PruneEH.cpp revision 321369
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/ADT/SmallPtrSet.h" 18#include "llvm/ADT/Statistic.h" 19#include "llvm/Analysis/CallGraph.h" 20#include "llvm/Analysis/CallGraphSCCPass.h" 21#include "llvm/Analysis/EHPersonalities.h" 22#include "llvm/IR/CFG.h" 23#include "llvm/IR/Constants.h" 24#include "llvm/IR/Function.h" 25#include "llvm/IR/InlineAsm.h" 26#include "llvm/IR/Instructions.h" 27#include "llvm/IR/IntrinsicInst.h" 28#include "llvm/IR/LLVMContext.h" 29#include "llvm/Support/raw_ostream.h" 30#include "llvm/Transforms/IPO.h" 31#include "llvm/Transforms/Utils/Local.h" 32#include <algorithm> 33using namespace llvm; 34 35#define DEBUG_TYPE "prune-eh" 36 37STATISTIC(NumRemoved, "Number of invokes removed"); 38STATISTIC(NumUnreach, "Number of noreturn calls optimized"); 39 40namespace { 41 struct PruneEH : public CallGraphSCCPass { 42 static char ID; // Pass identification, replacement for typeid 43 PruneEH() : CallGraphSCCPass(ID) { 44 initializePruneEHPass(*PassRegistry::getPassRegistry()); 45 } 46 47 // runOnSCC - Analyze the SCC, performing the transformation if possible. 48 bool runOnSCC(CallGraphSCC &SCC) override; 49 50 }; 51} 52static bool SimplifyFunction(Function *F, CallGraph &CG); 53static void DeleteBasicBlock(BasicBlock *BB, CallGraph &CG); 54 55char PruneEH::ID = 0; 56INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh", 57 "Remove unused exception handling info", false, false) 58INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass) 59INITIALIZE_PASS_END(PruneEH, "prune-eh", 60 "Remove unused exception handling info", false, false) 61 62Pass *llvm::createPruneEHPass() { return new PruneEH(); } 63 64static bool runImpl(CallGraphSCC &SCC, CallGraph &CG) { 65 SmallPtrSet<CallGraphNode *, 8> SCCNodes; 66 bool MadeChange = false; 67 68 // Fill SCCNodes with the elements of the SCC. Used for quickly 69 // looking up whether a given CallGraphNode is in this SCC. 70 for (CallGraphNode *I : SCC) 71 SCCNodes.insert(I); 72 73 // First pass, scan all of the functions in the SCC, simplifying them 74 // according to what we know. 75 for (CallGraphNode *I : SCC) 76 if (Function *F = I->getFunction()) 77 MadeChange |= SimplifyFunction(F, CG); 78 79 // Next, check to see if any callees might throw or if there are any external 80 // functions in this SCC: if so, we cannot prune any functions in this SCC. 81 // Definitions that are weak and not declared non-throwing might be 82 // overridden at linktime with something that throws, so assume that. 83 // If this SCC includes the unwind instruction, we KNOW it throws, so 84 // obviously the SCC might throw. 85 // 86 bool SCCMightUnwind = false, SCCMightReturn = false; 87 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); 88 (!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) { 89 Function *F = (*I)->getFunction(); 90 if (!F) { 91 SCCMightUnwind = true; 92 SCCMightReturn = true; 93 } else if (!F->hasExactDefinition()) { 94 SCCMightUnwind |= !F->doesNotThrow(); 95 SCCMightReturn |= !F->doesNotReturn(); 96 } else { 97 bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow(); 98 bool CheckReturn = !SCCMightReturn && !F->doesNotReturn(); 99 // Determine if we should scan for InlineAsm in a naked function as it 100 // is the only way to return without a ReturnInst. Only do this for 101 // no-inline functions as functions which may be inlined cannot 102 // meaningfully return via assembly. 103 bool CheckReturnViaAsm = CheckReturn && 104 F->hasFnAttribute(Attribute::Naked) && 105 F->hasFnAttribute(Attribute::NoInline); 106 107 if (!CheckUnwind && !CheckReturn) 108 continue; 109 110 for (const BasicBlock &BB : *F) { 111 const TerminatorInst *TI = BB.getTerminator(); 112 if (CheckUnwind && TI->mayThrow()) { 113 SCCMightUnwind = true; 114 } else if (CheckReturn && isa<ReturnInst>(TI)) { 115 SCCMightReturn = true; 116 } 117 118 for (const Instruction &I : BB) { 119 if ((!CheckUnwind || SCCMightUnwind) && 120 (!CheckReturnViaAsm || SCCMightReturn)) 121 break; 122 123 // Check to see if this function performs an unwind or calls an 124 // unwinding function. 125 if (CheckUnwind && !SCCMightUnwind && I.mayThrow()) { 126 bool InstMightUnwind = true; 127 if (const auto *CI = dyn_cast<CallInst>(&I)) { 128 if (Function *Callee = CI->getCalledFunction()) { 129 CallGraphNode *CalleeNode = CG[Callee]; 130 // If the callee is outside our current SCC then we may throw 131 // because it might. If it is inside, do nothing. 