[Bug c++/60237] New: isnan fails with -ffast-math

[Bug c++/60237] New: isnan fails with -ffast-math

[nathanael.schaeffer at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60237

            Bug ID: 60237
           Summary: isnan fails with -ffast-math
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nathanael.schaeffer at gmail dot com

With -ffast-math, isnan should return true if passed a NaN value.
Otherwise, how is isnan different than (x!=x) ?

isnan worked as expected with gcc 4.7, but does not with 4.8.1 and 4.8.2

How can I check if x is a NaN in a portable way (not presuming any compilation
option) ?

[Bug c++/60237] isnan fails with -ffast-math

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60237

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Well, -ffast-math implies -ffinite-math-only, so the compiler is assuming no
NaNs or infinites are used as arguments/return values of any expression.  So,
if you have a program that produces NaNs anyway, you shouldn't be building it
with -ffast-math, at least not with -ffinite-math-only.

[Bug c++/60237] isnan fails with -ffast-math

[nathanael.schaeffer at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60237

--- Comment #2 from N Schaeffer <nathanael.schaeffer at gmail dot com> ---
Thank you for your answer.

My program (which is a computational fluid dynamics solver) is not supposed to
produce NaNs. However, when it does (which means something went wrong), I would
like to abort the program and return an error instead of continuing crunching
NaNs.
I also want it to run as fast as possible (hence the -ffast-math option).

I would argue that: if printf("%f",x) outputs "NaN", isnan(x) should also be
returning true.

Do you have a suggestion concerning my last question:
How can I check if x is NaN in a portable way (not presuming any compilation
option) ?

[Bug c++/60237] isnan fails with -ffast-math

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60237

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to N Schaeffer from comment #2)
> Do you have a suggestion concerning my last question:
> How can I check if x is NaN in a portable way (not presuming any compilation
> option) ?

This should bypass software optimizations. But if the hardware is put in a mode
that does strange things with NaN, it will be harder to work around.

int my_isnan(double x){
  volatile double y=x;
  return y!=y;
}

[Bug c++/60237] isnan fails with -ffast-math

[nathanael.schaeffer at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60237

--- Comment #4 from N Schaeffer <nathanael.schaeffer at gmail dot com> ---
int my_isnan(double x){
  volatile double y=x;
  return y!=y;
}

is translated to:
   0x0000000000406cf0 <+0>:     movsd  QWORD PTR [rsp-0x8],xmm0
   0x0000000000406cf6 <+6>:     xor    eax,eax
   0x0000000000406cf8 <+8>:     movsd  xmm1,QWORD PTR [rsp-0x8]
   0x0000000000406cfe <+14>:    movsd  xmm0,QWORD PTR [rsp-0x8]
   0x0000000000406d04 <+20>:    comisd xmm1,xmm0
   0x0000000000406d08 <+24>:    setne  al
   0x0000000000406d0b <+27>:    ret

which also fails to detect NaN, which is right according to the documented
behaviour of comisd:
http://www.jaist.ac.jp/iscenter-new/mpc/altix/altixdata/opt/intel/vtune/doc/users_guide/mergedProjects/analyzer_ec/mergedProjects/reference_olh/mergedProjects/instructions/instruct32_hh/vc44.htm

[Bug c++/60237] isnan fails with -ffast-math

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60237

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Dup.  You can try building with -fno-builtin-isnan, but no - that's not a
"portable" way.

*** This bug has been marked as a duplicate of bug 25975 ***

[Bug c++/60237] isnan fails with -ffast-math

[nathanael.schaeffer at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60237

--- Comment #6 from N Schaeffer <nathanael.schaeffer at gmail dot com> ---
-fno-builtin-isnan is also interesting, thanks.

Is there somewhere a rationale for not making isnan() find NaN's with
-ffinite-math-only ?

[Bug c++/60237] isnan fails with -ffast-math

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60237

--- Comment #7 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to N Schaeffer from comment #6)
> Is there somewhere a rationale for not making isnan() find NaN's with
> -ffinite-math-only ?

finite-math-only is basically a promise that isinf and isnan always return
false...

I have inline library functions that do: if (isnan(x)) ...
When compiled with -ffinite-math-only, I want this eliminated as dead code,
that's the reason I made the function inline in the first place.

Another way to cheat:
int myisnan(double)__asm__("isnan");