c - Is memcpy() usually faster than strcpy()?

Is memcpy() usually faster than strcpy() (on most real platforms)? (I assume that size of the string is known.)

If I remember i386 assembler correctly, there are loop instructions which copy a given number of bytes or words. So it is the fastest way, while strcpy() i386 assembler implementation would use manual checking for '\0' in a plain loop.

So I feel that on x86 memcpy() is faster than strcpy().

What's about other architectures?

Answer

If you know the size of the data to be copied, then memcpy() should be as fast or faster than strcpy(). Otherwise, memcpy() can't be used alone, and strcpy() should be as fast or faster than strlen() followed by memcpy().

However...

A lot of implementations of memcpy() and/or strcpy() and/or strlen() are designed to efficiently handle large amounts of data. This often means additional startup overhead (e.g. determining alignment, setting up SIMD, cache management, etc) and makes these implementations bad (slow) for copying small amounts of data (which is far more likely in well written code). Because of this, "should be as fast or faster" does not necessarily imply "is as fast or faster". For example, for small amounts of data an memcpy() optimised for large amounts of data may be significantly slower than a strcpy() that wasn't optimised for large amounts of data.

Also note that the main problem here is that generic code (e.g. memcpy() and strcpy()) can't be optimised for a specific case. The best solution would have been to have multiple functions - e.g. memcpy_small() that's optimised for copying small amounts of data and memcpy_large() that's optimised for bad code that failed avoid copying a large amount of data.