PruneEH.cpp revision 309124
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/Statistic.h" 20#include "llvm/Support/raw_ostream.h" 21#include "llvm/Analysis/CallGraph.h" 22#include "llvm/Analysis/CallGraphSCCPass.h" 23#include "llvm/Analysis/EHPersonalities.h" 24#include "llvm/IR/CFG.h" 25#include "llvm/IR/Constants.h" 26#include "llvm/IR/Function.h" 27#include "llvm/IR/InlineAsm.h" 28#include "llvm/IR/Instructions.h" 29#include "llvm/IR/IntrinsicInst.h" 30#include "llvm/IR/LLVMContext.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->isDeclaration() || F->isInterposable()) { 94 // Note: isInterposable (as opposed to hasExactDefinition) is fine above, 95 // since we're not inferring new attributes here, but only using existing, 96 // assumed to be correct, function attributes. 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 (CallGraphNode *I : SCC) { 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 (CallGraphNode *I : SCC) { 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, CG); 176 } 177 178 return MadeChange; 179} 180 181 182bool PruneEH::runOnSCC(CallGraphSCC &SCC) { 183 if (skipSCC(SCC)) 184 return false; 185 CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph(); 186 return runImpl(SCC, CG); 187} 188 189 190// SimplifyFunction - Given information about callees, simplify the specified 191// function if we have invokes to non-unwinding functions or code after calls to 192// no-return functions. 193static bool SimplifyFunction(Function *F, CallGraph &CG) { 194 bool MadeChange = false; 195 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) { 196 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator())) 197 if (II->doesNotThrow() && canSimplifyInvokeNoUnwind(F)) { 198 BasicBlock *UnwindBlock = II->getUnwindDest(); 199 removeUnwindEdge(&*BB); 200 201 // If the unwind block is now dead, nuke it. 202 if (pred_empty(UnwindBlock)) 203 DeleteBasicBlock(UnwindBlock, CG); // Delete the new BB. 204 205 ++NumRemoved; 206 MadeChange = true; 207 } 208 209 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) 210 if (CallInst *CI = dyn_cast<CallInst>(I++)) 211 if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) { 212 // This call calls a function that cannot return. Insert an 213 // unreachable instruction after it and simplify the code. Do this 214 // by splitting the BB, adding the unreachable, then deleting the 215 // new BB. 216 BasicBlock *New = BB->splitBasicBlock(I); 217 218 // Remove the uncond branch and add an unreachable. 219 BB->getInstList().pop_back(); 220 new UnreachableInst(BB->getContext(), &*BB); 221 222 DeleteBasicBlock(New, CG); // Delete the new BB. 223 MadeChange = true; 224 ++NumUnreach; 225 break; 226 } 227 } 228 229 return MadeChange; 230} 231 232/// DeleteBasicBlock - remove the specified basic block from the program, 233/// updating the callgraph to reflect any now-obsolete edges due to calls that 234/// exist in the BB. 235static void DeleteBasicBlock(BasicBlock *BB, CallGraph &CG) { 236 assert(pred_empty(BB) && "BB is not dead!"); 237 238 Instruction *TokenInst = nullptr; 239 240 CallGraphNode *CGN = CG[BB->getParent()]; 241 for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) { 242 --I; 243 244 if (I->getType()->isTokenTy()) { 245 TokenInst = &*I; 246 break; 247 } 248 249 if (auto CS = CallSite (&*I)) { 250 const Function *Callee = CS.getCalledFunction(); 251 if (!Callee || !Intrinsic::isLeaf(Callee->getIntrinsicID())) 252 CGN->removeCallEdgeFor(CS); 253 else if (!Callee->isIntrinsic()) 254 CGN->removeCallEdgeFor(CS); 255 } 256 257 if (!I->use_empty()) 258 I->replaceAllUsesWith(UndefValue::get(I->getType())); 259 } 260 261 if (TokenInst) { 262 if (!isa<TerminatorInst>(TokenInst)) 263 changeToUnreachable(TokenInst->getNextNode(), /*UseLLVMTrap=*/false); 264 } else { 265 // Get the list of successors of this block. 266 std::vector<BasicBlock *> Succs(succ_begin(BB), succ_end(BB)); 267 268 for (unsigned i = 0, e = Succs.size(); i != e; ++i) 269 Succs[i]->removePredecessor(BB); 270 271 BB->eraseFromParent(); 272 } 273} 274