LLVM API Documentation
00001 //===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===// 00002 // 00003 // The LLVM Compiler Infrastructure 00004 // 00005 // This file was developed by the LLVM research group and is distributed under 00006 // the University of Illinois Open Source License. See LICENSE.TXT for details. 00007 // 00008 //===----------------------------------------------------------------------===// 00009 // 00010 // This file implements a simple interprocedural pass which walks the 00011 // call-graph, turning invoke instructions into calls, iff the callee cannot 00012 // throw an exception. It implements this as a bottom-up traversal of the 00013 // call-graph. 00014 // 00015 //===----------------------------------------------------------------------===// 00016 00017 #include "llvm/Transforms/IPO.h" 00018 #include "llvm/CallGraphSCCPass.h" 00019 #include "llvm/Function.h" 00020 #include "llvm/Intrinsics.h" 00021 #include "llvm/Instructions.h" 00022 #include "llvm/Analysis/CallGraph.h" 00023 #include "llvm/ADT/Statistic.h" 00024 #include <set> 00025 #include <algorithm> 00026 using namespace llvm; 00027 00028 namespace { 00029 Statistic<> NumRemoved("prune-eh", "Number of invokes removed"); 00030 00031 struct PruneEH : public CallGraphSCCPass { 00032 /// DoesNotThrow - This set contains all of the functions which we have 00033 /// determined cannot throw exceptions. 00034 std::set<CallGraphNode*> DoesNotThrow; 00035 00036 // runOnSCC - Analyze the SCC, performing the transformation if possible. 00037 bool runOnSCC(const std::vector<CallGraphNode *> &SCC); 00038 }; 00039 RegisterOpt<PruneEH> X("prune-eh", "Remove unused exception handling info"); 00040 } 00041 00042 ModulePass *llvm::createPruneEHPass() { return new PruneEH(); } 00043 00044 00045 bool PruneEH::runOnSCC(const std::vector<CallGraphNode *> &SCC) { 00046 CallGraph &CG = getAnalysis<CallGraph>(); 00047 00048 // First, check to see if any callees might throw or if there are any external 00049 // functions in this SCC: if so, we cannot prune any functions in this SCC. 00050 // If this SCC includes the unwind instruction, we KNOW it throws, so 00051 // obviously the SCC might throw. 00052 // 00053 bool SCCMightThrow = false; 00054 for (unsigned i = 0, e = SCC.size(); !SCCMightThrow && i != e; ++i) { 00055 Function *F = SCC[i]->getFunction(); 00056 if (F == 0 || (F->isExternal() && !F->getIntrinsicID())) { 00057 SCCMightThrow = true; 00058 } else { 00059 // Check to see if this function performs an unwind or calls an 00060 // unwinding function. 00061 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) { 00062 if (isa<UnwindInst>(BB->getTerminator())) { // Uses unwind! 00063 SCCMightThrow = true; 00064 break; 00065 } 00066 00067 // Invoke instructions don't allow unwinding to continue, so we are 00068 // only interested in call instructions. 00069 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 00070 if (CallInst *CI = dyn_cast<CallInst>(I)) { 00071 if (Function *Callee = CI->getCalledFunction()) { 00072 CallGraphNode *CalleeNode = CG[Callee]; 00073 // If the callee is outside our current SCC, or if it is not 00074 // known to throw, then we might throw also. 00075 if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end()&& 00076 !DoesNotThrow.count(CalleeNode)) { 00077 SCCMightThrow = true; 00078 break; 00079 } 00080 00081 } else { 00082 // Indirect call, it might throw. 00083 SCCMightThrow = true; 00084 break; 00085 } 00086 } 00087 if (SCCMightThrow) break; 00088 } 00089 } 00090 } 00091 bool MadeChange = false; 00092 00093 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 00094 // If the SCC can't throw, remember this for callers... 00095 if (!SCCMightThrow) 00096 DoesNotThrow.insert(SCC[i]); 00097 00098 // Convert any invoke instructions to non-throwing functions in this node 00099 // into call instructions with a branch. This makes the exception blocks 00100 // dead. 00101 if (Function *F = SCC[i]->getFunction()) 00102 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) 00103 if (InvokeInst *II = dyn_cast<InvokeInst>(I->getTerminator())) 00104 if (Function *F = II->getCalledFunction()) 00105 if (DoesNotThrow.count(CG[F])) { 00106 // Insert a call instruction before the invoke... 00107 std::string Name = II->getName(); II->setName(""); 00108 Value *Call = new CallInst(II->getCalledValue(), 00109 std::vector<Value*>(II->op_begin()+3, 00110 II->op_end()), 00111 Name, II); 00112 00113 // Anything that used the value produced by the invoke instruction 00114 // now uses the value produced by the call instruction. 00115 II->replaceAllUsesWith(Call); 00116 II->getUnwindDest()->removePredecessor(II->getParent()); 00117 00118 // Insert a branch to the normal destination right before the 00119 // invoke. 00120 new BranchInst(II->getNormalDest(), II); 00121 00122 // Finally, delete the invoke instruction! 00123 I->getInstList().pop_back(); 00124 00125 ++NumRemoved; 00126 MadeChange = true; 00127 } 00128 } 00129 00130 return MadeChange; 00131 } 00132