c库的rand/random随机数产生函数性能差？

有网文称c标准库的rand/random随机数产生函数性能极差。一直信以为真，但从没做过验证。最近因其他因缘，写了些代码专门验证rand/random的性能。结果大出意料，颠覆之前的成见。

结论如下：

1) rand/random性极佳。在64位机器上，其性能大约比简单自增略低30%(32位的自增比64位性能高出1倍以上)！

2) srand/srandom性能极差极差。绝对不能每次调用rand之前都调用srand。这么做不仅没必要，还会极大降低性能，性能只有调用rand的1%！！！

3) rand文档中提到的实现示例也实际实现存在差别，尤其是srand实现！

4) rand的实现起始就是简单的乘法和取模，简单的随机数实现在性能上几乎无法超越系统自带的标准实现！

5) 网上的东西很多真是不靠谱！！！

下面测试代码，代码在64/32位机器都能运行。

编译命令：g++ -o3 -o test random.cpp

#include <stdio.h>
#include <stdint.h>
#include <vector>
#include <algorithm>

#include <stdlib.h>
#include <math.h>

#include <sys/time.h>


#define NUM_RAND_SEED    100


class Random
{
    public:
        static int srandom(size_t randSeedNum = NUM_RAND_SEED);

        static size_t random();       

    private:
        static bool    m_bInit;
        static size_t  m_count;
        static std::vector<size_t> m_randSeeds;
};

bool    Random::m_bInit = false;
size_t  Random::m_count = 0;
std::vector<size_t>  Random::m_randSeeds;
   
int Random::srandom( size_t randSeedNum )
{
    m_randSeeds.clear();
    
    for(size_t i=0; i< randSeedNum; ++i){
        m_randSeeds.push_back( i );
    }
    
    std::random_shuffle(m_randSeeds.begin(), m_randSeeds.end());
    m_bInit = true;

    printf("Random::srandom\n");
    return 0;
}

size_t Random::random()
{
    if( ! m_bInit ) {
        srandom();
    }

    static size_t size = m_randSeeds.size();
    return 16777619 * m_randSeeds[ m_count ++ % size ];
    
    //return 16777619 * m_randSeeds[ m_count ++ % NUM_RAND_SEED ];
    //return 16777619 * m_randSeeds[ (m_count ++) & 0xffL ];
}

// 简单随机数
int MyRandom()
{
    static struct timeval tv;
    static size_t iCount = 0;
    
    tv.tv_usec += 54321;
    if( tv.tv_usec > 1000000){
        tv.tv_usec -= 1000000;
    }
    if( iCount++ % 1000 == 0 ){
        gettimeofday(&tv, NULL);
    }

    return tv.tv_usec;
}

// 自增
int Inc()
{
    static size_t iCount = 0;
    
    return iCount++;
}

//////////////////////////////////////////////

struct timeval stStartTv;

//return past time. uint: us
long PostTime(struct timeval *pstStartTv)
{
    struct timeval  stEndTv;
    gettimeofday(&stEndTv, NULL);
    struct timeval* pstCurrTv = &stEndTv;

    long sec, usec = 0;
    sec = pstCurrTv->tv_sec - pstStartTv->tv_sec;
    if ((usec = pstCurrTv->tv_usec - pstStartTv->tv_usec) < 0) {
        sec--;
        usec += 1000000;
    }
    usec += sec*1000000;

    return usec;
}

void LogPastTime(struct timeval *pstStartTv, const char* sStep)
{
    long usec = PostTime(pstStartTv);

    printf("%s: Past time: %ld ms\n", sStep, usec / 1000);
}

#define STAT_NUM 100

// 自增函数
void TestInc(size_t count)
{
    gettimeofday(&stStartTv, NULL);
    size_t arrCount[STAT_NUM] = {0};
    printf("Test Inc...\n");

    for(size_t i=0; i<count; ++i){
        size_t rand = Inc();
        arrCount[ rand % STAT_NUM ]++;
    }

    printf("Total count: %lu\n", count);
    for(size_t i=0; i<STAT_NUM; ++i){
        printf("%lu: count=%lu, ratio=%f\n", i, arrCount[i], arrCount[i] * 1.0 / count);
    }

    LogPastTime(&stStartTv, "Inc");
    printf("Test Inc.\n");
}

// 简单自增
void TestInc2(size_t count)
{
    gettimeofday(&stStartTv, NULL);
    size_t arrCount[STAT_NUM] = {0};
    printf("Test Inc...\n");

    static size_t icount = 0;
    for(size_t i=0; i<count; ++i){
        size_t rand = icount ++;
        arrCount[ rand % STAT_NUM ]++;
    }

    printf("Total count: %lu\n", count);
    for(size_t i=0; i<STAT_NUM; ++i){
        printf("%lu: count=%lu, ratio=%f\n", i, arrCount[i], arrCount[i] * 1.0 / count);
    }