132 if (SCCNodes.count(CalleeNode) > 0) 133 InstMightUnwind = false; 134 } 135 } 136 SCCMightUnwind |= InstMightUnwind; 137 } 138 if (CheckReturnViaAsm && !SCCMightReturn) 139 if (auto ICS = ImmutableCallSite(&I)) 140 if (const auto *IA = dyn_cast<InlineAsm>(ICS.getCalledValue())) 141 if (IA->hasSideEffects()) 142 SCCMightReturn = true; 143 } 144 145 if (SCCMightUnwind && SCCMightReturn) 146 break; 147 } 148 } 149 } 150 151 // If the SCC doesn't unwind or doesn't throw, note this fact. 152 if (!SCCMightUnwind || !SCCMightReturn) 153 for (CallGraphNode *I : SCC) { 154 Function *F = I->getFunction(); 155 156 if (!SCCMightUnwind && !F->hasFnAttribute(Attribute::NoUnwind)) { 157 F->addFnAttr(Attribute::NoUnwind); 158 MadeChange = true; 159 } 160 161 if (!SCCMightReturn && !F->hasFnAttribute(Attribute::NoReturn)) { 162 F->addFnAttr(Attribute::NoReturn); 163 MadeChange = true; 164 } 165 } 166 167 for (CallGraphNode *I : SCC) { 168 // Convert any invoke instructions to non-throwing functions in this node 169 // into call instructions with a branch. This makes the exception blocks 170 // dead. 171 if (Function *F = I->getFunction()) 172 MadeChange |= SimplifyFunction(F, CG); 173 } 174 175 return MadeChange; 176} 177 178 179bool PruneEH::runOnSCC(CallGraphSCC &SCC) { 180 if (skipSCC(SCC)) 181 return false; 182 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph(); 183 return runImpl(SCC, CG); 184} 185 186 187// SimplifyFunction - Given information about callees, simplify the specified 188// function if we have invokes to non-unwinding functions or code after calls to 189// no-return functions. 190static bool SimplifyFunction(Function *F, CallGraph &CG) { 191 bool MadeChange = false; 192 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) { 193 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator())) 194 if (II->doesNotThrow() && canSimplifyInvokeNoUnwind(F)) { 195 BasicBlock *UnwindBlock = II->getUnwindDest(); 196 removeUnwindEdge(&*BB); 197 198 // If the unwind block is now dead, nuke it. 199 if (pred_empty(UnwindBlock)) 200 DeleteBasicBlock(UnwindBlock, CG); // Delete the new BB. 201 202 ++NumRemoved; 203 MadeChange = true; 204 } 205 206 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) 207 if (CallInst *CI = dyn_cast<CallInst>(I++)) 208 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) { 209 // This call calls a function that cannot return. Insert an 210 // unreachable instruction after it and simplify the code. Do this 211 // by splitting the BB, adding the unreachable, then deleting the 212 // new BB. 213 BasicBlock *New = BB->splitBasicBlock(I); 214 215 // Remove the uncond branch and add an unreachable. 216 BB->getInstList().pop_back(); 217 new UnreachableInst(BB->getContext(), &*BB); 218 219 DeleteBasicBlock(New, CG); // Delete the new BB. 220 MadeChange = true; 221 ++NumUnreach; 222 break; 223 } 224 } 225 226 return MadeChange; 227} 228 229/// DeleteBasicBlock - remove the specified basic block from the program, 230/// updating the callgraph to reflect any now-obsolete edges due to calls that 231/// exist in the BB. 232static void DeleteBasicBlock(BasicBlock *BB, CallGraph &CG) { 233 assert(pred_empty(BB) && "BB is not dead!"); 234 235 Instruction *TokenInst = nullptr; 236 237 CallGraphNode *CGN = CG[BB->getParent()]; 238 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) { 239 --I; 240 241 if (I->getType()->isTokenTy()) { 242 TokenInst = &*I; 243 break; 244 } 245 246 if (auto CS = CallSite (&*I)) { 247 const Function *Callee = CS.getCalledFunction(); 248 if (!Callee || !Intrinsic::isLeaf(Callee->getIntrinsicID())) 249 CGN->removeCallEdgeFor(CS); 250 else if (!Callee->isIntrinsic()) 251 CGN->removeCallEdgeFor(CS); 252 } 253 254 if (!I->use_empty()) 255 I->replaceAllUsesWith(UndefValue::get(I->getType())); 256 } 257 258 if (TokenInst) { 259 if (!isa<TerminatorInst>(TokenInst)) 260 changeToUnreachable(TokenInst->getNextNode(), /*UseLLVMTrap=*/false); 261 } else { 262 // Get the list of successors of this block. 263 std::vector<BasicBlock *> Succs(succ_begin(BB), succ_end(BB)); 264 265 for (unsigned i = 0, e = Succs.size(); i != e; ++i) 266 Succs[i]->removePredecessor(BB); 267 268 BB->eraseFromParent(); 269 } 270} 271