Re: [patch] Disable old loop optimizer

[Mark Mitchell <mark at codesourcery dot com>]

Zdenek Dvorak wrote:
> Hello,
>
> this patch disables old loop optimizer and enables the invariant motion
> of the new rtl loop optimizer instead by default.  This is the first
> step towards complete removal of the old loop optimizer, which I would
> very much like to happen in 4.1 (since it would enable some pretty
> significant cleanups of the compiler, like removal of loop notes,
> removal of rtl level branch prediction pass, maybe removal of libcall
> notes, etc.).

You did post this dangerously close to April 1st, but I think you were 
quite serious.  I've been watching for replies.

I think that Jim Wilson recently indicated that there were ways in which 
the old RTL loop optimizer still did useful things.

Can we expect that all of the old RTL loop optimizer will be useless? 
Certainly, one would hope that high-level loop transformations (like 
unrolling) would be performed at the tree level.  And, loop-invariant 
code motion will be handled by the new RTL loop optimizer.  Is there 
anything else of interest?

I'm not keen on the disablement.  If we're going to disable it, let's 
just remove it.  If, on the other hand, it's still useful, then perhaps 
we shouldn't be disabling it.

Personally, I think your patch is probably a good idea, once the patches 
on which it depends have been merged.  However, I'm certainly not going 
to approve the patch in the absence of consensus.  Perhaps my tentative 
endorsement will prompt any conscientious objetors to speak up!

--
Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com
(916) 791-8304