Re: Patch to allow Ada to work with tree-ssa

[Daniel Berlin <dberlin at dberlin dot org>]

On Jun 22, 2004, at 4:28 PM, Richard Kenner wrote:

>     Since Richard Henderson and Zack seem to think it's best, why 
> don't you
>     just create new tree nodes for these variable-length cases?  That 
> seems
>     to be the consensus point of view on how to solve the problem.
>
> Except that the authors of the optimizers haven't weighed in here yet
> and they were very much against adding new nodes to GIMPLE, which is
> what this would be doing.  Having played with some of that code, I can
> tell you that it won't be pleasant to add all the extra cases to
> deal with the additional nodes.

As a part who writes optimizers, as a general principle, i'm very 
against adding nodes to GIMPLE.
However, if it's a node that is just a variable sized analogue of a 
node that already exists in the grammar, and retains the same basic 
grammar as the old node, it's not that a big a deal.

IE if you allow VARIABLE_ARRAY_REF (or whatever) *only* where you 
currently allow array_ref, it's not that big a deal from an optimizer 
perspective.

It's when you start making significant actual grammar changes that make 
GIMPLE much more complex, that the optimizers run into problems.

The most painful changes in the optimizers and other walkers i've had 
to deal with were when we started allowing things like ADDR_EXPR's in 
call arguments (at one point that wasn't allowed), when we started 
allowing INDIRECT_REFS on both sides of the same expression (at one 
point that wasn't allowed), etc.

In other words, things that significantly modified the shape of 
expressions you would see.

Having a VARIABLE_ARRAY_REF (or whatever it was, i keep forgetting) 
exactly where you allow array_ref, doesn't change the shape of 
expressions you see much, if at all.

--Dan