Naturally, fixed-width integer arithmetics can overflow. Chibi handles
it pretty well in general, but one case was missing: it is negation of
the minimal negative number that can be represented as a fixnum. That is,
sexp_fx_neg() must not be applied to sexp_make_fixnum(SEXP_MIN_FIXNUM)
because it overflows and returns an identical fixnum back.
sexp_fx_neg() itself seems to be used right in the current code, but
sexp_fx_abs()--which is defined in terms of sexp_fx_neg()--could be
applied to the forbidden number when used to retrieve an unboxed value
via the sexp_unbox_fixnum(sexp_fx_abs(x)) pattern. So I have added a
separate macro that safely calculates unboxed absolute value of a fixnum,
and replaced sexp_unbox_fixnum(sexp_fx_abs(x)) usages with it.
Current implementation uses two-bit tag for fixnums, plus we need one
bit for the sign, so fixnums have (machine word - 3) significant bits.
Regression tests cover word sizes of 16, 32, 64, and 128 bits (for the
sake of past- and future-proofness).
sexp_bignum_expt() does not have a regression test because we need to
check it with negative exponents like -2^29, so the base must be over
at least 2^(2^29) for the differences to be visible. Fun fact: bignum
representation of such number takes around 1/32 of the available user-
space memory, which makes testing on anything except 32-bit systems
unreasonable (4 TB of RAM anyone?)
