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