LLVM 源码分析（五）BasicBlockPass

BasicBlockPass 主要增加
doInitaliztion(Function &) ;doFinalization(Function &);runOnBasicBlock();
注意参数为Funtion & 和FunctionPass 的Module 引用有区别

其实都和前面相似 BasicBlockPass 比FunctionPass粒度更小
使用方式如下

namespace{
struct Hello1 : public BasicBlockPass {
static char ID; // Pass identification, replacement for typeid
Hello1() : BasicBlockPass(ID) {}

virtual bool runOnBasicBlock(BasicBlock &BB) {
errs() << "Hello1: ";
errs().write_escaped(BB.getName()) << '\n';
return false;
}
bool doInitialization(Function &F){
// By default, don't do anything.
errs()<<"BB doInitialization  test";
return false;
}
};
}

char Hello1::ID = 0;
static RegisterPass<Hello1> Z("hello1", "Hello World Pass");

下面是一段著名的dead code 删除代码

//===- DCE.cpp - Code to perform dead code elimination --------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements dead inst elimination and dead code elimination.
//
// Dead Inst Elimination performs a single pass over the function removing
// instructions that are obviously dead.  Dead Code Elimination is similar, but
// it rechecks instructions that were used by removed instructions to see if
// they are newly dead.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "dce"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Instruction.h"
#include "llvm/Pass.h"
#include "llvm/Support/InstIterator.h"
#include "llvm/Target/TargetLibraryInfo.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;

STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
STATISTIC(DCEEliminated, "Number of insts removed");

namespace {
//===--------------------------------------------------------------------===//
// DeadInstElimination pass implementation
//
struct DeadInstElimination : public BasicBlockPass {
static char ID; // Pass identification, replacement for typeid
DeadInstElimination() : BasicBlockPass(ID) {
initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
}
virtual bool runOnBasicBlock(BasicBlock &BB) {
TargetLibraryInfo *TLI = getAnalysisIfAvailable<TargetLibraryInfo>();
bool Changed = false;
for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
Instruction *Inst = DI++;
if (isInstructionTriviallyDead(Inst, TLI)) {
Inst->eraseFromParent();
Changed = true;
++DIEEliminated;
}
}
return Changed;
}

virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
}
};
}

char DeadInstElimination::ID = 0;
INITIALIZE_PASS(DeadInstElimination, "die",
"Dead Instruction Elimination", false, false)

Pass *llvm::createDeadInstEliminationPass() {
return new DeadInstElimination();
}