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. Thanks!
+6
A:
There is no -O4 in 3.2.3. Everything above -O3 results in -O3 being chosen.
tstenner
2010-08-03 18:59:18
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?
2010-08-04 19:36:26
Some build systems insert the compiler settings into the binary, but there's no easy, generic way.
tstenner
2010-08-04 19:48:58
+3
A:
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.
Lukasz Stelmach
2010-08-03 20:01:12