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/Visibility.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   };
  38   RegisterOpt<UnreachableBlockElim>
  39   X("unreachableblockelim", "Remove unreachable blocks from the CFG");
  40 }
  41
  42 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
  43   return new UnreachableBlockElim();
  44 }
  45
  46 bool UnreachableBlockElim::runOnFunction(Function &F) {
  47   std::set<BasicBlock*> Reachable;
  48
  49   // Mark all reachable blocks.
  50   for (df_ext_iterator<Function*> I = df_ext_begin(&F, Reachable),
  51          E = df_ext_end(&F, Reachable); I != E; ++I)
  52     /* Mark all reachable blocks */;
  53
  54   // Loop over all dead blocks, remembering them and deleting all instructions
  55   // in them.
  56   std::vector<BasicBlock*> DeadBlocks;
  57   for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
  58     if (!Reachable.count(I)) {
  59       BasicBlock *BB = I;
  60       DeadBlocks.push_back(BB);
  61       while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
  62         PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
  63         BB->getInstList().pop_front();
  64       }
  65       for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
  66         (*SI)->removePredecessor(BB);
  67       BB->dropAllReferences();
  68     }
  69
  70   if (DeadBlocks.empty()) return false;
  71
  72   // Actually remove the blocks now.
  73   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
  74     F.getBasicBlockList().erase(DeadBlocks[i]);
  75
  76   return true;
  77 }