4 ms·

I would be using SSE or AVX instructions for memcpy.

• adrian_b 3y ago
  On recent Intel and AMD CPUs, SSE or AVX instructions can beat "rep movsb" only in certain ranges of copy sizes. On all CPUs, "rep movsb" is slower for very short copies. The threshold under which "rep movsb" becomes slower depends on the CPU model. For example, on a Zen 3 the threshold is slightly above 2 kilobytes, so for copies up to 2 kB one should use SSE/AVX and above 2 kB one should use "rep movsb". On Zen 3 there is a second range where "rep movsb" is slower, approximately between 1 MB and 20 MB (i.e. when the operands are in the L3 cache memory). For any larger copies "rep movsb" is again faster. So depending on the size of the copy and on the CPU model, an optimized memcpy should choose either "rep movsb" or SSE/AVX for each copy. A simplified criterion that should be acceptable on most recent CPUs would be to always use "rep movsb" for sizes of one memory page (4 kB) or more and to use SSE/AVX for the shorter copies.