On Wed, Feb 14, 2018 at 12:09:57PM +0100, 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> > > > 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. > > ./cc1 -quiet t.c -O -frounding-math -fassociative-math > cc1: warning: -fassociative-math disabled; other options take precedence

You need ./cc1 -quiet t.c -O -fassociative-math -fno-trapping-math -fno-signed-zeros -frounding-math > So _maybe_ we should disable these patterns for !flag_associative_math > when dealing with FP? We do, this is in block with: /* We can't reassociate floating-point unless -fassociative-math or fixed-point plus or minus because of saturation to +-Inf. */ (if ((!FLOAT_TYPE_P (type) || flag_associative_math) && !FIXED_POINT_TYPE_P (type)) But that doesn't mean you can't request associative math and rounding math at the same time. Jakub