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 // Definitions that are weak and not declared non-throwing might be
68 // overridden at linktime with something that throws, so assume that.
69 // If this SCC includes the unwind instruction, we KNOW it throws, so
70 // obviously the SCC might throw.
72 bool SCCMightUnwind = false, SCCMightReturn = false;
73 for (unsigned i = 0, e = SCC.size();
74 (!SCCMightUnwind || !SCCMightReturn) && i != e; ++i) {
75 Function *F = SCC[i]->getFunction();
77 SCCMightUnwind = true;
78 SCCMightReturn = true;
79 } else if (F->isDeclaration() || F->hasWeakLinkage()) {
80 SCCMightUnwind |= !F->doesNotThrow();
81 SCCMightReturn |= !F->doesNotReturn();
83 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
84 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
86 if (!CheckUnwind && !CheckReturn)
89 // Check to see if this function performs an unwind or calls an
90 // unwinding function.
91 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
92 if (CheckUnwind && isa<UnwindInst>(BB->getTerminator())) {
94 SCCMightUnwind = true;
95 } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
96 SCCMightReturn = true;
99 // Invoke instructions don't allow unwinding to continue, so we are
100 // only interested in call instructions.
101 if (CheckUnwind && !SCCMightUnwind)
102 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
103 if (CallInst *CI = dyn_cast<CallInst>(I)) {
104 if (CI->doesNotThrow()) {
105 // This call cannot throw.
106 } else if (Function *Callee = CI->getCalledFunction()) {
107 CallGraphNode *CalleeNode = CG[Callee];
108 // If the callee is outside our current SCC then we may
109 // throw because it might.
110 if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end()){
111 SCCMightUnwind = true;
115 // Indirect call, it might throw.
116 SCCMightUnwind = true;
120 if (SCCMightUnwind && SCCMightReturn) break;
125 // If the SCC doesn't unwind or doesn't throw, note this fact.
126 if (!SCCMightUnwind || !SCCMightReturn)
127 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
128 ParameterAttributes NewAttributes = ParamAttr::None;
131 NewAttributes |= ParamAttr::NoUnwind;
133 NewAttributes |= ParamAttr::NoReturn;
135 const PAListPtr &PAL = SCC[i]->getFunction()->getParamAttrs();
136 SCC[i]->getFunction()->setParamAttrs(PAL.addAttr(0, NewAttributes));
139 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
140 // Convert any invoke instructions to non-throwing functions in this node
141 // into call instructions with a branch. This makes the exception blocks
143 if (Function *F = SCC[i]->getFunction())
144 MadeChange |= SimplifyFunction(F);
151 // SimplifyFunction - Given information about callees, simplify the specified
152 // function if we have invokes to non-unwinding functions or code after calls to
153 // no-return functions.
154 bool PruneEH::SimplifyFunction(Function *F) {
155 bool MadeChange = false;
156 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
157 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
158 if (II->doesNotThrow()) {
159 SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
160 // Insert a call instruction before the invoke.
161 CallInst *Call = CallInst::Create(II->getCalledValue(),
162 Args.begin(), Args.end(), "", II);
164 Call->setCallingConv(II->getCallingConv());
165 Call->setParamAttrs(II->getParamAttrs());
167 // Anything that used the value produced by the invoke instruction
168 // now uses the value produced by the call instruction.
169 II->replaceAllUsesWith(Call);
170 BasicBlock *UnwindBlock = II->getUnwindDest();
171 UnwindBlock->removePredecessor(II->getParent());
173 // Insert a branch to the normal destination right before the
175 BranchInst::Create(II->getNormalDest(), II);
177 // Finally, delete the invoke instruction!
178 BB->getInstList().pop_back();
180 // If the unwind block is now dead, nuke it.
181 if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
182 DeleteBasicBlock(UnwindBlock); // Delete the new BB.
188 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
189 if (CallInst *CI = dyn_cast<CallInst>(I++))
190 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
191 // This call calls a function that cannot return. Insert an
192 // unreachable instruction after it and simplify the code. Do this
193 // by splitting the BB, adding the unreachable, then deleting the
195 BasicBlock *New = BB->splitBasicBlock(I);
197 // Remove the uncond branch and add an unreachable.
198 BB->getInstList().pop_back();
199 new UnreachableInst(BB);
201 DeleteBasicBlock(New); // Delete the new BB.
211 /// DeleteBasicBlock - remove the specified basic block from the program,
212 /// updating the callgraph to reflect any now-obsolete edges due to calls that
214 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
215 assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
216 CallGraph &CG = getAnalysis<CallGraph>();
218 CallGraphNode *CGN = CG[BB->getParent()];
219 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
221 if (CallInst *CI = dyn_cast<CallInst>(I)) {
222 if (Function *Callee = CI->getCalledFunction())
223 CGN->removeCallEdgeTo(CG[Callee]);
224 } else if (InvokeInst *II = dyn_cast<InvokeInst>(I)) {
225 if (Function *Callee = II->getCalledFunction())
226 CGN->removeCallEdgeTo(CG[Callee]);
229 I->replaceAllUsesWith(UndefValue::get(I->getType()));
232 // Get the list of successors of this block.
233 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
235 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
236 Succs[i]->removePredecessor(BB);
238 BB->eraseFromParent();