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();
}