UnreachableBlockElim.cpp revision 089efffd7d1ca0d10522ace38d36e0a67f4fac2d 
1//===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// 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"
32using namespace llvm;
33
34namespace {
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}
42char UnreachableBlockElim::ID = 0;
43static RegisterPass<UnreachableBlockElim>
44X("unreachableblockelim", "Remove unreachable blocks from the CFG");
45
46FunctionPass *llvm::createUnreachableBlockEliminationPass() {
47  return new UnreachableBlockElim();
48}
49
50bool 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}
82