[Scm-discuss] 5f2 regression: r4rstest.scm fails on 32-bit

[Steve VanDevender]

Steve VanDevender writes:
 > This has been bugging me for a while too.  Here are some even more
 > precise cases down to successive integers that, when converted to real
 > numbers, print or do not (I also enabled one of the debugging printfs in
 > pdbl2str):
 > 
 > > (- (expt 2 49) 1)
 > 562949953421311
 > > 562949953421311.
 > mantissa = 1 -> #x1ffffffffffff0; e2 = 49 -> -4; point = -1; ndig = 53 -> 53
 > 5.62949953421311e14
 > > (expt 2 49)
 > 562949953421312
 > > 562949953421312.
 > mantissa = 0.5 -> #x10000000000000; e2 = 50 -> -3; point = 0; ndig = 53 -> 53
 > 
 > ;ERROR: "/home/stevev/scm/Transcen.scm": round-quotient: Wrong type in arg1 
 > (0 . 0)
 > ; in scope:
 > ;   (z1 z2)  procedure expt
 > ; defined by load: "/home/stevev/scm/Transcen.scm"

I've narrowed this down a bit further.  When SCM attempts to print
562949953421312. it calls scm_round_quotient at line 182 of scl.c:

179         /* try first with starved precision */
180         {
181           SCM den = scm_ash(MAKINUM(1L), MAKINUM(point - e2));
182           num = product(mant, VELTS(pows5)[- (long) point]);
183           bigrecy(mant);
184           quo = scm_round_quotient(num, den);
185           if (pmantexp2dbl(quo, point) != f) {

And then in scm_round_quotient it fails the assert at line 322:

319     #ifdef BIGDIG
320       if (NINUMP(num)) {
321         long w;
322         ASRTER(NIMP(num) && BIGP(num), num, ARG1, s_rquotient);
323         if (NINUMP(den)) {
324           ASRTGO(NIMP(den) && BIGP(den), badden);
325           return divbigbig(BDIGITS(num), NUMDIGS(num), BDIGITS(den), 
NUMDIGS(den),
326                            BIGSIGN(num) ^ BIGSIGN(den), 3);
327         }

I can't say I can really follow what the code is doing here, but from
the error SCM throws:

;ERROR: "/home/stevev/scm/Init5f2.scm": round-quotient: Wrong type in arg1 (0 . 
0)

This suggests that the parameter "num" now refers to a recycled bignum.
Unfortunately I don't really understand *how* it got into that state
from reading the code.