[PATCH] D24716: [Polly] DeLICM/DePRE (WIP)

[Sven Verdoolaege via llvm-commits]

On Sat, Jan 21, 2017 at 11:20:12PM +0000, Michael Kruse via Phabricator wrote:
> Meinersbur marked 39 inline comments as done.
> Meinersbur added inline comments.
> 
> 
> ================
> Comment at: lib/Transform/DeLICM.cpp:654
> +void simplify(IslPtr<isl_set> &Set) {
> +  Set = give(isl_set_compute_divs(Set.take()));
> +  Set = give(isl_set_detect_equalities(Set.take()));
> ----------------
> grosser wrote:
> > compute_divs can sometimes make a map a lot more complex. Did you had a case where this is necessary.
> > 
> > We also should ask sven at some point if he can provide a simplify function for isl that does all simplifications in the right order.
> I am not aware that this can make sets more complex, it just provides an additional explicit representation of a div that should make follow-up computations faster and was sometimes necessary for coalescing.

It can result in an increase in the number of disjuncts.
See 3.7 Simplification and Quantifier Elimination of
"Presburger Formulas and Polyhedral Compilation".
The number of integer divisions in the result may also
be greater than the number of existentially quantified variables
in the input.

skimo