Question

How many GCC optimization levels are there?

I tried gcc -O1, gcc -O2, gcc -O3, and gcc -O4

If I use a really large number, it won’t work.

However, I have tried

gcc -O100

and it compiled.

How many optimization levels are there?

Answer 1

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.