    LogPastTime(&stStartTv, "Inc");
    printf("Test Inc.\n");
}

// 
void TestMyRandom(size_t count)
{
    Random::srandom();  // not cala time

    gettimeofday(&stStartTv, NULL);
    size_t arrCount[STAT_NUM] = {0};
    printf("Test My Random...\n");

    for(size_t i=0; i<count; ++i){
        size_t rand = Random::random();
        arrCount[ rand % STAT_NUM ]++;
    }

    printf("Total count: %lu\n", count);
    for(size_t i=0; i<STAT_NUM; ++i){
        printf("%lu: count=%lu, ratio=%f\n", i, arrCount[i], arrCount[i] * 1.0 / count);
    }

    LogPastTime(&stStartTv, "MyRandom");
    printf("Test My Random.\n");
}

// 简单随机数
void TestSimpleRandom(size_t count)
{
    gettimeofday(&stStartTv, NULL);
    size_t arrCount[STAT_NUM] = {0};
    printf("Test Simple Random...\n");

    for(size_t i=0; i<count; ++i){
        size_t rand = MyRandom();
        arrCount[ rand % STAT_NUM ]++;
    }

    printf("Total count: %lu\n", count);
    for(size_t i=0; i<STAT_NUM; ++i){
        printf("%lu: count=%lu, ratio=%f\n", i, arrCount[i], arrCount[i] * 1.0 / count);
    }

    LogPastTime(&stStartTv, "Simple Random");
    printf("Test Simple Random.\n");
}

// random
void TestRandom(size_t count)
{ 
    gettimeofday(&stStartTv, NULL);
    size_t arrCount[STAT_NUM] = {0};
    printf("Test Random...\n");

    for(size_t i=0; i<count; ++i){
        size_t rand = random();
        arrCount[ rand % STAT_NUM ]++;
    }

    printf("Total count: %lu\n", count);
    for(size_t i=0; i<STAT_NUM; ++i){
        printf("%lu: count=%lu, ratio=%f\n", i, arrCount[i], arrCount[i] * 1.0 / count);
    }

    LogPastTime(&stStartTv, "Sys Random");
    printf("Test Random.\n");
}

// rand
void TestRand(size_t count)
{ 
    gettimeofday(&stStartTv, NULL);
    size_t arrCount[STAT_NUM] = {0};
    printf("Test Rand...\n");

    for(size_t i=0; i<count; ++i){
        size_t r = rand();
        arrCount[ r % STAT_NUM ]++;
    }

    printf("Total count: %lu\n", count);
    for(size_t i=0; i<STAT_NUM; ++i){
        printf("%lu: count=%lu, ratio=%f\n", i, arrCount[i], arrCount[i] * 1.0 / count);
    }

    LogPastTime(&stStartTv, "Sys Rand");
    printf("Test Rand.\n");
}

// 调用srand 和rand
void TestRand2(size_t count)
{ 
    gettimeofday(&stStartTv, NULL);
    size_t arrCount[STAT_NUM] = {0};
    printf("Test Rand (and srand)...\n");

    for(size_t i=0; i<count; ++i){
        srand(i);
        size_t r = rand();
        arrCount[ r % STAT_NUM ]++;
    }

    printf("Total count: %lu\n", count);
    for(size_t i=0; i<STAT_NUM; ++i){
        printf("%lu: count=%lu, ratio=%f\n", i, arrCount[i], arrCount[i] * 1.0 / count);
    }

    LogPastTime(&stStartTv, "Sys Rand");
    printf("Test Rand.\n");
}


int main(int argc, char** argv)
{
    size_t count = 10000;
    if( argc > 2){
        count = strtol(argv[2], NULL, 0);
    }

    if( argc < 2){
        printf("Usage: %s mode [count]\n", argv[0]);
        exit(0);
    }

    int mode = strtol(argv[1], NULL, 0);
    switch( mode )
    {
        case 0:
            TestInc(count);
            break;

        case 1:
            TestMyRandom(count);
            break;
    
        case 2:
            TestSimpleRandom(count);
            break;
     
        case 3:
            TestRandom(count);
            break;
 
        case 4:
            TestRand(count);
            break;
 
        case 5:
            TestRand2(count);
            break;
  
        case 6:
            TestInc2(count);
            break;
            
            
        default:
            printf("Unsupport mode: %d\n", mode);
    }

    return 0;
}