[Bug target/57363] New: IBM long double: adding NaN and number raises inexact exception

[Bug target/57363] New: IBM long double: adding NaN and number raises inexact exception

[azanella at linux dot vnet.ibm.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57363

            Bug ID: 57363
           Summary: IBM long double: adding NaN and number raises inexact
                    exception
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: azanella at linux dot vnet.ibm.com

For IBM long double, adding a normal number to a NaN raises an inexact
exception. Adding any number to NaN should not raise any exception. The
following testcase triggers the issue (the testcase is meant to run a gnu
compatible libc):

--------------------------------------------------------------
$ cat gcc_testcase.c
#include <math.h>
#include <fenv.h>
#include <stdio.h>

double
sum (double x, long double y)
{
  return x + y;
}

int main ()
{
  feenableexcept (FE_INEXACT);

  double x = __builtin_nan ("");
  long double y = 1.1L;

  printf ("%e\n", sum (x, y));

  return 0;
}
$ gcc -O3 -m64 -fno-inline gcc_testcase.c -o gcc_testcase -lm
$ ./gcc_testcase 
Floating point exception (core dumped)
--------------------------------------------------------------

The issue is in __gcc_qadd implementation at
libgcc/config/rs6000/ibm-ldouble.c, 
if the number if non finite, there is not check if it a NaN before actually
summing all the components. A possible solution would be to add an extra test
and return the first sum if the number if not infinity:

Index: libgcc/config/rs6000/ibm-ldouble.c
===================================================================
--- libgcc/config/rs6000/ibm-ldouble.c  (revision 199159)
+++ libgcc/config/rs6000/ibm-ldouble.c  (working copy)
@@ -104,6 +104,8 @@

   if (nonfinite (z))
     {
+      if (z != inf())
+       return z;
       z = cc + aa + c + a;
       if (nonfinite (z))
        return z;

[Bug target/57363] IBM long double: adding NaN and number raises inexact exception

[rth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57363

Richard Henderson <rth at gcc dot gnu.org> changed:

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

--- Comment #1 from Richard Henderson <rth at gcc dot gnu.org> ---
(In reply to Adhemerval Zanella from comment #0)
> Adding any number to NaN should not raise any exception.

Not quite true -- QNaN should not raise any exception,
but SNaN should raise Invalid.

[Bug target/57363] IBM long double: adding NaN and number raises inexact exception

[azanella at linux dot vnet.ibm.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57363

--- Comment #2 from Adhemerval Zanella <azanella at linux dot vnet.ibm.com> ---
You are correct and I meant in my first comment QNaN for the NaN work. And I
believe the patch is still correct: for IBM long double SNaN will be
represented
by first double being the NaN (the second double is ignored) and the first sum
(z = a + c) will trigger exception if any.

[Bug target/57363] IBM long double: adding NaN and number raises inexact exception

[uweigand at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57363

Ulrich Weigand <uweigand at gcc dot gnu.org> changed:

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

--- Comment #3 from Ulrich Weigand <uweigand at gcc dot gnu.org> ---
Hi Adhemerval, I'm also seeing that this patch fixes some glibc failures.

What's the status of this?  Were you planning to submit it for inclusion?

B.t.w. I'm wondering if we don't need to use

+      if (fabs (z) != inf())
+       return z;

instead; z could still be minus infinity, right?

[Bug target/57363] IBM long double: adding NaN and number raises inexact exception

[uweigand at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57363

Ulrich Weigand <uweigand at gcc dot gnu.org> changed:

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

--- Comment #4 from Ulrich Weigand <uweigand at gcc dot gnu.org> ---
Fixed.
http://gcc.gnu.org/ml/gcc-cvs/2013-12/msg00087.html