Slow memcpy Performance

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This may seem like a stupid/obvious question to some of you, but I'm still learning so please be gentle haha.

I'm writing an application without the CRT, so I have to implement my own memcpy function. After doing everything and getting it working, I noticed the application was performing significantly slower than it's CRT counterpart. After a while I tracked it down to my custom memcpy function.

void* _memcpy(void* destination, void* source, size_t num)
{
    char* d = (char*)destination;
    char* s = (char*)source;
    while (num--)
        *d++ = *s++;
    return destination;
}

My friend told me this was a complete sh*t implementation, so I'm posting this here to ask how I could at least improve it to meet the performance of it's CRT counterpart. And also to get an explanation of why it's so slow

memcpy c++
1 Answers

First thing first. Computers handle things in words. Typical word size is 4 or 8 bytes long (except on some 8 bit micros). If you can copy a word at a time, things will be much faster.

There are complications though. Many processors don't like misaligned access so each copy should on word boundaries.

Other optimizations might include pre fetching data but these start becoming more complicated.

Take a look a newlib-nano's implementation for inspiration. https://github.com/eblot/newlib/blob/master/newlib/libc/string/memcpy.c

user_2329180
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