And it also requires that you pay attention to your evolving toolset. I've worked on x86 code that was hand-optimized back in 1995. Once I reverse-engineered it and handed it to Visual C++ the code gen'ned by VC killed it, by like a factor of 2x.

I emailed the code to the guy who originally wrote it and his response was brief, "The compiler wasn't that smart before". But yet the code sat there for like 10 years, untouched -- largely because people don't like to touch x86 assembly code that is working.

The moral, you may get a benefit today, but you don't get any future benefits of the investment at MS, Intel, the Gcc folks, etc..., who are improving the compilers. Sometimes it is the right call to hand optimize. But when you do, I suggest having a C/C++ version alongside of it that one can test aganist it whenever the compiler revs.