src/llvm/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 <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<ResumeInst>(BB->getTerminator())) {
 105           // Uses unwind / resume!
 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 }