Re: Optimal Optimizations: A Very Preliminary Report

[Diego Novillo <dnovillo at redhat dot com>]

On Mon, 2003-08-11 at 21:05, Scott Robert Ladd wrote:

> In the vast majority of cases, anecdotal assertions lack any formal 
> proof of their validity, and, more often than not, the suggested 
> "improvement" is ineffective or detrimental. It has become increasingly 
> obvious that no one -- myself included -- knows precisely how all these 
> GCC options work together in generating executable code.
> 
I haven't visited your site, but from this message you seem to
concentrate on one axis of the problem: the combination of the different
optimization options.

As I see it, the problem has at least three dimensions:

     1. Selection of optimization options
     2. Ordering of the different passes
     3. Programs used as benchmarks

Ideally, one would explore that 3 dimensional space and cut slices to
stick in the different -Ox.  GCC will let you play with #1 (to a certain
extent) and #3, but you cannot change #2.

There may even be a 4th dimension: input language.  There are
optimizations that are language specific that help certain languages but
not others (escape analysis, dispatch table optimizations).

I seem to remember reading/hearing that just the selection and ordering
is either NP-hard or intractable.  But that was years ago and I can't
find a reference now, so I take it with a grain of salt.

In any case, I think what you're doing is great and I'm very interested
in the results.  What are the applications that you're using for your
study?


Diego.