lib/Transforms/IPO/PruneEH.cpp

   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 #define DEBUG_TYPE "prune-eh"
  18 #include "llvm/Transforms/IPO.h"
  19 #include "llvm/CallGraphSCCPass.h"
  20 #include "llvm/Constants.h"
  21 #include "llvm/Function.h"
  22 #include "llvm/LLVMContext.h"
  23 #include "llvm/Instructions.h"
  24 #include "llvm/IntrinsicInst.h"
  25 #include "llvm/Analysis/CallGraph.h"
  26 #include "llvm/ADT/SmallPtrSet.h"
  27 #include "llvm/ADT/SmallVector.h"
  28 #include "llvm/ADT/Statistic.h"
  29 #include "llvm/Support/CFG.h"
  30 #include "llvm/Support/Compiler.h"
  31 #include <set>
  32 #include <algorithm>
  33 using namespace llvm;
  34
  35 STATISTIC(NumRemoved, "Number of invokes removed");
  36 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
  37
  38 namespace {
  39   struct VISIBILITY_HIDDEN PruneEH : public CallGraphSCCPass {
  40     static char ID; // Pass identification, replacement for typeid
  41     PruneEH() : CallGraphSCCPass(&ID) {}
  42
  43     // runOnSCC - Analyze the SCC, performing the transformation if possible.
  44     bool runOnSCC(std::vector<CallGraphNode *> &SCC);
  45
  46     bool SimplifyFunction(Function *F);
  47     void DeleteBasicBlock(BasicBlock *BB);
  48   };
  49 }
  50
  51 char PruneEH::ID = 0;
  52 static RegisterPass<PruneEH>
  53 X("prune-eh", "Remove unused exception handling info");
  54
  55 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
  56
  57
  58 bool PruneEH::runOnSCC(std::vector<CallGraphNode *> &SCC) {
  59   SmallPtrSet<CallGraphNode *, 8> SCCNodes;
  60   CallGraph &CG = getAnalysis<CallGraph>();
  61   bool MadeChange = false;
  62
  63   // Fill SCCNodes with the elements of the SCC.  Used for quickly
  64   // looking up whether a given CallGraphNode is in this SCC.
  65   for (unsigned i = 0, e = SCC.size(); i != e; ++i)
  66     SCCNodes.insert(SCC[i]);
  67
  68   // First pass, scan all of the functions in the SCC, simplifying them
  69   // according to what we know.
  70   for (unsigned i = 0, e = SCC.size(); i != e; ++i)
  71     if (Function *F = SCC[i]->getFunction())
  72       MadeChange |= SimplifyFunction(F);
  73
  74   // Next, check to see if any callees might throw or if there are any external
  75   // functions in this SCC: if so, we cannot prune any functions in this SCC.
  76   // Definitions that are weak and not declared non-throwing might be
  77   // overridden at linktime with something that throws, so assume that.
  78   // If this SCC includes the unwind instruction, we KNOW it throws, so
  79   // obviously the SCC might throw.
  80   //
  81   bool SCCMightUnwind = false, SCCMightReturn = false;
  82   for (unsigned i = 0, e = SCC.size();
  83        (!SCCMightUnwind || !SCCMightReturn) && i != e; ++i) {
  84     Function *F = SCC[i]->getFunction();
  85     if (F == 0) {
  86       SCCMightUnwind = true;
  87       SCCMightReturn = true;
  88     } else if (F->isDeclaration() || F->mayBeOverridden()) {
  89       SCCMightUnwind |= !F->doesNotThrow();
  90       SCCMightReturn |= !F->doesNotReturn();
  91     } else {
  92       bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
  93       bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
  94
  95       if (!CheckUnwind && !CheckReturn)
  96         continue;
  97
  98       // Check to see if this function performs an unwind or calls an
  99       // unwinding function.
 100       for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
 101         if (CheckUnwind && isa<UnwindInst>(BB->getTerminator())) {
 102           // Uses unwind!
 103           SCCMightUnwind = true;
 104         } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
 105           SCCMightReturn = true;
 106         }
 107
 108         // Invoke instructions don't allow unwinding to continue, so we are
 109         // only interested in call instructions.
 110         if (CheckUnwind && !SCCMightUnwind)
 111           for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
 112             if (CallInst *CI = dyn_cast<CallInst>(I)) {
 113               if (CI->doesNotThrow()) {
 114                 // This call cannot throw.
 115               } else if (Function *Callee = CI->getCalledFunction()) {
 116                 CallGraphNode *CalleeNode = CG[Callee];
 117                 // If the callee is outside our current SCC then we may
 118                 // throw because it might.
 119                 if (!SCCNodes.count(CalleeNode)) {
 120                   SCCMightUnwind = true;
 121                   break;
 122                 }
 123               } else {
 124                 // Indirect call, it might throw.
