[Bug c/79973] New: atanh() constant-folds differently than it would evaluate at runtime

[arigo at tunes dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79973

            Bug ID: 79973
           Summary: atanh() constant-folds differently than it would
                    evaluate at runtime
           Product: gcc
           Version: 6.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arigo at tunes dot org
  Target Milestone: ---

Created attachment 40930
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40930&action=edit
reproducer, linux x86-64, gcc -lm

atanh(x), where x is a variable that happens to contain the value 0.3,
evaluates to a slightly different result than atanh(0.3) constant-folded at
computation time.  While this is allowed---and this may be yet another
duplicate of Bug 323---I still find it surprizing when there is no extra
computation involved and I'm running on a x86-64 (and not the 32-bit x86 fpu). 
See the reproducer.

Platform: Linux 64, gcc 6.3.1 20170109.

Compile the reproducer without any optimization (gcc -lm filename.c).  To
reproduce with optimizations, you need to work harder to hide the value of x
from the atanh(x), but the same occurs then.