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

[Jakub Jelinek <jakub at redhat dot com>]

On Tue, Feb 13, 2018 at 03:28:25PM -0400, Marc Glisse wrote:
> 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.

Yeah, exactly, e.g.

/* { dg-do compile } */
/* { dg-options "-Ofast -frounding-math" } */

float
foo (void)
{
  float a = 9.999999974752427078783512115478515625e-7f;
  float b = 1024.0f;
  float c = 2048.0f;
  return a + b + c;
}

would no longer be optimized into a single addition rather than 2.

	Jakub