Hi,
I think I’ve found a bug in the handling of __builtin_nans() in GCC, but I am
aware that this is a tricky area, so before claiming so I would like to check
with the experts. Consider the following code:
$ cat v.c
#include <stdint.h>
#include <stdio.h>
#if 1
typedef double GFC_REAL_8;
#else
#define GFC_REAL_8 double
#endif
GFC_REAL_8 foo (void)
{
return __builtin_nans("");
}
int main (void) {
double x;
x = __builtin_nans ("");
printf("==> %lX\n", *(uint64_t *) &x);
x = foo ();
printf("==> %lX\n", *(uint64_t *) &x);
}
$ gcc v.c -W -Wall && ./a.out
==> 7FF4000000000000
==> 7FF8000000000000
My expectation is: the x variable should be assigned a signalling NaN, both
times, and therefore the code should output the same value twice. But as you
can see, the second time the NaN is converted to quiet.
What is even more troubling: this behavior only happens if GFC_REAL_8 is
typedef’ed to double. If we use the double type directly (change #if 1 to #if
0) then the output is as expected:
==> 7FF4000000000000
==> 7FF4000000000000
What is even more worrying is that, if you keep the typedef, but change the
function to go through a variable, then the signalling nan is returned
correctly:
typedef double GFC_REAL_8;
GFC_REAL_8 foo (void)
{
GFC_REAL_8 x = __builtin_nans("");
return x;
}
---------
The reason I ended up in this rabbit hole is that I am implementing some
handling of signalling NaNs in libgfortran. Could someone either confirm that
the behavior observed above is a bug, or if not, kindly explain to me why it
happens?
Thanks,
FX
