multi-character constant优化比较

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有这样一段代码，比较字符串，然后根据key是否相同，给对应变量赋值。

版本1

最常规的写法：

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int Record::set1(int itemIdx, const char* key, const char* value) {
    uint64_t num = *(uint64_t*)value;
    if (0 == strcmp(key, "record_type")) {
        this->record_type = num;
    } else if (0 == strcmp(key, "record_time")) {
        this->record_time_start = num;
    } else if (0 == strcmp(key, "rule_major_type")) {
        this->rule_major_type = num;
    } else if (0 == strcmp(key, "rule_minor_type")) {
        this->rule_minor_type = num;
    } else if (0 == strcmp(key, "src_group")) {
        this->src_group = num;
    } else if (0 == strcmp(key, "src_type")) {
        this->src_type = num;
    } else if (0 == strcmp(key, "src_branch_id")) {
        this->src_branch_id = num;
    } else if (0 == strcmp(key, "src_ip")) {
        strncpy(this->src_ip, value, sizeof(this->src_ip));
    } else if (0 == strcmp(key, "dst_group")) {
        this->dst_group = num;
    } else if (0 == strcmp(key, "dst_type")) {
        this->dst_type = num;
    } else if (0 == strcmp(key, "dst_branch_id")) {
        this->dst_branch_id = num;
    } else if (0 == strcmp(key, "dst_ip")) {
        strncpy(this->dst_ip, value, sizeof(this->dst_ip));
    }
    return 0;
}

版本2

感觉前面比较第一个字符，就能排除很多无效功，于是做了第二个版本：

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int Record::set2(int itemIdx, const char* key, const char* value) {
    uint64_t num = *(uint64_t*)value;
    switch (key[0]) {
        case 'r':
            if (0 == strcmp(key, "record_type")) {
                this->record_type = num;
            } else if (0 == strcmp(key, "record_time")) {
                this->record_time_start = num;
            } else if (0 == strcmp(key, "rule_major_type")) {
                this->rule_major_type = num;
            } else if (0 == strcmp(key, "rule_minor_type")) {
                this->rule_minor_type = num;
            }
            break;
        case 's':
            if (0 == strcmp(key, "src_group")) {
                this->src_group = num;
            } else if (0 == strcmp(key, "src_type")) {
                this->src_type = num;
            } else if (0 == strcmp(key, "src_branch_id")) {
                this->src_branch_id = num;
            } else if (0 == strcmp(key, "src_ip")) {
                strncpy(this->src_ip, value, sizeof(this->src_ip));
            }
            break;
        case 'd':
            if (0 == strcmp(key, "dst_group")) {
                this->dst_group = num;
            } else if (0 == strcmp(key, "dst_type")) {
                this->dst_type = num;
            } else if (0 == strcmp(key, "dst_branch_id")) {
                this->dst_branch_id = num;
            } else if (0 == strcmp(key, "dst_ip")) {
                strncpy(this->dst_ip, value, sizeof(this->dst_ip));
            }
            break;
    }
    return 0;
}

版本3

N年前看公司的一个模块，里面用到了multi-character constant的写法(这种写法编译器会报Warning)，可以将小于等于4个的字符常量转换成一个整数表示，小端平台，rule不能写成rule，需要反过来，变成elur，这样，multi-character constant当成一个整数后，可以直接整数比较。论上是比调用strcmp调用要高效。

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int Record::set3(int itemIdx, const char* key, const char* value) {
    uint64_t num = *(uint64_t*)value;
    int isSrc = 0;
    switch (*(int32_t*)(key)) {
        case 'ocer':
            if ('t_dr' == *(int32_t*)(key+4) && 'e' == key[10]) {
                if ('py' == *(int16_t*)(key+8)) {
                    this->record_type = num;
                } else if ('mi' == *(int16_t*)(key+8)) {
                    this->record_time_start = num;
                }
            }
            break;
        case 'elur':
            if ('t_ro' == *(int32_t*)(key+8) && 'py' == *(int16_t*)(key+12) && 'e' == key[14]) {
                if ('jam_' == *(int32_t*)(key+4)) {
                    this->rule_major_type = num;
                } else if ('nim_' == *(int32_t*)(key+4)) {
                    this->rule_minor_type = num;
                }
            }
            break;
        case '_crs':
            isSrc = 1;
        case '_tsd':
            if ('narb' == *(int32_t*)(key+4) && 'i_hc' == *(int32_t*)(key+8) && 'd' == key[12]) {
                if (isSrc) {
                    this->src_branch_id = num;
                } else {
                    this->dst_branch_id = num;
                }
            }else if ('uorp' == *(int32_t*)(key+4) && 'p' == key[8]) {
                if (isSrc) {
                    this->src_group = num;
                } else {
                    this->dst_group = num;
                }
            } else if ('epyt' == *(int32_t*)(key+4)) {
                if (isSrc) {
                    this->src_type = num;
                } else {
                    this->dst_type = num;
                }
            } else if ('pi' == *(int16_t*)(key+4)) {
                if (isSrc) {
                    strncpy(this->src_ip, value, sizeof(this->src_ip));
                } else {
                    strncpy(this->dst_ip, value, sizeof(this->dst_ip));
                }
            }
            break;
    }
    return 0;
}

验证一下2亿条数据，执行第一个版本耗时2.3s，执行第二个版本耗时2.0s，第三个版本耗时1.9s，性能稳定：

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6152  D kvdreader.cpp:161(readBlock)    | set1 cost 2671ms, set2 cost 2162ms, set3 cost 1961ms, total count 209712500
6152  D kvdreader.cpp:161(readBlock)    | set1 cost 2307ms, set2 cost 2017ms, set3 cost 1960ms, total count 209693500
6152  D kvdreader.cpp:161(readBlock)    | set1 cost 2290ms, set2 cost 2013ms, set3 cost 1948ms, total count 209705800
6152  D kvdreader.cpp:161(readBlock)    | set1 cost 2293ms, set2 cost 2015ms, set3 cost 1946ms, total count 209712400
6152  D kvdreader.cpp:161(readBlock)    | set1 cost 2298ms, set2 cost 2011ms, set3 cost 1945ms, total count 209708800
6152  D kvdreader.cpp:161(readBlock)    | set1 cost 2298ms, set2 cost 2008ms, set3 cost 1950ms, total count 209688100
6152  D kvdreader.cpp:161(readBlock)    | set1 cost 2285ms, set2 cost 2005ms, set3 cost 1942ms, total count 209554400
6152  D kvdreader.cpp:161(readBlock)    | set1 cost 2294ms, set2 cost 2006ms, set3 cost 1945ms, total count 209563300
6152  D kvdreader.cpp:161(readBlock)    | set1 cost 2294ms, set2 cost 2041ms, set3 cost 1951ms, total count 209686400
6152  D kvdreader.cpp:161(readBlock)    | set1 cost 2285ms, set2 cost 2008ms, set3 cost 1949ms, total count 209688200

注：首次执行的时候，数据还不在cache中，导致set1的时间有偏差，看第一条为2671ms，理论上应该在2307ms左右，若先调用set3，set3的第一次为2622ms，而不是1961ms。

可以看出，效果意义并不大，我一天最多才处理2亿条，一天节约0.5s的时间，代码却比较难维护，还是，不要太计较CPU性能，不要太在意细节，不要提前优化。