lib/CodeGen/UnreachableBlockElim.cpp

   1 //===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file was developed by the LLVM research group and is distributed under
   6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This pass is an extremely simple version of the SimplifyCFG pass.  Its sole
  11 // job is to delete LLVM basic blocks that are not reachable from the entry
  12 // node.  To do this, it performs a simple depth first traversal of the CFG,
  13 // then deletes any unvisited nodes.
  14 //
  15 // Note that this pass is really a hack.  In particular, the instruction
  16 // selectors for various targets should just not generate code for unreachable
  17 // blocks.  Until LLVM has a more systematic way of defining instruction
  18 // selectors, however, we cannot really expect them to handle additional
  19 // complexity.
  20 //
  21 //===----------------------------------------------------------------------===//
  22
  23 #include "llvm/CodeGen/Passes.h"
  24 #include "llvm/Constant.h"
  25 #include "llvm/Instructions.h"
  26 #include "llvm/Function.h"
  27 #include "llvm/Pass.h"
  28 #include "llvm/Type.h"
  29 #include "llvm/Support/CFG.h"
  30 #include "llvm/Support/Compiler.h"
  31 #include "llvm/ADT/DepthFirstIterator.h"
  32 using namespace llvm;
  33
  34 namespace {
  35   class VISIBILITY_HIDDEN UnreachableBlockElim : public FunctionPass {
  36     virtual bool runOnFunction(Function &F);
  37   public:
  38     static char ID; // Pass identification, replacement for typeid
  39     UnreachableBlockElim() : FunctionPass((intptr_t)&ID) {}
  40   };
  41   char UnreachableBlockElim::ID = 0;
  42   RegisterPass<UnreachableBlockElim>
  43   X("unreachableblockelim", "Remove unreachable blocks from the CFG");
  44 }
  45
  46 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
  47   return new UnreachableBlockElim();
  48 }
  49
  50 bool UnreachableBlockElim::runOnFunction(Function &F) {
  51   std::set<BasicBlock*> Reachable;
  52
  53   // Mark all reachable blocks.
  54   for (df_ext_iterator<Function*> I = df_ext_begin(&F, Reachable),
  55          E = df_ext_end(&F, Reachable); I != E; ++I)
  56     /* Mark all reachable blocks */;
  57
  58   // Loop over all dead blocks, remembering them and deleting all instructions
  59   // in them.
  60   std::vector<BasicBlock*> DeadBlocks;
  61   for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
  62     if (!Reachable.count(I)) {
  63       BasicBlock *BB = I;
  64       DeadBlocks.push_back(BB);
  65       while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
  66         PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
  67         BB->getInstList().pop_front();
  68       }
  69       for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
  70         (*SI)->removePredecessor(BB);
  71       BB->dropAllReferences();
  72     }
  73
  74   if (DeadBlocks.empty()) return false;
  75
  76   // Actually remove the blocks now.
  77   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
  78     F.getBasicBlockList().erase(DeadBlocks[i]);
  79
  80   return true;
  81 }