测试结果：
gcc7/gcc9：
    优化：g++ foo.cc -o foo -O2 -fopenmp
    malloc: 0ms
    new: 900ms
vs2017：
    优化：默认Release
    new/malloc: 200ms

看上去vs的运行结果比较正常，怎么解释这个？

测试代码如下：
#include <chrono>
#include <iostream>
#include <string.h>

using namespace std::chrono;

void test_copy() {
    const long N = 1 * 1024 * 1024 * 1024L;
    char *src = (char *)malloc(N);
    if (!src) {
        printf("src null\n");
        return;
    }
    char *dst = (char *)malloc(N);
    if (!dst) {
        printf("dst null\n");
        return;
    }
    // char *src = new char[N];
    // char *dst = new char[N];

    {
        auto begin = steady_clock::now();
#pragma omp parallel for
        for (long i = 0; i < N; i++) {
            src[i] = i;
        }
        auto end = steady_clock::now();
        auto duration = duration_cast<milliseconds>(end - begin);
        std::cout << "assign duration time: " << duration.count() << "ms\n";
    }

    auto begin = steady_clock::now();
    memcpy(dst, src, N);
    auto end = steady_clock::now();
    auto duration = duration_cast<milliseconds>(end - begin);
    std::cout << "memcpy duration time: " << duration.count() << "ms\n";

    // delete [] src;
    // delete [] dst;
    free(src);
    free(dst);
}

int main(int argc, char* argv[])
{
    test_copy();

    return 0;
}
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学到了，gcc优化这块确实是绞尽脑汁，同样的O2优化，vc就不会在这里做优化
- 来自 水木社区APP v3.5.7
【 在 z16166 的大作中提到: 】
: 看汇编代码。
: 注意四个now()之间的代码。
:
: 编译器检测出通过memcpy给dst赋值了但没用，没调用memcpy
: 要想不让它优化掉memcpy，可以在后面printf引用一下dst里的某处内容，或者给dst加volatile。
:
: 可以查一下SecureZeroMemory怎么来的，就是因为用memset()清空内存中的密码这个操作，会被优化掉。
:
: Notes
: memset may be optimized away (under the as-if rules) if the object modified by this function is not accessed again for the rest of its lifetime (e.g., gcc bug 8537). For that reason, this function cannot be used to scrub memory (e.g., to fill an array that stored a password with zeroes).
:
: This optimization is prohibited for memset_explicit and memset_s: they are guaranteed to perform the memory write.
:
: Third-party solutions for that include FreeBSD explicit_bzero or Microsoft SecureZeroMemory.
: https://en.cppreference.com/w/c/string/byte/memset
:
:
:
:
: --
: 每天半小时节奏光剑，预防老年痴呆
--
FROM 220.202.218.*