[RFC] Meta-description for tree and gimple folding

[Marc Glisse]

On Mon, 3 Mar 2014, Richard Biener wrote:

>> How do I restrict some subexpression to have
>> a single use?
>
> This kind of restrictions come via the valueize() hook - simply
> valueize to NULL_TREE to make the match fail (for example
> SSA_NAME_OCCURS_IN_ABNORMAL_PHI could be made fail that way).

Shouldn't that single-use property depend more on the transformation and 
less on where it is called from? a+b-b -> a is always going to be a good 
idea (well, register pressure aside), even if a+b is used in many other 
places. But if you are using a*b elsewhere, turning a*b+c into FMA doesn't 
make so much sense.

Well, we can always call has_single_use on some @i if it is an SSA_NAME.

>> If I write a COND_EXPR matcher, could it generate code for phiopt as
>> well?
>
> Not sure, what do you have in mind specifically?

fold-const.c has the equivalent of:
(define_match_and_simplify abs
   (COND_EXPR (LT_EXPR @0 zero_p) (NEGATE_EXPR @0) @0)
   (ABS_EXPR @0))

(it would help to be able to write LT_EXPR|LE_EXPR, maybe even to try 
automatically simplify(!a)?c:b for a?b:c)
which works well on trees, but requires more complicated code in phiopt 
(same for min/max).

>> How do you handle a
>> transformation that currently tries to recursively fold something else and
>> does the main transformation only if that simplified?
>
> And doesn't do the other folding (because it's not in the IL literally?)?
> Similar to the cst without overflow case, by writing custom C code
> and allowing that to signal failure.

I am not sure if it will be easy to write code that works for generic and 
gimple. I'll see...

-- 
Marc Glisse