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      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 <algorithm>
     31 using namespace llvm;
     32 
     33 STATISTIC(NumRemoved, "Number of invokes removed");
     34 STATISTIC(NumUnreach, "Number of noreturn calls optimized");
     35 
     36 namespace {
     37   struct PruneEH : public CallGraphSCCPass {
     38     static char ID; // Pass identification, replacement for typeid
     39     PruneEH() : CallGraphSCCPass(ID) {
     40       initializePruneEHPass(*PassRegistry::getPassRegistry());
     41     }
     42 
     43     // runOnSCC - Analyze the SCC, performing the transformation if possible.
     44     bool runOnSCC(CallGraphSCC &SCC);
     45 
     46     bool SimplifyFunction(Function *F);
     47     void DeleteBasicBlock(BasicBlock *BB);
     48   };
     49 }
     50 
     51 char PruneEH::ID = 0;
     52 INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
     53                 "Remove unused exception handling info", false, false)
     54 INITIALIZE_AG_DEPENDENCY(CallGraph)
     55 INITIALIZE_PASS_END(PruneEH, "prune-eh",
     56                 "Remove unused exception handling info", false, false)
     57 
     58 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
     59 
     60 
     61 bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
     62   SmallPtrSet<CallGraphNode *, 8> SCCNodes;
     63   CallGraph &CG = getAnalysis<CallGraph>();
     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 (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
     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 (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
     74     if (Function *F = (*I)->getFunction())
     75       MadeChange |= SimplifyFunction(F);
     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 == 0) {
     89       SCCMightUnwind = true;
     90       SCCMightReturn = true;
     91     } else if (F->isDeclaration() || F->mayBeOverridden()) {
     92       SCCMightUnwind |= !F->doesNotThrow();
     93       SCCMightReturn |= !F->doesNotReturn();
     94     } else {
     95       bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
     96       bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
     97 
     98       if (!CheckUnwind && !CheckReturn)
     99         continue;
    100 
    101       // Check to see if this function performs an unwind or calls an
    102       // unwinding function.
    103       for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
    104         if (CheckUnwind && isa<UnwindInst>(BB->getTerminator())) {
    105           // Uses unwind!
    106           SCCMightUnwind = true;
    107         } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
    108           SCCMightReturn = true;
    109         }
    110 
    111         // Invoke instructions don't allow unwinding to continue, so we are
    112         // only interested in call instructions.
    113         if (CheckUnwind && !SCCMightUnwind)
    114           for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
    115             if (CallInst *CI = dyn_cast<CallInst>(I)) {
    116               if (CI->doesNotThrow()) {
    117                 // This call cannot throw.
    118               } else if (Function *Callee = CI->getCalledFunction()) {
    119                 CallGraphNode *CalleeNode = CG[Callee];
    120                 // If the callee is outside our current SCC then we may
    121                 // throw because it might.
    122                 if (!SCCNodes.count(CalleeNode)) {
    123                   SCCMightUnwind = true;
    124                   break;
    125                 }
    126               } else {
    127                 // Indirect call, it might throw.
    128                 SCCMightUnwind = true;
    129                 break;
    130               }
    131             }
    132         if (SCCMightUnwind && SCCMightReturn) break;
    133       }
    134     }
    135   }
    136 
    137   // If the SCC doesn't unwind or doesn't throw, note this fact.
    138   if (!SCCMightUnwind || !SCCMightReturn)
    139     for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
    140       Attributes NewAttributes = Attribute::None;
    141 
    142       if (!SCCMightUnwind)
    143         NewAttributes |= Attribute::NoUnwind;
    144       if (!SCCMightReturn)
    145         NewAttributes |= Attribute::NoReturn;
    146 
    147       Function *F = (*I)->getFunction();
    148       const AttrListPtr &PAL = F->getAttributes();
    149       const AttrListPtr &NPAL = PAL.addAttr(~0, NewAttributes);
    150       if (PAL != NPAL) {
    151         MadeChange = true;
    152         F->setAttributes(NPAL);
    153       }
    154     }
    155 
    156   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
    157     // Convert any invoke instructions to non-throwing functions in this node
    158     // into call instructions with a branch.  This makes the exception blocks
    159     // dead.
