6 Compiling with optimization
GCC is an optimizing compiler. It provides a wide range of
options which aim to increase the speed, or reduce the size, of the
executable files it generates.
Optimization is a complex process. For each high-level command in the
source code there are usually many possible combinations of machine
instructions that can be used to achieve the appropriate final result.
The compiler must consider these possibilities and choose among them.
In general, different code must be generated for different processors,
as they use incompatible assembly and machine languages. Each type of
processor also has its own characteristics--some CPUs provide a large
number of registers for holding intermediate results of
calculations, while others must store and fetch intermediate results
from memory. Appropriate code must be generated in each case.
Furthermore, different amounts of time are needed for different
instructions, depending on how they are ordered. GCC takes all these factors
into account and tries to produce the fastest executable for a given
system when compiling with optimization.