To be pedantic, there are 8 different valid -O options you can give to gcc, though there are some that mean the same thing.
The original version of this answer stated there were 7 options. GCC has since added -Og
to bring the total to 8.
From the man page:
-O
(Same as-O1
)-O0
(do no optimization, the default if no optimization level is specified)-O1
(optimize minimally)-O2
(optimize more)-O3
(optimize even more)-Ofast
(optimize very aggressively to the point of breaking standard compliance)-Og
(Optimize debugging experience. -Og enables optimizations that do not interfere with debugging. It should be the
optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization
while maintaining fast compilation and a good debugging experience.)-Os
(Optimize for size.-Os
enables all-O2
optimizations that do not typically increase code size. It also performs further optimizations
designed to reduce code size.
-Os
disables the following optimization flags:-falign-functions -falign-jumps -falign-loops -falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays -ftree-vect-loop-version
)
There may also be platform specific optimizations, as @pauldoo notes, OS X has -Oz
.