Re: criteria.html open issues

[Geoff Keating <geoffk at geoffk dot org>]

"Zack Weinberg" <zackw@stanford.edu> writes:

> On Sun, May 27, 2001 at 11:46:51PM -0400, dewar@gnat.com wrote:
> > I do not understand the philosophy behind only enabling strict aliasing
> > at -O3. Generally we often find -O3 to be a disadvantage because of the
> > excessive inlining which can cause icache penalties. But strict aliasing
> > seems like something that should come along with -O2.
> 
> Reality injection: -fstrict-aliasing is enabled at -O2 in all
> snapshots since 2.96, and will be in 3.0.  It was turned off on the
> 2.95 branch to give people more time to fix their code.  It is our
> understanding that most free software projects have either fixed their
> code or wired in a -fno-strict-aliasing where appropriate.
> 
> -O3 turns on inlining of all functions and register renaming.  I
> believe the intent is to move -frename-registers to -O2 once it no
> longer causes such severe problems with debugging (i.e. once DWARF2
> with location lists is the default on most common platforms).
> 
> We haven't really had a sensible policy about what -O levels mean,

Actually, we do.  They're fully documented in the Info documentation:

`-O'
`-O1'
     Optimize.  Optimizing compilation takes somewhat more time, and a
     lot more memory for a large function.

     Without `-O', the compiler's goal is to reduce the cost of
     compilation and to make debugging produce the expected results.
...
`-O2'
     Optimize even more.  GCC performs nearly all supported
     optimizations that do not involve a space-speed tradeoff.  The
     compiler does not perform loop unrolling or function inlining when
     you specify `-O2'.  As compared to `-O', this option increases
     both compilation time and the performance of the generated code.
...
`-O3'
     Optimize yet more.  `-O3' turns on all optimizations specified by
     `-O2' and also turns on the `inline-functions' option.

Actually, this is slightly out-of-date.  -O3 switches on the
optimisations that will trade space for speed.

...
`-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.


> except that -O2 should be a reasonable default choice (which it isn't
> always; -O or -Os have been reported to be better under a rather broad
> set of conditions).
...

-- 
- Geoffrey Keating <geoffk@geoffk.org>