UnreachableBlockElim.cpp revision fc3c82a804c9b5eda24d96c21f7e6ff66bc2c529 
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/Function.h"
25#include "llvm/Pass.h"
26#include "llvm/Support/CFG.h"
27#include "Support/DepthFirstIterator.h"
28using namespace llvm;
29
30namespace {
31  class UnreachableBlockElim : public FunctionPass {
32    virtual bool runOnFunction(Function &F);
33  };
34  RegisterOpt<UnreachableBlockElim>
35  X("unreachableblockelim", "Remove unreachable blocks from the CFG");
36}
37
38FunctionPass *llvm::createUnreachableBlockEliminationPass() {
39  return new UnreachableBlockElim();
40}
41
42bool UnreachableBlockElim::runOnFunction(Function &F) {
43  std::set<BasicBlock*> Reachable;
44
45  // Mark all reachable blocks.
46  for (df_ext_iterator<Function*> I = df_ext_begin(&F, Reachable),
47         E = df_ext_end(&F, Reachable); I != E; ++I)
48    /* Mark all reachable blocks */;
49
50  // Loop over all dead blocks, remembering them and deleting all instructions
51  // in them.
52  std::vector<BasicBlock*> DeadBlocks;
53  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
54    if (!Reachable.count(I)) {
55      DeadBlocks.push_back(I);
56      for (succ_iterator SI = succ_begin(&*I), E = succ_end(&*I); SI != E; ++SI)
57        (*SI)->removePredecessor(I);
58      I->dropAllReferences();
59    }
60
61  if (DeadBlocks.empty()) return false;
62
63  // Actually remove the blocks now.
64  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
65    F.getBasicBlockList().erase(DeadBlocks[i]);
66
67  return true;
68}
69