This is one of those questions where you really, honestly, do need to look at a very low level. Back in the ancient days, I worked at IBM doing benchmarking fo

This is one of those questions where you really, honestly, do need to look at a very low level. Back in the ancient days, I worked at IBM doing benchmarking for an OS project that was never released. We were using PPC601 Sandalfoots (Sandalfeet?) as dev machines. A perennial fight was devs writing their own memcpy using dst++ = src++ loops rather than the one in the library, which was written by one of my coworkers and consisted of 3 pages of assembly that used at least 18 registers. The simple loop was something like X cycles/byte, while the library version was P + (Q cycles/byte) but the difference was such that the crossover point was about 8 bytes. So, scraping out the simple memcpy implementations from the code was about a weekly thing for me. At this point, we discovered that our C compiler would pass structs by value (This was the early-ish days of ANSI C and was a surprise to some of my older coworkers.) and benchmarked that. And discovered that its copy code was worse than the simple dst++ = src++ loops. By about a factor of 4. (The simple loop would be optimized to work with word-sized ints, while the compiler was generating code that copied each byte individually.) If you are doing something where this matters, something like VTune is very important. So is the ability to convince people who do stupid things to stop doing the stupid things.