 125                 SCCMightUnwind = true;
 126                 break;
 127               }
 128             }
 129         if (SCCMightUnwind && SCCMightReturn) break;
 130       }
 131     }
 132   }
 133
 134   // If the SCC doesn't unwind or doesn't throw, note this fact.
 135   if (!SCCMightUnwind || !SCCMightReturn)
 136     for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
 137       Attributes NewAttributes = Attribute::None;
 138
 139       if (!SCCMightUnwind)
 140         NewAttributes |= Attribute::NoUnwind;
 141       if (!SCCMightReturn)
 142         NewAttributes |= Attribute::NoReturn;
 143
 144       const AttrListPtr &PAL = SCC[i]->getFunction()->getAttributes();
 145       const AttrListPtr &NPAL = PAL.addAttr(~0, NewAttributes);
 146       if (PAL != NPAL) {
 147         MadeChange = true;
 148         SCC[i]->getFunction()->setAttributes(NPAL);
 149       }
 150     }
 151
 152   for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
 153     // Convert any invoke instructions to non-throwing functions in this node
 154     // into call instructions with a branch.  This makes the exception blocks
 155     // dead.
 156     if (Function *F = SCC[i]->getFunction())
 157       MadeChange |= SimplifyFunction(F);
 158   }
 159
 160   return MadeChange;
 161 }
 162
 163
 164 // SimplifyFunction - Given information about callees, simplify the specified
 165 // function if we have invokes to non-unwinding functions or code after calls to
 166 // no-return functions.
 167 bool PruneEH::SimplifyFunction(Function *F) {
 168   bool MadeChange = false;
 169   for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
 170     if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
 171       if (II->doesNotThrow()) {
 172         SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
 173         // Insert a call instruction before the invoke.
 174         CallInst *Call = CallInst::Create(II->getCalledValue(),
 175                                           Args.begin(), Args.end(), "", II);
 176         Call->takeName(II);
 177         Call->setCallingConv(II->getCallingConv());
 178         Call->setAttributes(II->getAttributes());
 179
 180         // Anything that used the value produced by the invoke instruction
 181         // now uses the value produced by the call instruction.  Note that we
 182         // do this even for void functions and calls with no uses so that the
 183         // callgraph edge is updated.
 184         II->replaceAllUsesWith(Call);
 185         BasicBlock *UnwindBlock = II->getUnwindDest();
 186         UnwindBlock->removePredecessor(II->getParent());
 187
 188         // Insert a branch to the normal destination right before the
 189         // invoke.
 190         BranchInst::Create(II->getNormalDest(), II);
 191
 192         // Finally, delete the invoke instruction!
 193         BB->getInstList().pop_back();
 194
 195         // If the unwind block is now dead, nuke it.
 196         if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
 197           DeleteBasicBlock(UnwindBlock);  // Delete the new BB.
 198
 199         ++NumRemoved;
 200         MadeChange = true;
 201       }
 202
 203     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
 204       if (CallInst *CI = dyn_cast<CallInst>(I++))
 205         if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
 206           // This call calls a function that cannot return.  Insert an
 207           // unreachable instruction after it and simplify the code.  Do this
 208           // by splitting the BB, adding the unreachable, then deleting the
 209           // new BB.
 210           BasicBlock *New = BB->splitBasicBlock(I);
 211
 212           // Remove the uncond branch and add an unreachable.
 213           BB->getInstList().pop_back();
 214           new UnreachableInst(BB->getContext(), BB);
 215
 216           DeleteBasicBlock(New);  // Delete the new BB.
 217           MadeChange = true;
 218           ++NumUnreach;
 219           break;
 220         }
 221   }
 222
 223   return MadeChange;
 224 }
 225
 226 /// DeleteBasicBlock - remove the specified basic block from the program,
 227 /// updating the callgraph to reflect any now-obsolete edges due to calls that
 228 /// exist in the BB.
 229 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
 230   assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
 231   CallGraph &CG = getAnalysis<CallGraph>();
 232
 233   CallGraphNode *CGN = CG[BB->getParent()];
 234   for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
 235     --I;
 236     if (CallInst *CI = dyn_cast<CallInst>(I)) {
 237       if (!isa<DbgInfoIntrinsic>(I))
 238         CGN->removeCallEdgeFor(CI);
 239     } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
 240       CGN->removeCallEdgeFor(II);
 241     if (!I->use_empty())
 242       I->replaceAllUsesWith(UndefValue::get(I->getType()));
 243   }
 244
 245   // Get the list of successors of this block.
 246   std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
 247
 248   for (unsigned i = 0, e = Succs.size(); i != e; ++i)
 249     Succs[i]->removePredecessor(BB);
 250
 251   BB->eraseFromParent();
 252 }