UnreachableBlockElim.cpp revision 5b3a4553c1da7e417a240379e2f510c77532c5c1 
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/ADT/DepthFirstIterator.h"
31using namespace llvm;
32
33namespace {
34  class UnreachableBlockElim : public FunctionPass {
35    virtual bool runOnFunction(Function &F);
36  };
37  RegisterOpt<UnreachableBlockElim>
38  X("unreachableblockelim", "Remove unreachable blocks from the CFG");
39}
40
41FunctionPass *llvm::createUnreachableBlockEliminationPass() {
42  return new UnreachableBlockElim();
43}
44
45bool UnreachableBlockElim::runOnFunction(Function &F) {
46  std::set<BasicBlock*> Reachable;
47
48  // Mark all reachable blocks.
49  for (df_ext_iterator<Function*> I = df_ext_begin(&F, Reachable),
50         E = df_ext_end(&F, Reachable); I != E; ++I)
51    /* Mark all reachable blocks */;
52
53  // Loop over all dead blocks, remembering them and deleting all instructions
54  // in them.
55  std::vector<BasicBlock*> DeadBlocks;
56  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
57    if (!Reachable.count(I)) {
58      BasicBlock *BB = I;
59      DeadBlocks.push_back(BB);
60      while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
61        PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
62        BB->getInstList().pop_front();
63      }
64      for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
65        (*SI)->removePredecessor(BB);
66      BB->dropAllReferences();
67    }
68
69  if (DeadBlocks.empty()) return false;
70
71  // Actually remove the blocks now.
72  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
73    F.getBasicBlockList().erase(DeadBlocks[i]);
74
75  return true;
76}
77