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Date: Tue, 25 Jan 2022 11:59:28 -0800
From: Steve Kargl <sgk@troutmask.apl.washington.edu>
To: FX <fxcoudert@gmail.com>
Cc: fortran@gcc.gnu.org
Subject: Re: New signaling NaN causes 12 testsuite failures
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On Tue, Jan 25, 2022 at 09:05:55AM +0100, FX wrote:
> 
> > Got the following in testsuite/gfortran/gfortran.log
> > 
> > NaN 7FFFA000000000000000
> > NaN 7FFFC000000000000000
> > NaN 7FFFA000000000000000
> > 

Could be a problem with __builtin_nansl().

#include <stdio.h>
#include <stdint.h>

int
main(void)
{
   union {       float x; uint32_t i; }    f;
   union {      double x; uint64_t i; }    d;
   union { long double x; uint64_t i[2]; } l;

   printf("Quiet NaN\n");
   f.x = __builtin_nanf("");
   printf("%f %x\n", f.x, f.i);
   d.x = __builtin_nan("");
   printf("%lf %lx\n", d.x, d.i);
   l.x = __builtin_nanl("");
   printf("%Lf %lx%lx\n", l.x, l.i[1], l.i[0]);

   printf("Signaling NaN\n");
   f.x = __builtin_nansf("");
   printf("%f %x\n", f.x, f.i);
   d.x = __builtin_nans("");
   printf("%lf %lx\n", d.x, d.i);
   l.x = __builtin_nansl("");
   printf("%Lf %lx%lx\n", l.x, l.i[1], l.i[0]);

   return 0;
} 

% ~/work/x/bin/gcc -o z a.c && ./z
Quiet NaN
nan 7fc00000
nan 7ff8000000000000
nan 7fffc000000000000000
Signaling NaN
nan 7fa00000
nan 7ff4000000000000
nan 7fffa000000000000000

s bit is 0, so the 7 is correct.  The width of the 
exponet is w = 8, 11, and 15 bits for float, double,
and long double.  The first significant bit, d, is
then 9, 12, and 16.
         s|----w---|d
7fc  --> 0111 1111 1100
7fa  --> 0111 1111 1010
         s|-----w-----| d
7ff8 --> 0111 1111 1111 1000
7ff4 --> 0111 1111 1111 0100
         s|-------w--------| d
7fffc -> 0111 1111 1111 1111 1100 <-- should be 7fff8?
7fffa -> 0111 1111 1111 1111 1010 <-- should be 7fff4?

What does linux/darwin show?

-- 
Steve