Performance Cost of a Memcopy in C/C++

Question

So whenever I write code I always think about the performance implications. I've often wondered, what is the "cost" of using a memcopy relative to other functions in terms of performance?

For example, I may be writing a sequence of numbers to a static buffer and concentrate on a frame within the buffer, in order to keep the frame once I get to the end of the buffer, I might memcopy all of it to the beginning OR I can implement an algorithm to amortize the computation.

Answer 1

memcpy is generally optimized to maximize memory bandwidth of large copies. Of course, it's not as fast as avoiding a copy completely, and for short copies of fixed size, direct assignment may be faster since memcpy has extra code to deal with odd lengths.

But when you need to copy a block of memory, it's hard to beat memcpy. It's highly portable and most compilers go to great lengths to make it fast, whether that's using SIMD instructions or maybe inlining.

Answer 2

It's ok to consider performance implications, but don't become too distracted from the real goal of writing good clean code. If you are inclined to obsess about performance even when you know better, try to focus on higher level implications, and ignore the bit-by-bit stuff such as memcpy, which you can trust the compiler and library authors to optimize.

Generally avoid premature optimization of this low-level kind because it consumes your time, the effects bubble up to infect the entire program, and without measurements, you cannot expect to achieve any performance gains.