lib/Transforms/IPO/PruneEH.cpp

   1 //===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements a simple interprocedural pass which walks the
  10 // call-graph, turning invoke instructions into calls, iff the callee cannot
  11 // throw an exception, and marking functions 'nounwind' if they cannot throw.
  12 // It implements this as a bottom-up traversal of the call-graph.
  13 //
  14 //===----------------------------------------------------------------------===//
  15
  16 #include "llvm/ADT/SmallPtrSet.h"
  17 #include "llvm/ADT/Statistic.h"
  18 #include "llvm/Analysis/CallGraph.h"
  19 #include "llvm/Analysis/CallGraphSCCPass.h"
  20 #include "llvm/Analysis/EHPersonalities.h"
  21 #include "llvm/Transforms/Utils/Local.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/LLVMContext.h"
  28 #include "llvm/Support/raw_ostream.h"
  29 #include "llvm/Transforms/IPO.h"
  30 #include <algorithm>
  31 using namespace llvm;
  32
  33 #define DEBUG_TYPE "prune-eh"
  34
  35 STATISTIC(NumRemoved, "Number of invokes removed");
  36 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
  37
  38 namespace {
  39   struct PruneEH : public CallGraphSCCPass {
  40     static char ID; // Pass identification, replacement for typeid
  41     PruneEH() : CallGraphSCCPass(ID) {
  42       initializePruneEHPass(*PassRegistry::getPassRegistry());
  43     }
  44
  45     // runOnSCC - Analyze the SCC, performing the transformation if possible.
  46     bool runOnSCC(CallGraphSCC &SCC) override;
  47
  48   };
  49 }
  50 static bool SimplifyFunction(Function *F, CallGraph &CG);
  51 static void DeleteBasicBlock(BasicBlock *BB, CallGraph &CG);
  52
  53 char PruneEH::ID = 0;
  54 INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
  55                 "Remove unused exception handling info", false, false)
  56 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
  57 INITIALIZE_PASS_END(PruneEH, "prune-eh",
  58                 "Remove unused exception handling info", false, false)
  59
  60 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
  61
  62 static bool runImpl(CallGraphSCC &SCC, CallGraph &CG) {
  63   SmallPtrSet<CallGraphNode *, 8> SCCNodes;
  64   bool MadeChange = false;
  65
  66   // Fill SCCNodes with the elements of the SCC.  Used for quickly
  67   // looking up whether a given CallGraphNode is in this SCC.
  68   for (CallGraphNode *I : SCC)
  69     SCCNodes.insert(I);
  70
  71   // First pass, scan all of the functions in the SCC, simplifying them
  72   // according to what we know.
  73   for (CallGraphNode *I : SCC)
  74     if (Function *F = I->getFunction())
  75       MadeChange |= SimplifyFunction(F, CG);
  76
  77   // Next, check to see if any callees might throw or if there are any external
  78   // functions in this SCC: if so, we cannot prune any functions in this SCC.
  79   // Definitions that are weak and not declared non-throwing might be
  80   // overridden at linktime with something that throws, so assume that.
  81   // If this SCC includes the unwind instruction, we KNOW it throws, so
  82   // obviously the SCC might throw.
  83   //
  84   bool SCCMightUnwind = false, SCCMightReturn = false;
  85   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end();
  86        (!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) {
  87     Function *F = (*I)->getFunction();
  88     if (!F) {
  89       SCCMightUnwind = true;
  90       SCCMightReturn = true;
  91     } else if (!F->hasExactDefinition()) {
  92       SCCMightUnwind |= !F->doesNotThrow();
  93       SCCMightReturn |= !F->doesNotReturn();
  94     } else {
  95       bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
  96       bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
  97       // Determine if we should scan for InlineAsm in a naked function as it
  98       // is the only way to return without a ReturnInst.  Only do this for
  99       // no-inline functions as functions which may be inlined cannot
 100       // meaningfully return via assembly.
 101       bool CheckReturnViaAsm = CheckReturn &&
 102                                F->hasFnAttribute(Attribute::Naked) &&
 103                                F->hasFnAttribute(Attribute::NoInline);
 104
 105       if (!CheckUnwind && !CheckReturn)
 106         continue;
 107
 108       for (const BasicBlock &BB : *F) {
 109         const Instruction *TI = BB.getTerminator();
 110         if (CheckUnwind && TI->mayThrow()) {
 111           SCCMightUnwind = true;
 112         } else if (CheckReturn && isa<ReturnInst>(TI)) {
 113           SCCMightReturn = true;
 114         }
 115
 116         for (const Instruction &I : BB) {
 117           if ((!CheckUnwind || SCCMightUnwind) &&
 118               (!CheckReturnViaAsm || SCCMightReturn))
 119             break;
 120
 121           // Check to see if this function performs an unwind or calls an
 122           // unwinding function.
 123           if (CheckUnwind && !SCCMightUnwind && I.mayThrow()) {
 124             bool InstMightUnwind = true;
 125             if (const auto *CI = dyn_cast<CallInst>(&I)) {
 126               if (Function *Callee = CI->getCalledFunction()) {
 127                 CallGraphNode *CalleeNode = CG[Callee];
 128                 // If the callee is outside our current SCC then we may throw
 129                 // because it might.  If it is inside, do nothing.
