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

[bnewbold]

Hello!

I've had trouble getting scm 5f2 to pass r4rstests.scm on several 32-bit 
platforms. 5f1 works fine, and the test that fails has not changed. The 
failure looks like, after building vanilla sources and using default 
configuration (except pointing to a slightly old system-wide copy of 
slib):

;ERROR: "r4rstest.scm": round-quotient: Wrong type in arg1 (0 . 0)
; in expression: (address@hidden>string address@hidden)
; in scope:
;   (num)  procedure <anon>
;   (estr)  procedure <anon>
;   (cnt)  procedure <anon>
;   (#<id loop:b75379d0> cnt)  procedure <anon>
[SNIP]
;   (6)
;   (success . address@hidden)
;   ()  procedure float-rw-range-test
;   (log2 slow-frexp float-precision float-print-test 
mult-float-print-test float-rw-range-test . address@hidden)
;   (f0.0 f0.5 f1.0 f2.0)  procedure <anon>
;   ()  procedure test-inexact-printing
; defined by load: "r4rstest.scm"

A minimal test to cause this failure on a 32-bit machine is:

  (display (string->number "1.e15"))


Commenting out the body of `bigrecy()` on 32-bit platforms gets 
r4rstests.scm to pass, which might help with debugging (patch attached).

Some armchair speculation:

It's not a simple size problem: "1.e12", "1.e20", and "1.e300" all work 
fine.

After some debugging with gdb, my hypothesis is that there is a "use after 
free()"-style bug with the new `bigrecy()` optmization, which results in 
the 'round-quotient' call at scl.c:184 happening after `num` has somehow 
accidentially been "recycled" and reset to an empty pair. This failure 
mode doesn't happen on 64-bit machines because `num` fits as an immediate 
integer, instead of being a pointer to a bignum (i'm probably mangling 
terminology here).

I don't know if the problem is actually a use-after-free situation (in 
which case it might be lurking on 64-bit platforms) or a problem with 
`bigrecy()` overwriting adjacent memory locations or something (in which 
case it wouldn't be).

--bryan

PS, I stumbled across this while updating the Debian package for scm.

bypass_bigrecy_32bit.patch
Description: Text Data