Re: [patch] Disable old loop optimizer

[Mark Mitchell <mark at codesourcery dot com>]

Zdenek Dvorak wrote:

> as far as I know, there are the following optimizations or optimization
> enhancements that are only handled by the old loop optimizer:
>
> 1) array prefetching: We now have speculative prefetching, which
>    partially replaces the functionality, but it is a bit cumbersome
>    to use (needs profile feedback).  Also, there are several possible
>    replacement patches:

I would certainly think that this could -- and should --be done at the 
tree level.

> 2) loop reversal (i.e. replacing "for (i = 0; i < n; i++) a[i]=i;" with
>    "for (i = n - 1; i >= 0; i--) a[i]=i;", thus causing us to compare with
>    zero instead of arbitrary bound, and possibly shorten the life range
>    of n).  Should be trivial within linear-transform framework, the major
>    problem is to decide when it is really useful.

Likewise.

> 3) ivopts do not handle induction variables with nonconstant step,
>    whereas the old loop optimizer does (PR 18316, patch
>    http://gcc.gnu.org/ml/gcc-patches/2005-02/msg00142.html needs
>    updating). 

Likewise.

So, that's good -- the things that are missing seem like things that we 
can add at the tree level.

My immediate reaction, however, is that we should add them first, before 
removing or disabling the RTL optimizer.  Otherwise, the risk is that we 
don't get the tree-level work done before GCC 4.1 gets close, and then 
we have performance regressions.  Meanwhile, the RTL code has not been 
receiving testing.

> My reason for proposing disabling of the old loop optimizer is that I
> would like to get some testing in this situation.

Then I'd suggest we add an option to enable the new code in lieu of the 
new, and ask people to test with that option turned on.

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