為什么stoi比沒有-O3的stringstream慢得多？

Question

今天我在談論C ++ 11中的新聞，如線程，to_string和stoi。

但事實上，所有這些在C ++ 98中已經成為可能。

然后我決定比較舊庫和新聞庫：

C ++ 11：

g ++ -std = c ++ 11 main.cpp

#include <iostream>
#include <string>
#include <ctime>

using namespace std;

int main()
{
    clock_t tStart = clock();
    string input = "50";

    for (int i = 0; i < 50000; i++)
    {
        int number = stoi(input);
    }

    cout << (double)(clock() - tStart) << endl;
    return 0;
}

C ++ 98

g ++ main.cpp

#include <iostream>
#include <string>
#include <sstream>
#include <ctime>

using namespace std;

int main()
{
    clock_t tStart = clock();
    string input = "50";
    stringstream ss;

    for (int i = 0; i < 50000; i++)
    {
        int number;
        ss << input;
        ss >> number;
    }

    cout << (double)(clock() - tStart) << endl;
    return 0;
}

屏幕截圖：

Windows 7輕型服務器：

Ubuntu 14.04：

Slackware Server 14.1通過SSH：

隨着優化

Windows 7輕型服務器：

Ubuntu 14.04：

Slackware Server 14.1通過SSH：

配置：

Windows 7輕型服務器： Intel（R）Core（TM）i5-4590 CPU @ 3.30GHz

Ubuntu 14.04： Intel（R）Core（TM）i7-2670QM CPU @ 2.20GHz

Slackware Server 14.1通過SSH： Intel（R）Core（TM）i3-4150 CPU @ 3.50GHz

問題：

stoi和stringstream有什么區別？

為什么以及何時在字符串lib（stoi，to_string）中使用新聞函數？

最后......使用-O3，stoi速度要快得多，而sstream則不然。 那么，-O3對stoi的作用比對sstream沒有做的更多？

Answer 1

我剛剛檢查過以下內容：

gcc -v：

Using built-in specs.
COLLECT_GCC=x86_64-alt-linux-gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-alt-linux/5/lto-wrapper
Target: x86_64-alt-linux
Configured with: ../configure --host=x86_64-alt-linux --build=x86_64-alt-linux --target=x86_64-alt-linux --prefix=/usr --exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc --datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib64 --libexecdir=/usr/libexec --localstatedir=/var/lib --sharedstatedir=/var/lib --mandir=/usr/share/man --infodir=/usr/share/info --disable-dependency-tracking --without-included-gettext --enable-shared --program-suffix=-5 --with-slibdir=/lib64 --with-bugurl=http://bugzilla.altlinux.org --enable-__cxa_atexit --enable-threads=posix --enable-checking=release --with-system-zlib --without-included-gettext --enable-multilib --enable-gnu-unique-object --enable-linker-build-id --with-linker-hash-style=gnu --with-arch_32=i586 --with-tune_32=generic --with-multilib-list=m64,m32,mx32 --enable-bootstrap --enable-languages=c,c++,fortran,objc,obj-c++,java,ada,go,lto --enable-plugin --enable-java-awt=gtk --with-native-libdir=/usr/lib64/gcj-5 --with-ecj-jar=/usr/share/java/ecj.jar --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-1.5.0.0/jre --enable-libgcj-multifile --disable-libjava-multilib --enable-java-maintainer-mode
Thread model: posix
gcc version 5.3.1 20151207 (ALT Linux 5.3.1-alt1) (GCC)

編碼：

#include <iostream>
#include <string>
#include <chrono>
#include <sstream>

using namespace std;
using namespace chrono;

void stringstream_test(const string& input, int repetitions) {
    stringstream ss;
    auto start = steady_clock::now();
    for (int i = 0; i < repetitions; i++) {
        int number;
        ss << input;
        ss >> number;
    }

    auto end = steady_clock::now();
    auto dur = duration_cast<nanoseconds>(end - start);
    cout << "stringstream_test completed in " << dur.count() << " nanoseconds." << endl;
}

void stoi_test(const string& input, int repetitions) {
    auto start = steady_clock::now();
    for (int i = 0; i < repetitions; i++) {
        int number = stoi(input);
    }
    auto end = steady_clock::now();
    auto dur = duration_cast<nanoseconds>(end - start);
    cout << "stoi_test         completed in " << dur.count() << " nanoseconds." << endl;
}

int main() {
    stringstream_test("50", 500000);
    stoi_test("50", 500000);
    return 0;
}

編譯為c++ -std=c++11 -O3 -o stoi_perf stoi_perf.cc 。

結果，收到的while true; do ./stoi_perf; done while true; do ./stoi_perf; done while true; do ./stoi_perf; done並重復幾次后停止：

stringstream_test completed in 10449080 nanoseconds.
stoi_test         completed in 10437559 nanoseconds.
stringstream_test completed in 9074869 nanoseconds.
stoi_test         completed in 9895661 nanoseconds.
stringstream_test completed in 11516788 nanoseconds.
stoi_test         completed in 12266627 nanoseconds.
stringstream_test completed in 10017085 nanoseconds.
stoi_test         completed in 9468441 nanoseconds.
stringstream_test completed in 9957401 nanoseconds.
stoi_test         completed in 9483004 nanoseconds.
stringstream_test completed in 10069845 nanoseconds.
stoi_test         completed in 9657653 nanoseconds.
stringstream_test completed in 9568359 nanoseconds.
stoi_test         completed in 9162406 nanoseconds.
stringstream_test completed in 9868536 nanoseconds.
stoi_test         completed in 9166439 nanoseconds.
stringstream_test completed in 9059762 nanoseconds.
stoi_test         completed in 11558076 nanoseconds.
stringstream_test completed in 11673084 nanoseconds.
stoi_test         completed in 13432386 nanoseconds.
...

正如您可能看到，兩個測試都給出了可比較的結果，有時候一個測試稍微好一點。 我的工作站很繁忙（有一個后台進程完全吃掉了六個中的一個或兩個核心）但即使在這樣的條件下，測試結果看起來也令我滿意。