Re: [RTL] Canonicalize commutative operations more

[Marc Glisse <marc.glisse@inria.fr>]

On Tue, 2 Apr 2013, Eric Botcazou wrote:

>> I am not sure about adding just a few rules. If I just say that lshift is
>> stronger than rshift, the relation is not an order (transitive) anymore.
>
> Why?  Can't you give them precedences in commutative_operand_precedence that
> preserve the transitivity?

I can, but then I am giving lshift higher priority than every other 
operation, not just rshift. And if I want to give (vec_select x 0) a 
higher precedence than (vec_select x 1) but lower than (vec_select 
(vec_concat a b) 1), the weights may become complicated, whereas the 
comparison function could just recurse. But I understand that it has 
advantages over an arbitrary order, so if I ever feel the need again I 
will try to play with precedence values.

I might also experiment with the new transformation feature of .md files 
to write a pattern once and have it expand to 2 patterns for the 2 orders.

> The fear (at least mine) is that, by canonicalizing everything, you will 
> make changes behind the back of back-ends that could disable some of 
> their patterns silently.

I wonder if those issues might in most cases be bugs in the back-ends 
(optimizations missed depending on the order), that the canonicalization 
would make more noticable (and thus easier to fix).

-- 
Marc Glisse