 130                 if (SCCNodes.count(CalleeNode) > 0)
 131                   InstMightUnwind = false;
 132               }
 133             }
 134             SCCMightUnwind |= InstMightUnwind;
 135           }
 136           if (CheckReturnViaAsm && !SCCMightReturn)
 137             if (auto ICS = ImmutableCallSite(&I))
 138               if (const auto *IA = dyn_cast<InlineAsm>(ICS.getCalledValue()))
 139                 if (IA->hasSideEffects())
 140                   SCCMightReturn = true;
 141         }
 142
 143         if (SCCMightUnwind && SCCMightReturn)
 144           break;
 145       }
 146     }
 147   }
 148
 149   // If the SCC doesn't unwind or doesn't throw, note this fact.
 150   if (!SCCMightUnwind || !SCCMightReturn)
 151     for (CallGraphNode *I : SCC) {
 152       Function *F = I->getFunction();
 153
 154       if (!SCCMightUnwind && !F->hasFnAttribute(Attribute::NoUnwind)) {
 155         F->addFnAttr(Attribute::NoUnwind);
 156         MadeChange = true;
 157       }
 158
 159       if (!SCCMightReturn && !F->hasFnAttribute(Attribute::NoReturn)) {
 160         F->addFnAttr(Attribute::NoReturn);
 161         MadeChange = true;
 162       }
 163     }
 164
 165   for (CallGraphNode *I : SCC) {
 166     // Convert any invoke instructions to non-throwing functions in this node
 167     // into call instructions with a branch.  This makes the exception blocks
 168     // dead.
 169     if (Function *F = I->getFunction())
 170       MadeChange |= SimplifyFunction(F, CG);
 171   }
 172
 173   return MadeChange;
 174 }
 175
 176
 177 bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
 178   if (skipSCC(SCC))
 179     return false;
 180   CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
 181   return runImpl(SCC, CG);
 182 }
 183
 184
 185 // SimplifyFunction - Given information about callees, simplify the specified
 186 // function if we have invokes to non-unwinding functions or code after calls to
 187 // no-return functions.
 188 static bool SimplifyFunction(Function *F, CallGraph &CG) {
 189   bool MadeChange = false;
 190   for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
 191     if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
 192       if (II->doesNotThrow() && canSimplifyInvokeNoUnwind(F)) {
 193         BasicBlock *UnwindBlock = II->getUnwindDest();
 194         removeUnwindEdge(&*BB);
 195
 196         // If the unwind block is now dead, nuke it.
 197         if (pred_empty(UnwindBlock))
 198           DeleteBasicBlock(UnwindBlock, CG);  // Delete the new BB.
 199
 200         ++NumRemoved;
 201         MadeChange = true;
 202       }
 203
 204     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
 205       if (CallInst *CI = dyn_cast<CallInst>(I++))
 206         if (CI->doesNotReturn() && !CI->isMustTailCall() &&
 207             !isa<UnreachableInst>(I)) {
 208           // This call calls a function that cannot return.  Insert an
 209           // unreachable instruction after it and simplify the code.  Do this
 210           // by splitting the BB, adding the unreachable, then deleting the
 211           // new BB.
 212           BasicBlock *New = BB->splitBasicBlock(I);
 213
 214           // Remove the uncond branch and add an unreachable.
 215           BB->getInstList().pop_back();
 216           new UnreachableInst(BB->getContext(), &*BB);
 217
 218           DeleteBasicBlock(New, CG);  // Delete the new BB.
 219           MadeChange = true;
 220           ++NumUnreach;
 221           break;
 222         }
 223   }
 224
 225   return MadeChange;
 226 }
 227
 228 /// DeleteBasicBlock - remove the specified basic block from the program,
 229 /// updating the callgraph to reflect any now-obsolete edges due to calls that
 230 /// exist in the BB.
 231 static void DeleteBasicBlock(BasicBlock *BB, CallGraph &CG) {
 232   assert(pred_empty(BB) && "BB is not dead!");
 233
 234   Instruction *TokenInst = nullptr;
 235
 236   CallGraphNode *CGN = CG[BB->getParent()];
 237   for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
 238     --I;
 239
 240     if (I->getType()->isTokenTy()) {
 241       TokenInst = &*I;
 242       break;
 243     }
 244
 245     if (auto *Call = dyn_cast<CallBase>(&*I)) {
 246       const Function *Callee = Call->getCalledFunction();
 247       if (!Callee || !Intrinsic::isLeaf(Callee->getIntrinsicID()))
 248         CGN->removeCallEdgeFor(*Call);
 249       else if (!Callee->isIntrinsic())
 250         CGN->removeCallEdgeFor(*Call);
 251     }
 252
 253     if (!I->use_empty())
 254       I->replaceAllUsesWith(UndefValue::get(I->getType()));
 255   }
 256
 257   if (TokenInst) {
 258     if (!TokenInst->isTerminator())
 259       changeToUnreachable(TokenInst->getNextNode(), /*UseLLVMTrap=*/false);
 260   } else {
 261     // Get the list of successors of this block.
 262     std::vector<BasicBlock *> Succs(succ_begin(BB), succ_end(BB));
 263
 264     for (unsigned i = 0, e = Succs.size(); i != e; ++i)
 265       Succs[i]->removePredecessor(BB);
 266
 267     BB->eraseFromParent();
 268   }
 269 }