lib/Transforms/IPO/PruneEH.cpp

//===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a simple interprocedural pass which walks the
// call-graph, turning invoke instructions into calls, iff the callee cannot
// throw an exception.  It implements this as a bottom-up traversal of the
// call-graph.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/IPO.h"
#include "llvm/CallGraphSCCPass.h"
#include "llvm/Constants.h"
#include "llvm/Function.h"
#include "llvm/Intrinsics.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/CFG.h"
#include <set>
#include <algorithm>
using namespace llvm;

namespace {
  Statistic<> NumRemoved("prune-eh", "Number of invokes removed");
  Statistic<> NumUnreach("prune-eh", "Number of noreturn calls optimized");

  struct PruneEH : public CallGraphSCCPass {
    /// DoesNotUnwind - This set contains all of the functions which we have
    /// determined cannot unwind.
    std::set<CallGraphNode*> DoesNotUnwind;

    /// DoesNotReturn - This set contains all of the functions which we have
    /// determined cannot return normally (but might unwind).
    std::set<CallGraphNode*> DoesNotReturn;

    // runOnSCC - Analyze the SCC, performing the transformation if possible.
    bool runOnSCC(const std::vector<CallGraphNode *> &SCC);

    bool SimplifyFunction(Function *F);
    void DeleteBasicBlock(BasicBlock *BB);
  };
  RegisterOpt<PruneEH> X("prune-eh", "Remove unused exception handling info");
}

ModulePass *llvm::createPruneEHPass() { return new PruneEH(); }


bool PruneEH::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
  CallGraph &CG = getAnalysis<CallGraph>();
  bool MadeChange = false;

  // First pass, scan all of the functions in the SCC, simplifying them
  // according to what we know.
  for (unsigned i = 0, e = SCC.size(); i != e; ++i)
    if (Function *F = SCC[i]->getFunction())
      MadeChange |= SimplifyFunction(F);

  // Next, check to see if any callees might throw or if there are any external
  // functions in this SCC: if so, we cannot prune any functions in this SCC.
  // If this SCC includes the unwind instruction, we KNOW it throws, so
  // obviously the SCC might throw.
  //
  bool SCCMightUnwind = false, SCCMightReturn = false;
  for (unsigned i = 0, e = SCC.size();
       (!SCCMightUnwind || !SCCMightReturn) && i != e; ++i) {
    Function *F = SCC[i]->getFunction();
    if (F == 0 || (F->isExternal() && !F->getIntrinsicID())) {
      SCCMightUnwind = true;
      SCCMightReturn = true;
    } else {
      if (F->isExternal())
        SCCMightReturn = true;

      // Check to see if this function performs an unwind or calls an
      // unwinding function.
      for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
        if (isa<UnwindInst>(BB->getTerminator())) {  // Uses unwind!
          SCCMightUnwind = true;
        } else if (isa<ReturnInst>(BB->getTerminator())) {
          SCCMightReturn = true;
        }

        // Invoke instructions don't allow unwinding to continue, so we are
        // only interested in call instructions.
        if (!SCCMightUnwind)
          for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
            if (CallInst *CI = dyn_cast<CallInst>(I)) {
              if (Function *Callee = CI->getCalledFunction()) {
                CallGraphNode *CalleeNode = CG[Callee];
                // If the callee is outside our current SCC, or if it is not
                // known to throw, then we might throw also.
                if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end()&&
                    !DoesNotUnwind.count(CalleeNode)) {
                  SCCMightUnwind = true;
                  break;
                }
              } else {
                // Indirect call, it might throw.
                SCCMightUnwind = true;
                break;
              }
            }
        if (SCCMightUnwind && SCCMightReturn) break;
      }
    }
  }

  // If the SCC doesn't unwind or doesn't throw, note this fact.
  if (!SCCMightUnwind)
    for (unsigned i = 0, e = SCC.size(); i != e; ++i)
      DoesNotUnwind.insert(SCC[i]);
  if (!SCCMightReturn)
    for (unsigned i = 0, e = SCC.size(); i != e; ++i)
      DoesNotReturn.insert(SCC[i]);

  for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
    // Convert any invoke instructions to non-throwing functions in this node
    // into call instructions with a branch.  This makes the exception blocks
    // dead.
    if (Function *F = SCC[i]->getFunction())
      MadeChange |= SimplifyFunction(F);
  }

  return MadeChange;
}


// SimplifyFunction - Given information about callees, simplify the specified
// function if we have invokes to non-unwinding functions or code after calls to
// no-return functions.
bool PruneEH::SimplifyFunction(Function *F) {
  CallGraph &CG = getAnalysis<CallGraph>();
  bool MadeChange = false;
  for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
    if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
      if (Function *F = II->getCalledFunction())
        if (DoesNotUnwind.count(CG[F])) {
          // Insert a call instruction before the invoke...
          std::string Name = II->getName();  II->setName("");
          CallInst *Call = new CallInst(II->getCalledValue(),
                                        std::vector<Value*>(II->op_begin()+3,
                                                            II->op_end()),
                                        Name, II);
          Call->setCallingConv(II->getCallingConv());

          // Anything that used the value produced by the invoke instruction
          // now uses the value produced by the call instruction.
          II->replaceAllUsesWith(Call);
          BasicBlock *UnwindBlock = II->getUnwindDest();
          UnwindBlock->removePredecessor(II->getParent());

          // Insert a branch to the normal destination right before the
          // invoke.
          new BranchInst(II->getNormalDest(), II);

          // Finally, delete the invoke instruction!
          BB->getInstList().pop_back();

          // If the unwind block is now dead, nuke it.
          if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
            DeleteBasicBlock(UnwindBlock);  // Delete the new BB.

          ++NumRemoved;
          MadeChange = true;
        }

    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
      if (CallInst *CI = dyn_cast<CallInst>(I++))
        if (Function *Callee = CI->getCalledFunction())
          if (DoesNotReturn.count(CG[Callee]) && !isa<UnreachableInst>(I)) {
            // This call calls a function that cannot return.  Insert an
            // unreachable instruction after it and simplify the code.  Do this
            // by splitting the BB, adding the unreachable, then deleting the
            // new BB.
            BasicBlock *New = BB->splitBasicBlock(I);

            // Remove the uncond branch and add an unreachable.
            BB->getInstList().pop_back();
            new UnreachableInst(BB);

            DeleteBasicBlock(New);  // Delete the new BB.
            MadeChange = true;
            ++NumUnreach;
            break;
          }

  }
  return MadeChange;
}

/// DeleteBasicBlock - remove the specified basic block from the program,
/// updating the callgraph to reflect any now-obsolete edges due to calls that
/// exist in the BB.
void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
  assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
  CallGraph &CG = getAnalysis<CallGraph>();

  CallGraphNode *CGN = CG[BB->getParent()];
  for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
    --I;
    if (CallInst *CI = dyn_cast<CallInst>(I)) {
      if (Function *Callee = CI->getCalledFunction())
        CGN->removeCallEdgeTo(CG[Callee]);
    } else if (InvokeInst *II = dyn_cast<InvokeInst>(I)) {
      if (Function *Callee = II->getCalledFunction())
        CGN->removeCallEdgeTo(CG[Callee]);
    }
    if (!I->use_empty())
      I->replaceAllUsesWith(UndefValue::get(I->getType()));
  }

  // Get the list of successors of this block.
  std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));

  for (unsigned i = 0, e = Succs.size(); i != e; ++i)
    Succs[i]->removePredecessor(BB);

  BB->eraseFromParent();
}