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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 #include "llvm/Transforms/IPO.h"
     18 #include "llvm/ADT/SmallPtrSet.h"
     19 #include "llvm/ADT/SmallVector.h"
     20 #include "llvm/ADT/Statistic.h"
     21 #include "llvm/Analysis/CallGraph.h"
     22 #include "llvm/Analysis/CallGraphSCCPass.h"
     23 #include "llvm/IR/CFG.h"
     24 #include "llvm/IR/Constants.h"
     25 #include "llvm/IR/Function.h"
     26 #include "llvm/IR/Instructions.h"
     27 #include "llvm/IR/IntrinsicInst.h"
     28 #include "llvm/IR/LLVMContext.h"
     29 #include <algorithm>
     30 using namespace llvm;
     31 
     32 #define DEBUG_TYPE "prune-eh"
     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       initializePruneEHPass(*PassRegistry::getPassRegistry());
     42     }
     43 
     44     // runOnSCC - Analyze the SCC, performing the transformation if possible.
     45     bool runOnSCC(CallGraphSCC &SCC) override;
     46 
     47     bool SimplifyFunction(Function *F);
     48     void DeleteBasicBlock(BasicBlock *BB);
     49   };
     50 }
     51 
     52 char PruneEH::ID = 0;
     53 INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
     54                 "Remove unused exception handling info", false, false)
     55 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
     56 INITIALIZE_PASS_END(PruneEH, "prune-eh",
     57                 "Remove unused exception handling info", false, false)
     58 
     59 Pass *llvm::createPruneEHPass() { return new PruneEH(); }
     60 
     61 
     62 bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
     63   SmallPtrSet<CallGraphNode *, 8> SCCNodes;
     64   CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
     65   bool MadeChange = false;
     66 
     67   // Fill SCCNodes with the elements of the SCC.  Used for quickly
     68   // looking up whether a given CallGraphNode is in this SCC.
     69   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
     70     SCCNodes.insert(*I);
     71 
     72   // First pass, scan all of the functions in the SCC, simplifying them
     73   // according to what we know.
     74   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
     75     if (Function *F = (*I)->getFunction())
     76       MadeChange |= SimplifyFunction(F);
     77 
     78   // Next, check to see if any callees might throw or if there are any external
     79   // functions in this SCC: if so, we cannot prune any functions in this SCC.
     80   // Definitions that are weak and not declared non-throwing might be
     81   // overridden at linktime with something that throws, so assume that.
     82   // If this SCC includes the unwind instruction, we KNOW it throws, so
     83   // obviously the SCC might throw.
     84   //
     85   bool SCCMightUnwind = false, SCCMightReturn = false;
     86   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end();
     87        (!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) {
     88     Function *F = (*I)->getFunction();
     89     if (!F) {
     90       SCCMightUnwind = true;
     91       SCCMightReturn = true;
     92     } else if (F->isDeclaration() || F->mayBeOverridden()) {
     93       SCCMightUnwind |= !F->doesNotThrow();
     94       SCCMightReturn |= !F->doesNotReturn();
     95     } else {
     96       bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
     97       bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
     98 
     99       if (!CheckUnwind && !CheckReturn)
    100         continue;
    101 
    102       // Check to see if this function performs an unwind or calls an
    103       // unwinding function.
    104       for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
    105         if (CheckUnwind && isa<ResumeInst>(BB->getTerminator())) {
    106           // Uses unwind / resume!
    107           SCCMightUnwind = true;
    108         } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
    109           SCCMightReturn = true;
    110         }
    111 
    112         // Invoke instructions don't allow unwinding to continue, so we are
    113         // only interested in call instructions.
    114         if (CheckUnwind && !SCCMightUnwind)
    115           for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
    116             if (CallInst *CI = dyn_cast<CallInst>(I)) {
    117               if (CI->doesNotThrow()) {
    118                 // This call cannot throw.
    119               } else if (Function *Callee = CI->getCalledFunction()) {
    120                 CallGraphNode *CalleeNode = CG[Callee];
    121                 // If the callee is outside our current SCC then we may
    122                 // throw because it might.
    123                 if (!SCCNodes.count(CalleeNode)) {
    124                   SCCMightUnwind = true;
    125                   break;
    126                 }
    127               } else {
    128                 // Indirect call, it might throw.
    129                 SCCMightUnwind = true;
    130                 break;
    131               }
    132             }
    133         if (SCCMightUnwind && SCCMightReturn) break;
    134       }
    135     }
    136   }
    137 
    138   // If the SCC doesn't unwind or doesn't throw, note this fact.
