On Wed, 14 Feb 2018, Richard Biener wrote:

On Tue, 13 Feb 2018, Marc Glisse wrote:

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.

./cc1 -quiet t.c -O -frounding-math -fassociative-math
cc1: warning: -fassociative-math disabled; other options take precedence

So _maybe_ we should disable these patterns for !flag_associative_math
when dealing with FP?

There is

 (if ((!FLOAT_TYPE_P (type) || flag_associative_math)
      && !FIXED_POINT_TYPE_P (type))

above, which I think covers this transformation.

Marc Glisse

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