gcc -O4 optimization flag

Question

What's the meaning of the -O4 optimization flag in gcc (3.2.3)? What's the difference to O3? When would you use one vs. the other?

The man pages only mention O, O0-3, Os, no word of the mysterious O4.

gcc 3.2.3 is positively archaic. Since then, the developers have released gcc 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4, and 4.5. You should really use a more current release. — Phil Miller
– Phil Miller, Commented Aug 3, 2010 at 20:35
But the answer is the same for everything at least up to 4.4 — tstenner
– tstenner, Commented Aug 3, 2010 at 20:47
Might be a leftover from the PGCC days which supported everything up to -O9. — Laurynas Biveinis
– Laurynas Biveinis, Commented Aug 5, 2010 at 8:10

tstenner · Accepted Answer · 2010-08-03 18:59:18Z

27

There is no -O4 in 3.2.3. Everything above -O3 results in -O3 being chosen.

answered Aug 3, 2010 at 18:59

10.4k10 gold badges61 silver badges94 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Ok that would explain it. Would you happen to know if I can tell what O flag was used when something was compiled if I have the binary?

Some build systems insert the compiler settings into the binary, but there's no easy, generic way.

Is this still relevant as of gcc 5.3.0?

Lukasz Stelmach · Accepted Answer · 2010-08-03 20:01:12Z

You can check what optimization are enabled for each level

gcc -c -Q -O3 --help=optimizers | grep enabled

It works at least for gcc 4.4.