Re: Combine four insns

["Vladimir N. Makarov" <vmakarov at redhat dot com>]

On 08/06/2010 01:22 PM, Steven Bosscher wrote:
> On Fri, Aug 6, 2010 at 6:45 PM, Paolo Bonzini<bonzini@gnu.org>  wrote:
>    
> (I know, it's the old argument we've had before: You have a real fix
> > > now for a real problem, others (incl. me) believe that to fix this
> > > problem in combine is a step away from sane instruction selection that
> > > GCC may not have for years to come...)
> > >        
> > In this case I actually think this argument is not going to work, since we
> > have not even a prototype of a sane instruction selection pass and not even
> > someone who is thinking about working on it.
> >      
>    
To be honest I am periodically thinking about working on it but I did 
not make a final decision yet and don't know when I can start.
> Agreed. There is the work from Preston Briggs, but that appears to
> have gone no-where, unfortunately.
>
>    
IMHO the code should have become public if we want to see a progress on 
the problem solution.  But it is a Google property and they probably 
decided not to give it gcc community.  Although I heard that Ken Zadeck 
has access to it.  We will see what the final result will be.

I only can guess from speculations on info I have that Preston's code is 
probably not so valuable for GCC because as I understand the backend was 
only specialized for x86/x86_64.

The real problem is not in implementing modern pattern matching itself 
(there are a lot free pattern matching code and tools for code 
selection).  The real problem is in how to feed all md files (in which 
implementation a lot of gcc community efforts were made) to pattern 
matcher (in any case most probably we will not need define_split in md 
files if we use a modern pattern matcher).

Another problem, most modern pattern matchers works on trees not on 
DAGs.  There are some solutions (heuristic or expensive optimal) for 
this problem.  The combiner already solves the problem in some way.

In any case I'd not expect, from a new code selection pass, big code or 
compile time improvements (e.g. code selection in LLVM is very expensive 
and takes more time than GCC combiner).  But a new code selection pass 
could be more readable and easy for maintenance.