Floating point exception in modulo

[Gwen Weinholt]

$ ikarus
Ikarus Scheme version 0.0.4-rc1+, 64-bit (revision 1806, build 2009-06-20)
Copyright (c) 2006-2009 Abdulaziz Ghuloum

> (modulo #xfffffffffffffff #x100000000)
4294967295
> (modulo #xffffffffffffffff #x100000000)
Floating point exception (core dumped)

[leppie (leppie)]

Works on 32-bit Cygwin. revision 1799

[Andreas Rottmann (rotty)]

Works on 64bit Debian GNU/Linux as well - revision 1806.

[Gwen Weinholt (weinholt)]

I found the bug on Debian GNU/Linux 5.0.1 amd64 and I've now reproduced it on another 64-bit machine with the same OS. Here's some extra info from ikarus compiled with -O0 -g3 (and not -DNDEBUG):

Ikarus Scheme version 0.0.4-rc1+, 64-bit (revision 1806, build 2009-06-24)
Copyright (c) 2006-2009 Abdulaziz Ghuloum

> (modulo #xffffffffffffffff #x100000000)
MMAP 0x00007f3bc84a7000 .. 0x00007f3bc84a7fff

Program received signal SIGFPE, Arithmetic exception.
0x00007f3bc98d058b in __gmpn_mod_1 () from /usr/lib/libgmp.so.3
(gdb) bt
#0 0x00007f3bc98d058b in __gmpn_mod_1 () from /usr/lib/libgmp.so.3
#1 0x0000000000412907 in ikrt_bnfx_modulo (x=139894750946565, y=34359738368) at ikarus-numerics.c:1821
#2 0x0000000000417dd0 in L_zero_args () at ikarus-enter.S:196
#3 0x0000000000000000 in ?? ()
(gdb) f 1
#1 0x0000000000412907 in ikrt_bnfx_modulo (x=139894750946565, y=34359738368) at ikarus-numerics.c:1821
1821 mp_limb_t m = mpn_mod_1(s2p, s2n, yint);
(gdb) p s2p
$1 = (mp_limb_t *) 0x7f3bc8efa508
(gdb) p *s2p
$2 = 18446744073709551615
(gdb) p s2n
$3 = 1
(gdb) p yint
$4 = 0

Looks like a division by zero. Here's an easier way to reproduce it that even works on the 32-bit build:
$ ikarus
Ikarus Scheme version 0.0.4-rc1+ (revision 1806, build 2009-06-23)
Copyright (c) 2006-2009 Abdulaziz Ghuloum

> (modulo 1 0)
Floating point exception (core dumped)

[Gwen Weinholt (weinholt)]

Looks like unfix() casts its value to 32 bits, which might be the trigger for the original problem on the 64-bit build:
#1 0x0000000000412907 in ikrt_bnfx_modulo (x=139894750946565, y=34359738368) at ikarus-numerics.c:1821
1798 ikptr
1799 ikrt_bnfx_modulo(ikptr x, ikptr y /*, ikpcb* pcb */){
1800 int yint = unfix(y);

#define wordshift ((wordsize == 4)?2:3)
#define fx_shift wordshift
#define unfix(x) (((long int)(x)) >> fx_shift)

(gdb) p (((long int) (34359738368)) >> 3)
$1 = 0
(gdb) p (((long long int) (34359738368)) >> 3)
$2 = 4294967296

[Gwen Weinholt (weinholt)]

Ok, it seems like gdb treats (long int) as 32 bits sometimes, depending on what program has been loaded possibly... so ignore that.
The problem is this:
1800 int yint = unfix(y);
If I change that to long int, then (modulo #xffffffffffffffff #x100000000) works correctly. But the division-by-zero can still be triggered if y==0, so a check needs to be added.

[Abdulaziz Ghuloum (aghuloum)]

Fixed in revision 1811. Thank you all for investigating the causes of this bug.

[Gwen Weinholt (weinholt)]

This bug is not completely fixed yet:

$ ikarus
Ikarus Scheme version 0.0.4-rc1+, 64-bit (revision 1815, build 2009-06-26)
Copyright (c) 2006-2009 Abdulaziz Ghuloum

> (modulo 1 0)
Floating point exception (core dumped)

[Abdulaziz Ghuloum (aghuloum)]

Fixed (again) in 1819. Thanks.