    139   if (!SCCMightUnwind || !SCCMightReturn)
    140     for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
    141       AttrBuilder NewAttributes;
    142 
    143       if (!SCCMightUnwind)
    144         NewAttributes.addAttribute(Attribute::NoUnwind);
    145       if (!SCCMightReturn)
    146         NewAttributes.addAttribute(Attribute::NoReturn);
    147 
    148       Function *F = (*I)->getFunction();
    149       const AttributeSet &PAL = F->getAttributes().getFnAttributes();
    150       const AttributeSet &NPAL = AttributeSet::get(
    151           F->getContext(), AttributeSet::FunctionIndex, NewAttributes);
    152 
    153       if (PAL != NPAL) {
    154         MadeChange = true;
    155         F->addAttributes(AttributeSet::FunctionIndex, NPAL);
    156       }
    157     }
    158 
    159   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
    160     // Convert any invoke instructions to non-throwing functions in this node
    161     // into call instructions with a branch.  This makes the exception blocks
    162     // dead.
    163     if (Function *F = (*I)->getFunction())
    164       MadeChange |= SimplifyFunction(F);
    165   }
    166 
    167   return MadeChange;
    168 }
    169 
    170 
    171 // SimplifyFunction - Given information about callees, simplify the specified
    172 // function if we have invokes to non-unwinding functions or code after calls to
    173 // no-return functions.
    174 bool PruneEH::SimplifyFunction(Function *F) {
    175   bool MadeChange = false;
    176   for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
    177     if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
    178       if (II->doesNotThrow()) {
    179         SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
    180         // Insert a call instruction before the invoke.
    181         CallInst *Call = CallInst::Create(II->getCalledValue(), Args, "", II);
    182         Call->takeName(II);
    183         Call->setCallingConv(II->getCallingConv());
    184         Call->setAttributes(II->getAttributes());
    185         Call->setDebugLoc(II->getDebugLoc());
    186 
    187         // Anything that used the value produced by the invoke instruction
    188         // now uses the value produced by the call instruction.  Note that we
    189         // do this even for void functions and calls with no uses so that the
    190         // callgraph edge is updated.
    191         II->replaceAllUsesWith(Call);
    192         BasicBlock *UnwindBlock = II->getUnwindDest();
    193         UnwindBlock->removePredecessor(II->getParent());
    194 
    195         // Insert a branch to the normal destination right before the
    196         // invoke.
    197         BranchInst::Create(II->getNormalDest(), II);
    198 
    199         // Finally, delete the invoke instruction!
    200         BB->getInstList().pop_back();
    201 
    202         // If the unwind block is now dead, nuke it.
    203         if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
    204           DeleteBasicBlock(UnwindBlock);  // Delete the new BB.
    205 
    206         ++NumRemoved;
    207         MadeChange = true;
    208       }
    209 
    210     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
    211       if (CallInst *CI = dyn_cast<CallInst>(I++))
    212         if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
    213           // This call calls a function that cannot return.  Insert an
    214           // unreachable instruction after it and simplify the code.  Do this
    215           // by splitting the BB, adding the unreachable, then deleting the
    216           // new BB.
    217           BasicBlock *New = BB->splitBasicBlock(I);
    218 
    219           // Remove the uncond branch and add an unreachable.
    220           BB->getInstList().pop_back();
    221           new UnreachableInst(BB->getContext(), BB);
    222 
    223           DeleteBasicBlock(New);  // Delete the new BB.
    224           MadeChange = true;
    225           ++NumUnreach;
    226           break;
    227         }
    228   }
    229 
    230   return MadeChange;
    231 }
    232 
    233 /// DeleteBasicBlock - remove the specified basic block from the program,
    234 /// updating the callgraph to reflect any now-obsolete edges due to calls that
    235 /// exist in the BB.
    236 void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
    237   assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
    238   CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
    239 
    240   CallGraphNode *CGN = CG[BB->getParent()];
    241   for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
    242     --I;
    243     if (CallInst *CI = dyn_cast<CallInst>(I)) {
    244       if (!isa<IntrinsicInst>(I))
    245         CGN->removeCallEdgeFor(CI);
    246     } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
    247       CGN->removeCallEdgeFor(II);
    248     if (!I->use_empty())
    249       I->replaceAllUsesWith(UndefValue::get(I->getType()));
    250   }
    251 
    252   // Get the list of successors of this block.
    253   std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
    254 
    255   for (unsigned i = 0, e = Succs.size(); i != e; ++i)
    256     Succs[i]->removePredecessor(BB);
    257 
    258   BB->eraseFromParent();
    259 }
    260