pow(2) returns slightly different result compared to other platforms' implementation

[Aleksej Lebedev]

Hi, everyone!

I'm porting ChezScheme that was recently opensourced to DragonFly.

I was able to compile it, but lots of tests are failing. At least some 
of them are due to (minor) bugs in DragonFly 
(http://bugs.dragonflybsd.org/issues/2951).

Also I noticed that one of the tests is failing because function pow(3) 
returns a different (wrong?) value compared to pow on other platforms.

On DragonFly:
a at kl:~/tmp$ uname -a
DragonFly kl.zta.lk 4.6-RELEASE DragonFly v4.6.0.10.g16fba-RELEASE #10: 
Wed Aug 17 14:26:31 CEST 2016 
root at kl.zta.lk:/usr/obj/usr/src/sys/X86_64_GENERIC  x86_64
a at kl:~/tmp$ cat powtest.c
#include <stdio.h>
#include <math.h>

void printrep(double x)
{
   unsigned long r = *(unsigned long *)&x;
   for (int i = 0; i != 64; ++i)
     putchar((r & (1lu << (63 - i))) ? '1' : '0');
   putchar('\n');
}

int main()
{
   double x, y;
   scanf("%lf%lf", &x, &y);
   printrep(pow(x, y));
   return 0;
}
a at kl:~/tmp$ cc powtest.c -lm
a at kl:~/tmp$ echo 10.0 -20.0 | ./a.out
0011101111000111100111001010000100001100100100100100001000100100

On Mac OSX the same program:
a at zdev:~/tmp$ uname -a
Darwin zdev.local 15.6.0 Darwin Kernel Version 15.6.0: Mon Aug 29 
20:21:34 PDT 2016; root:xnu-3248.60.11~1/RELEASE_X86_64 x86_64
a at zdev:~/tmp$ cc powtest.c
a at zdev:~/tmp$ echo 10.0 -20.0 | ./a.out
0011101111000111100111001010000100001100100100100100001000100011

I am not an expert in floating point arithmetic, more importantly not an 
expert in IEEE754. Maybe this is not a bug. Looks like a different 
rounding, but I'm not sure if it is allowed by IEE754. Does anyone know 
if it's OK? I can simply use powl instead of pow, this will make the 
test pass, but I would like not to create a workaround if this is really 
a bug.

-- 
Aleksej Lebedev