From: jmc AT gnu DOT ai DOT mit DOT edu
Subject: Re: A few simple Questions
To: dj AT ctron DOT com (DJ Delorie)
Date: Mon, 8 Aug 1994 15:59:27 -0400 (EDT)
Cc: wudavid AT ecf DOT toronto DOT edu, djgpp AT sun DOT soe DOT clarkson DOT edu

>>   Another Q is when you specify -O4 what exactly does this do?  I 
>> know that it is Optimization level 4, but the docs don't describe O's that
>> high.  Is it about the same as O2? 
>
>-O4 enables some expensive optimizations.  The gcc manual tells which
>are which, but "cc1 -version -O4" will tell you what options are enabled.
>

Let me clarify a little. -O turns on simple optimizations. -O2 turns on
everything but -funroll-loops and -finline-functions. (-fomit-frame-pointer
will get turned on for machines that don't have a problem debugging
without frame pointers).

-O3 as of 2.6.0 will turn on -finline-functions. Anything above that does
nothing more. The code for checking is real simple and is just == 2, >= 3
checks. -O4 won't do anything more than -O3 in reality then.

James