Re: Loop optimizer issues

[Daniel Berlin <dberlin at dberlin dot org>]

On Tuesday, July 29, 2003, at 1:56 PM, Zack Weinberg wrote:

> Diego Novillo <dnovillo@redhat.com> writes:
>
> > I've been thinking about this proposal and I am not convinced that it
> > would be a good idea to merge the two branches.
> ...
>
> You give some good reasons from the point of view of the people
> working on the tree-ssa branch.
>
> Unfortunately, I think the larger picture says something different:
>
>     * rtlopt is not getting merged into mainline for 3.4 unless we
>       make a special exception to the rules.
>     * tree-ssa is slated to be merged very early in 3.5 stage 1.
>     * At that point tree-ssa may well include the total removal of
>       the RTL loop optimizer, which would leave rtlopt with nothing
>       to merge into.
>     * But tree-ssa can usefully incorporate ideas from rtlopt if
>       they're made visible to it now.

This is an assumption.
The RTL loop optimizer is not SSA based, and we want most, if not all, 
of our loop opts to be SSA based.  This is a large strike against it 
IMHO.
It may be nice to share code with the RTL loop optimizer, but i'd 
rather see nice SSA based loop optimizations that are easy to maintain 
(by themselves), fast, and good at what they do.

> To me, that says that there are effectively three choices:
>
>     * Merge rtlopt into tree-ssa now and deal with the pain.
>     * Merge rtlopt into mainline now and deal with the pain.
>     * Throw rtlopt away now.
>
> Given what Zdenek said about the state of play on his branch, I am
> inclined toward option 1.

I would be inclined toward option 2, particularly since there is no 
evidence that that rtlopt furthers the ssa part of tree-ssa, or ever 
will.

> I am not opposed to option 2 if we all
> agree to make a push and finish fixing the thing on the mainline; this
> would have the advantage of getting a better loop optimizer for 3.4
> but risks release delays.  Option 3 seems like a heinous waste.
>
> zw