ClickHouse/base/memcpy

Internal implementation of memcpy function.

It has the following advantages over libc-supplied implementation:

• it is linked statically, so the function is called directly, not through a PLT (procedure lookup table of shared library);
• it is linked statically, so the function can have position-dependent code;
• your binaries will not depend on glibc's memcpy, that forces dependency on specific symbol version like memcpy@@GLIBC_2.14 and consequently on specific version of glibc library;
• you can include memcpy.h directly and the function has the chance to be inlined, which is beneficial for small but unknown at compile time sizes of memory regions;
• this version of memcpy pretend to be faster (in our benchmarks, the difference is within few percents).

Currently it uses the implementation from Linwei (skywind3000@163.com). Look at https://www.zhihu.com/question/35172305 for discussion.

Drawbacks:

• only use SSE 2, doesn't use wider (AVX, AVX 512) vector registers when available;
• no CPU dispatching; doesn't take into account actual cache size.

Also worth to look at:

• simple implementation from Facebook: https://github.com/facebook/folly/blob/master/folly/memcpy.S
• implementation from Agner Fog: http://www.agner.org/optimize/
• glibc source code.