1//===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 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. 14// 15//===----------------------------------------------------------------------===// 16 17#include "llvm/Transforms/IPO.h" 18#include "llvm/ADT/SmallPtrSet.h" 19#include "llvm/ADT/SmallVector.h" 20#include "llvm/ADT/Statistic.h" 21#include "llvm/Support/raw_ostream.h" 22#include "llvm/Analysis/CallGraph.h" 23#include "llvm/Analysis/CallGraphSCCPass.h" 24#include "llvm/Analysis/EHPersonalities.h" 25#include "llvm/IR/CFG.h" 26#include "llvm/IR/Constants.h" 27#include "llvm/IR/Function.h" 28#include "llvm/IR/InlineAsm.h" 29#include "llvm/IR/Instructions.h" 30#include "llvm/IR/IntrinsicInst.h" 31#include "llvm/IR/LLVMContext.h" 32#include "llvm/Transforms/Utils/Local.h" 33#include <algorithm> 34using namespace llvm; 35 36#define DEBUG_TYPE "prune-eh" 37 38STATISTIC(NumRemoved, "Number of invokes removed"); 39STATISTIC(NumUnreach, "Number of noreturn calls optimized"); 40 41namespace { 42 struct PruneEH : public CallGraphSCCPass { 43 static char ID; // Pass identification, replacement for typeid 44 PruneEH() : CallGraphSCCPass(ID) { 45 initializePruneEHPass(*PassRegistry::getPassRegistry()); 46 } 47 48 // runOnSCC - Analyze the SCC, performing the transformation if possible. 49 bool runOnSCC(CallGraphSCC &SCC) override; 50 51 bool SimplifyFunction(Function *F); 52 void DeleteBasicBlock(BasicBlock *BB); 53 }; 54} 55 56char PruneEH::ID = 0; 57INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh", 58 "Remove unused exception handling info", false, false) 59INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass) 60INITIALIZE_PASS_END(PruneEH, "prune-eh", 61 "Remove unused exception handling info", false, false) 62 63Pass *llvm::createPruneEHPass() { return new PruneEH(); } 64 65 66bool PruneEH::runOnSCC(CallGraphSCC &SCC) { 67 SmallPtrSet<CallGraphNode *, 8> SCCNodes; 68 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph(); 69 bool MadeChange = false; 70 71 // Fill SCCNodes with the elements of the SCC. Used for quickly 72 // looking up whether a given CallGraphNode is in this SCC. 73 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) 74 SCCNodes.insert(*I); 75 76 // First pass, scan all of the functions in the SCC, simplifying them 77 // according to what we know. 78 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) 79 if (Function *F = (*I)->getFunction()) 80 MadeChange |= SimplifyFunction(F); 81 82 // Next, check to see if any callees might throw or if there are any external 83 // functions in this SCC: if so, we cannot prune any functions in this SCC. 84 // Definitions that are weak and not declared non-throwing might be 85 // overridden at linktime with something that throws, so assume that. 86 // If this SCC includes the unwind instruction, we KNOW it throws, so 87 // obviously the SCC might throw. 88 // 89 bool SCCMightUnwind = false, SCCMightReturn = false; 90 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); 91 (!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) { 92 Function *F = (*I)->getFunction(); 93 if (!F) { 94 SCCMightUnwind = true; 95 SCCMightReturn = true; 96 } else if (F->isDeclaration() || F->mayBeOverridden()) { 97 SCCMightUnwind |= !F->doesNotThrow(); 98 SCCMightReturn |= !F->doesNotReturn(); 99 } else { 100 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow(); 101 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn(); 102 // Determine if we should scan for InlineAsm in a naked function as it 103 // is the only way to return without a ReturnInst. Only do this for 104 // no-inline functions as functions which may be inlined cannot 105 // meaningfully return via assembly. 106 bool CheckReturnViaAsm = CheckReturn && 107 F->hasFnAttribute(Attribute::Naked) && 108 F->hasFnAttribute(Attribute::NoInline); 109 110 if (!CheckUnwind && !CheckReturn) 111 continue; 112 113 for (const BasicBlock &BB : *F) { 114 const TerminatorInst *TI = BB.getTerminator(); 115 if (CheckUnwind && TI->mayThrow()) { 116 SCCMightUnwind = true; 117 } else if (CheckReturn && isa<ReturnInst>(TI)) { 118 SCCMightReturn = true; 119 } 120 121 for (const Instruction &I : BB) { 122 if ((!CheckUnwind || SCCMightUnwind) && 123 (!CheckReturnViaAsm || SCCMightReturn)) 124 break; 125 126 // Check to see if this function performs an unwind or calls an 127 // unwinding function. 