llvm/ADT/SmallVector.h"
25 #include "llvm/ADT/Statistic.h"
26 #include "llvm/Support/CFG.h"
27 #include "llvm/Support/Compiler.h"
32 STATISTIC(NumRemoved, "Number of invokes removed");
33 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
36 struct VISIBILITY_HIDDEN PruneEH : public CallGraphSCCPass {
37 static char ID; // Pass identification, replacement for typeid
38 PruneEH() : CallGraphSCCPass((intptr_t)&ID) {}
40 // runOnSCC - Analyze the SCC, performing the transformation if possible.
41 bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
43 bool SimplifyFunction(Function *F);
44 void DeleteBasicBlock(BasicBlock *BB);
49 static RegisterPass<PruneEH>
50 X("prune-eh", "Remove unused exception handling info");
52 Pass *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();
75 SCCMightUnwind = true;
76 SCCMightReturn = true;
77 } else if (F->isDeclaration()) {
78 SCCMightUnwind |= !F->doesNotThrow();
79 SCCMightReturn |= !F->doesNotReturn();
81 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
82 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
84 if (!CheckUnwind && !CheckReturn)
87 // Check to see if this function performs an unwind or calls an
88 // unwinding function.
89 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
90 if (CheckUnwind && isa<UnwindInst>(BB->getTerminator())) {
92 SCCMightUnwind = true;
93 } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
94 SCCMightReturn = true;
97 // Invoke instructions don't allow unwinding to continue, so we are
98 // only interested in call instructions.
99 if (CheckUnwind && !SCCMightUnwind)
100 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
101 if (CallInst *CI = dyn_cast<CallInst>(I)) {
102 if (CI->doesNotThrow()) {
103 // This call cannot throw.
104 } else if (Function *Callee = CI->getCalledFunction()) {
105 CallGraphNode *CalleeNode = CG[Callee];
106 // If the callee is outside our current SCC then we may
107 // throw because it might.
108 if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end()){
109 SCCMightUnwind = true;
113 // Indirect call, it might throw.
114 SCCMightUnwind = true;
118 if (SCCMightUnwind && SCCMightReturn) break;
123 // If the SCC doesn't unwind or doesn't throw, note this fact.
124 if (!SCCMightUnwind || !SCCMightReturn)
125 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
126 ParameterAttributes NewAttributes = ParamAttr::None;
129 NewAttributes |= ParamAttr::NoUnwind;
131 NewAttributes |= ParamAttr::NoReturn;
133 const PAListPtr &PAL = SCC[i]->getFunction()->getParamAttrs();
134 SCC[i]->getFunction()->setParamAttrs(PAL.addAttr(0, NewAttributes));
137 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
138 // Convert any invoke instructions to non-throwing functions in this node
139 // into call instructions with a branch. This makes the exception blocks
141 if (Function *F = SCC[i]->getFunction())
142 MadeChange |= SimplifyFunction(F);
149 // SimplifyFunction - Given information about callees, simplify the specified
150 // function if we have invokes to non-unwinding functions or code after calls to
151 // no-return functions.
152 bool PruneEH::SimplifyFunction(Function *F) {
153 bool MadeChange = false;
154 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
155 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
156 if (II->doesNotThrow()) {
157 SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
158 // Insert a call instruction before the invoke.
159 CallInst *Call = CallInst::Create(II->getCalledValue(),
160 Args.begin(), Args.end(), "", II);
162 Call->setCallingConv(II->getCallingConv());
163 Call->setParamAttrs(II->getParamAttrs());
165 // Anything that used the value produced by the invoke instruction
166 // now uses the value produced by the call instruction.
167 II->replaceAllUsesWith(Call);
168 BasicBlock *UnwindBlock = II->getUnwindDest();
169 UnwindBlock->removePredecessor(II->getParent());
171 // Insert a branch to the normal destination right before the
173 BranchInst::Create(II->getNormalDest(), II);
175 // Finally, delete the invoke instruction!
176 BB->getInstList().pop_back();
178 // If the unwind block is now dead, nuke it.
179 if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
180 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
186 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
187 if (CallInst *CI = dyn_cast<CallInst>(I++))
188 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
189 // This call calls a function that cannot return. Insert an
190 // unreachable instruction after it and simplify the code. Do this
191 // by splitting the BB, adding the unreachable, then deleting the
193 BasicBlock *New = BB->splitBasicBlock(I);
195 // Remove the uncond branch and add an unreachable.
196 BB->getInstList().pop_back();
197 new UnreachableInst(BB);
199 DeleteBasicBlock(New); // Delete the new BB.
209 /// DeleteBasicBlock - remove the specified basic block from the program,
210 /// updating the callgraph to reflect any now-obsolete edges due to calls that
212 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
213 assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
214 CallGraph &CG = getAnalysis<CallGraph>();
216 CallGraphNode *CGN = CG[BB->getParent()];
217 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
219 if (CallInst *CI = dyn_cast<CallInst>(I)) {
220 if (Function *Callee = CI->getCalledFunction())
221 CGN->removeCallEdgeTo(CG[Callee]);
222 } else if (InvokeInst *II = dyn_cast<InvokeInst>(I)) {
223 if (Function *Callee = II->getCalledFunction())
224 CGN->removeCallEdgeTo(CG[Callee]);
227 I->replaceAllUsesWith(UndefValue::get(I->getType()));
230 // Get the list of successors of this block.
231 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
233 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
234 Succs[i]->removePredecessor(BB);
236 BB->eraseFromParent();