ANSII C, optimization, and "hardware registers"

[Emperor]

If you are using an optimizing compiler, and your code deals with
hardware registers and such, it is usually a good idea to turn off
all optimizations. Nearly all optimizing transformations applied
to a program will cause certain kinds of programs that deal with
hardware to fail. Since, however, 99% of code written does not
deal directly with hardware, and a good optimizer can double the
speed of the resulting code, the optimizer is worth keeping around.
				Walter Bright