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Re: fold() can't fold simple expressions?

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)),

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?


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