On Tue, 13 Feb 2018, Richard Biener wrote:

On February 13, 2018 6:51:29 PM GMT+01:00, Jakub Jelinek <ja...@redhat.com> 

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

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