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//===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass is an extremely simple version of the SimplifyCFG pass. Its sole
// job is to delete LLVM basic blocks that are not reachable from the entry
// node. To do this, it performs a simple depth first traversal of the CFG,
// then deletes any unvisited nodes.
//
// Note that this pass is really a hack. In particular, the instruction
// selectors for various targets should just not generate code for unreachable
// blocks. Until LLVM has a more systematic way of defining instruction
// selectors, however, we cannot really expect them to handle additional
// complexity.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/Passes.h"
#include "llvm/Constant.h"
#include "llvm/Instructions.h"
#include "llvm/Function.h"
#include "llvm/Pass.h"
#include "llvm/Support/CFG.h"
#include "Support/DepthFirstIterator.h"
using namespace llvm;
namespace {
class UnreachableBlockElim : public FunctionPass {
virtual bool runOnFunction(Function &F);
};
RegisterOpt<UnreachableBlockElim>
X("unreachableblockelim", "Remove unreachable blocks from the CFG");
}
FunctionPass *llvm::createUnreachableBlockEliminationPass() {
return new UnreachableBlockElim();
}
bool UnreachableBlockElim::runOnFunction(Function &F) {
std::set<BasicBlock*> Reachable;
// Mark all reachable blocks.
for (df_ext_iterator<Function*> I = df_ext_begin(&F, Reachable),
E = df_ext_end(&F, Reachable); I != E; ++I)
/* Mark all reachable blocks */;
// Loop over all dead blocks, remembering them and deleting all instructions
// in them.
std::vector<BasicBlock*> DeadBlocks;
for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
if (!Reachable.count(I)) {
BasicBlock *BB = I;
DeadBlocks.push_back(BB);
while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
BB->getInstList().pop_front();
}
for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
(*SI)->removePredecessor(BB);
BB->dropAllReferences();
}
if (DeadBlocks.empty()) return false;
// Actually remove the blocks now.
for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
F.getBasicBlockList().erase(DeadBlocks[i]);
return true;
}
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