Re: [PATCH] FIx endless match.pd recursion on cst1 + cst2 + cst3 (PR tree-optimization/84334)

[Marc Glisse <marc dot glisse at inria dot fr>]

On Tue, 13 Feb 2018, Richard Biener wrote:

> On February 13, 2018 6:51:29 PM GMT+01:00, Jakub Jelinek <jakub@redhat.com> wrote:
> > Hi!
> >
> > On the following testcase, we recurse infinitely, because
> > we have float re-association enabled, but also rounding-math, so
> > we try to optimize (cst1 + cst2) + cst3 as (cst2 + cst3) + cst1
> > but (cst2 + cst3) doesn't simplify and we try again and optimize
> > it as (cst3 + cst1) + cst2 and then (cst1 + cst2) + cst3 and so on
> > forever.  If @0 is not a CONSTANT_CLASS_P, there is not a problem,
> > if it is, the code just checks if we can actually simplify the
> > operation between cst2 and cst3 into a constant.
>
> Is there a reason to try simplifying at all for constant @0?

Yes. cst2+cst3 might simplify (the operation happens to be exact and not 
require rounding), which leaves us with only one addition instead of 2.

On the other hand, mixing -frounding-math with reassociation seems strange 
to me, and likely not worth optimizing for.

--
Marc Glisse