    160     if (Function *F = (*I)->getFunction())
    161       MadeChange |= SimplifyFunction(F);
    162   }
    163 
    164   return MadeChange;
    165 }
    166 
    167 
    168 // SimplifyFunction - Given information about callees, simplify the specified
    169 // function if we have invokes to non-unwinding functions or code after calls to
    170 // no-return functions.
    171 bool PruneEH::SimplifyFunction(Function *F) {
    172   bool MadeChange = false;
    173   for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
    174     if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
    175       if (II->doesNotThrow()) {
    176         SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
    177         // Insert a call instruction before the invoke.
    178         CallInst *Call = CallInst::Create(II->getCalledValue(), Args, "", II);
    179         Call->takeName(II);
    180         Call->setCallingConv(II->getCallingConv());
    181         Call->setAttributes(II->getAttributes());
    182         Call->setDebugLoc(II->getDebugLoc());
    183 
    184         // Anything that used the value produced by the invoke instruction
    185         // now uses the value produced by the call instruction.  Note that we
    186         // do this even for void functions and calls with no uses so that the
    187         // callgraph edge is updated.
    188         II->replaceAllUsesWith(Call);
    189         BasicBlock *UnwindBlock = II->getUnwindDest();
    190         UnwindBlock->removePredecessor(II->getParent());
    191 
    192         // Insert a branch to the normal destination right before the
    193         // invoke.
    194         BranchInst::Create(II->getNormalDest(), II);
    195 
    196         // Finally, delete the invoke instruction!
    197         BB->getInstList().pop_back();
    198 
    199         // If the unwind block is now dead, nuke it.
    200         if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
    201           DeleteBasicBlock(UnwindBlock);  // Delete the new BB.
    202 
    203         ++NumRemoved;
    204         MadeChange = true;
    205       }
    206 
    207     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
    208       if (CallInst *CI = dyn_cast<CallInst>(I++))
    209         if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
    210           // This call calls a function that cannot return.  Insert an
    211           // unreachable instruction after it and simplify the code.  Do this
    212           // by splitting the BB, adding the unreachable, then deleting the
    213           // new BB.
    214           BasicBlock *New = BB->splitBasicBlock(I);
    215 
    216           // Remove the uncond branch and add an unreachable.
    217           BB->getInstList().pop_back();
    218           new UnreachableInst(BB->getContext(), BB);
    219 
    220           DeleteBasicBlock(New);  // Delete the new BB.
    221           MadeChange = true;
    222           ++NumUnreach;
    223           break;
    224         }
    225   }
    226 
    227   return MadeChange;
    228 }
    229 
    230 /// DeleteBasicBlock - remove the specified basic block from the program,
    231 /// updating the callgraph to reflect any now-obsolete edges due to calls that
    232 /// exist in the BB.
    233 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
    234   assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
    235   CallGraph &CG = getAnalysis<CallGraph>();
    236 
    237   CallGraphNode *CGN = CG[BB->getParent()];
    238   for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
    239     --I;
    240     if (CallInst *CI = dyn_cast<CallInst>(I)) {
    241       if (!isa<IntrinsicInst>(I))
    242         CGN->removeCallEdgeFor(CI);
    243     } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
    244       CGN->removeCallEdgeFor(II);
    245     if (!I->use_empty())
    246       I->replaceAllUsesWith(UndefValue::get(I->getType()));
    247   }
    248 
    249   // Get the list of successors of this block.
    250   std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
    251 
    252   for (unsigned i = 0, e = Succs.size(); i != e; ++i)
    253     Succs[i]->removePredecessor(BB);
    254 
    255   BB->eraseFromParent();
    256 }
    257