128 if (CheckUnwind && !SCCMightUnwind && I.mayThrow()) { 129 bool InstMightUnwind = true; 130 if (const auto *CI = dyn_cast<CallInst>(&I)) { 131 if (Function *Callee = CI->getCalledFunction()) { 132 CallGraphNode *CalleeNode = CG[Callee]; 133 // If the callee is outside our current SCC then we may throw 134 // because it might. If it is inside, do nothing. 135 if (SCCNodes.count(CalleeNode) > 0) 136 InstMightUnwind = false; 137 } 138 } 139 SCCMightUnwind |= InstMightUnwind; 140 } 141 if (CheckReturnViaAsm && !SCCMightReturn) 142 if (auto ICS = ImmutableCallSite(&I)) 143 if (const auto *IA = dyn_cast<InlineAsm>(ICS.getCalledValue())) 144 if (IA->hasSideEffects()) 145 SCCMightReturn = true; 146 } 147 148 if (SCCMightUnwind && SCCMightReturn) 149 break; 150 } 151 } 152 } 153 154 // If the SCC doesn't unwind or doesn't throw, note this fact. 155 if (!SCCMightUnwind || !SCCMightReturn) 156 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) { 157 Function *F = (*I)->getFunction(); 158 159 if (!SCCMightUnwind && !F->hasFnAttribute(Attribute::NoUnwind)) { 160 F->addFnAttr(Attribute::NoUnwind); 161 MadeChange = true; 162 } 163 164 if (!SCCMightReturn && !F->hasFnAttribute(Attribute::NoReturn)) { 165 F->addFnAttr(Attribute::NoReturn); 166 MadeChange = true; 167 } 168 } 169 170 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) { 171 // Convert any invoke instructions to non-throwing functions in this node 172 // into call instructions with a branch. This makes the exception blocks 173 // dead. 174 if (Function *F = (*I)->getFunction()) 175 MadeChange |= SimplifyFunction(F); 176 } 177 178 return MadeChange; 179} 180 181 182// SimplifyFunction - Given information about callees, simplify the specified 183// function if we have invokes to non-unwinding functions or code after calls to 184// no-return functions. 185bool PruneEH::SimplifyFunction(Function *F) { 186 bool MadeChange = false; 187 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) { 188 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator())) 189 if (II->doesNotThrow() && canSimplifyInvokeNoUnwind(F)) { 190 BasicBlock *UnwindBlock = II->getUnwindDest(); 191 removeUnwindEdge(&*BB); 192 193 // If the unwind block is now dead, nuke it. 194 if (pred_empty(UnwindBlock)) 195 DeleteBasicBlock(UnwindBlock); // Delete the new BB. 196 197 ++NumRemoved; 198 MadeChange = true; 199 } 200 201 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) 202 if (CallInst *CI = dyn_cast<CallInst>(I++)) 203 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) { 204 // This call calls a function that cannot return. Insert an 205 // unreachable instruction after it and simplify the code. Do this 206 // by splitting the BB, adding the unreachable, then deleting the 207 // new BB. 208 BasicBlock *New = BB->splitBasicBlock(I); 209 210 // Remove the uncond branch and add an unreachable. 211 BB->getInstList().pop_back(); 212 new UnreachableInst(BB->getContext(), &*BB); 213 214 DeleteBasicBlock(New); // Delete the new BB. 215 MadeChange = true; 216 ++NumUnreach; 217 break; 218 } 219 } 220 221 return MadeChange; 222} 223 224/// DeleteBasicBlock - remove the specified basic block from the program, 225/// updating the callgraph to reflect any now-obsolete edges due to calls that 226/// exist in the BB. 227void PruneEH::DeleteBasicBlock(BasicBlock *BB) { 228 assert(pred_empty(BB) && "BB is not dead!"); 229 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph(); 230 231 Instruction *TokenInst = nullptr; 232 233 CallGraphNode *CGN = CG[BB->getParent()]; 234 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) { 235 --I; 236 237 if (I->getType()->isTokenTy()) { 238 TokenInst = &*I; 239 break; 240 } 241 242 if (auto CS = CallSite (&*I)) { 243 const Function *Callee = CS.getCalledFunction(); 244 if (!Callee || !Intrinsic::isLeaf(Callee->getIntrinsicID())) 245 CGN->removeCallEdgeFor(CS); 246 else if (!Callee->isIntrinsic()) 247 CGN->removeCallEdgeFor(CS); 248 } 249 250 if (!I->use_empty()) 251 I->replaceAllUsesWith(UndefValue::get(I->getType())); 252 } 253 254 if (TokenInst) { 255 if (!isa<TerminatorInst>(TokenInst)) 256 changeToUnreachable(TokenInst->getNextNode(), /*UseLLVMTrap=*/false); 257 } else { 258 // Get the list of successors of this block. 259 std::vector<BasicBlock *> Succs(succ_begin(BB), succ_end(BB)); 260 261 for (unsigned i = 0, e = Succs.size(); i != e; ++i) 262 Succs[i]->removePredecessor(BB); 263 264 BB->eraseFromParent(); 265 } 266} 267