fold() can't fold simple expressions?

[Aldy Hernandez]

On 09/14/2016 09:32 AM, Jakub Jelinek wrote:
> On Wed, Sep 14, 2016 at 09:25:30AM -0400, Aldy Hernandez wrote:
>> Hi folks.  I'm working on better range information with Macleod, and I've
>> been playing with folding arbitrary range expressions, which I expect fold()
>> to ahem...fold.
>>
>> I'm surprised that even seemingly simple trees can't be folded after they've
>> been built, because AFAICT, fold actually just works by recognizing patterns
>> it recognizes at the top level, not by recursing down to the sub-trees.
>
> Yes, that is how fold is designed.
>
>> For example, I was surprised at this:
>>
>> #define INT(N) build_int_cst (integer_type_node, (N))
>> tree x = build2 (PLUS_EXPR, integer_type_node,
>> 		 build2 (MULT_EXPR, integer_type_node, INT(20), INT(3)),
>> 			 INT(10));
>
> You should be folding it when building it, i.e. fold_build2 in both cases.
> In the FEs where we don't want to fold everything immediately, we then have
> c_fully_fold and cp_fully_fold that fold things recursively.
>
> So, the question is why do you want to fold recursively in the middle-end,
> instead of folding when building the expressions.

Well, I can use fold_buildN to build it, but I'll be folding something 
like "20 * ssa_38 + 10" which fold() can't do anything about.  It's only 
at a later time that I'll substitute ssa_38 <== 3.  At which point, I 
basically need to refold everything, no?

Aldy