- SEXP_STACK had an off by one sexp_type_field_len_base past the top of stack
- SEXP_EXCEPTION claimed 6 slots but only 5 were present
- sexp_type_struct should have had "dl" slot at end
First we check for C99 support in Makefile.detect, looking for the
header we need and verifying whether it is the right one by using
a definition required by C99 standard to be present in that header.
uintN_t types are optional, but implementations are required to
provide corresponding limit #defines for the types they support,
so we can check for this with preprocessor only.
Finally, we define SEXP_UINTN_DEFINED for any sexp_uintN_t we have
so that the code can use #ifs to check for exact integer support.
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?)
