[Bug other/63387] New: Optimize pairs of isnan() calls into a single isunordered()

["jzwinck at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63387

            Bug ID: 63387
           Summary: Optimize pairs of isnan() calls into a single
                    isunordered()
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jzwinck at gmail dot com

Whenever a program does (isnan(x) || isnan(y)), GCC 4.7 and 4.9 in the best
case emits code like this on x86-64:

  ucomisd %xmm0, %xmm0 ; set parity flag if x is NAN
  setp    %dl          ; copy parity flag to %edx
  ucomisd %xmm1, %xmm1 ; set parity flag if y is NAN
  setp    %al          ; copy parity flag to %eax
  orl     %edx, %eax   ; OR one byte of each result into a full-width register

This is suboptimal because ucomisd already has the effect of checking if
*either* of its operands is NAN, so this code is equivalent:

  xorl    %eax, %eax
  ucomisd %xmm1, %xmm0
  setp    %al

And indeed that's what you get if you help GCC along by calling isunordered(x,
y) instead.  But no one thinks to do that, because isnan() is much more widely
used and it isn't an obvious thing at the source level to change pairs of
isnan() calls to isunordered().  So the optimizer should do it.

Someone already suggested that in GCC 5 it might be as simple as this (which
seems correct and safe to me):

  (simplify (or (unordered @0 @0) (unordered @1 @1)) (unordered @0 @1))

A discussion of this issue is here:
http://stackoverflow.com/questions/26053934/is-it-feasible-to-add-this-optimization-to-gcc