LLVM 初探(1) 简单Pass编写

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环境配置

llvm版本4.0.0，使用官网的prebuilt binary安装

跟着llvm-cookbook的教程做了下，发现教程那种方法需要重新编译llvm源码

于是上网搜了下，找到一个repo llvm-pass-skeleton，可以在不重新编译全部源码的情况下生成Pass

FuncBlockCount Pass

Pass源码如下

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#include "llvm/Pass.h"
#include "llvm/IR/Function.h" 
#include "llvm/Support/raw_ostream.h" 
using namespace llvm; 

namespace { 
    struct FuncBlockCount : public FunctionPass {  
        static char ID;  
        FuncBlockCount() : FunctionPass(ID) {}  
        bool runOnFunction(Function &F) override {    
            errs() << "Function " << F.getName() << '\n';    
            return false;  
            }           
        };        
}       
char FuncBlockCount::ID = 0;       
static RegisterPass<FuncBlockCount> X("funcblockcount", "Function Block Count", false, false);

编译出libFuncBlockCountPass. so

创建一个文件 sample.c

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int foo(int n,int m) {
    int sum = 0;
    int c0;
    for(c0 = n; c0 >0 ;c0--){
        int c1 = m;
        for(; c1>0 ; c1--) {
            sum += c0 > c1 ?1:0;
        }
    }
    return sum;
}

使用

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clang -O0 -S -emit-llvm sample.c -o sample.ll

生成IR

然后用opt运行Pass，结果如下，输出函数名字

FuncBlockCount Pass v2

源码如下

这里提一下getAnalysisUsage这个函数，在llvm cookbook里面并没有提到，但是如果没有加这个函数的话，后面用opt跑的时候会报错

至于这个函数的作用，可以看【LLVM】Pass之间的依赖

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#include "llvm/Pass.h"
#include "llvm/IR/Function.h" 
#include "llvm/Support/raw_ostream.h" 
#include "llvm/Analysis/LoopInfo.h"
using namespace llvm; 

namespace { 
    struct FuncBlockCount : public FunctionPass {  
        static char ID;  
        void countBlocksInLoop(Loop* L,unsigned nest){
            unsigned num_Blocks = 0;
            Loop::block_iterator bb;
            for(bb = L->block_begin(); bb!=L->block_end();++bb)
                num_Blocks++;
            errs()<<"Loop level "<<nest<<" has "<<num_Blocks<<" blocks\n";
            std::vector<Loop*> subLoops = L->getSubLoops();
            Loop::iterator j,f;
            for( j = subLoops.begin(), f = subLoops.end(); j != f; ++j)
                countBlocksInLoop(*j, nest+1);
        }
        FuncBlockCount() : FunctionPass(ID) {}  
        bool runOnFunction(Function &F) override {    
            LoopInfo* LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
            errs() << "Function " << F.getName() << '\n';    
            for(Loop *L : *LI)
                countBlocksInLoop(L, 0);
            return false;  
        }
        virtual void getAnalysisUsage(AnalysisUsage& AU) const override {
            AU.addRequired<LoopInfoWrapperPass>();
        }           
};
     
}       
char FuncBlockCount::ID = 0;       
static RegisterPass<FuncBlockCount> X("funcblockcount", "Function Block Count", false, false);

修改sample.c源码为

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int main(int argc, char **argv) {  
    int i, j, k, t = 0;  
    for(i = 0; i < 10; i++) {    
        for(j = 0; j < 10; j++) {      
            for(k = 0; k < 10; k++) {        
                t++;      
            }   
        }    
        for(j = 0; j < 10; j++) {      
            t++;    
        }
    }  
    for(i = 0; i < 20; i++) {    
        for(j = 0; j < 20; j++) {      
            t++;    
        }    
        for(j = 0; j < 20; j++) {      
            t++;    
        }  
    }  
    return t; 
}

使用

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clang –O0 –S –emit-llvm sample.c –o sample.ll

来生成IR

输出的结果

opcodeCounter

源码如下

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#define DEBUG_TYPE "opcodeCounter"
#include "llvm/Pass.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/raw_ostream.h"
#include <map>
using namespace llvm;

namespace {

struct CountOpcode: public FunctionPass{
    std::map<std::string, int> opcodeCounter;
    static char ID;
    CountOpcode() : FunctionPass(ID) {}
    virtual bool runOnFunction (Function& F){
        llvm::outs() << "Function " <<F.getName() << "\n";
        for( Function::iterator bb = F.begin(), e = F.end();bb!=e;++bb){
            for(BasicBlock::iterator i = bb->begin(),ee = bb->end();i!=ee;++i){
                if(opcodeCounter.find(i->getOpcodeName()) == opcodeCounter.end()){
                    opcodeCounter[i->getOpcodeName()] = 1;
                }else{
                    opcodeCounter[i->getOpcodeName()] += 1;
                }
            }
        }
        std::map<std::string, int>::iterator i = opcodeCounter.begin();
        std::map<std::string, int>::iterator e = opcodeCounter.end();
        while(i!=e){
            llvm::outs()<<i->first <<": "<<i->second<<"\n";
            i++;
        }
        llvm::outs()<<"\n";
        opcodeCounter.clear();
        return false;
    }   
};

}

char CountOpcode::ID = 0;
static RegisterPass<CountOpcode> X("opcodeCounter","Count the number of opcode in a functions");

这个Pass是统计各个opcode的数量

测试代码 testcode.c

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int func(int a,int b){
    int sum = 0;
    int iter;
    for(iter = 0; iter<a;iter++) {
        int iter1;
        for(iter1 = 0; iter1<b;iter1++){
            sum += iter> iter1 ? 1: 0;
        }
    }
    return sum;
}

运行结果