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Avoid overflow when doing sexp_fx_abs()
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?)
This commit is contained in:
ilammy
parent
3700b86470
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a1ec8ff493
3 changed files with 70 additions and 6 deletions
12
bignum.c
12
bignum.c
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@ -29,7 +29,7 @@ sexp sexp_make_bignum (sexp ctx, sexp_uint_t len) {
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sexp sexp_fixnum_to_bignum (sexp ctx, sexp a) {
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sexp res = sexp_make_bignum(ctx, 1);
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if (!sexp_exceptionp(res)) {
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sexp_bignum_data(res)[0] = sexp_unbox_fixnum(sexp_fx_abs(a));
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sexp_bignum_data(res)[0] = sexp_unbox_fx_abs(a);
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sexp_bignum_sign(res) = sexp_fx_sign(a);
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}
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return res;
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@ -326,9 +326,9 @@ sexp sexp_bignum_add_fixnum (sexp ctx, sexp a, sexp b) {
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sexp_gc_preserve1(ctx, c);
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c = sexp_copy_bignum(ctx, NULL, a, 0);
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if (sexp_bignum_sign(c) == sexp_fx_sign(b))
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c = sexp_bignum_fxadd(ctx, c, sexp_unbox_fixnum(sexp_fx_abs(b)));
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c = sexp_bignum_fxadd(ctx, c, sexp_unbox_fx_abs(b));
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else
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c = sexp_bignum_fxsub(ctx, c, sexp_unbox_fixnum(sexp_fx_abs(b)));
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c = sexp_bignum_fxsub(ctx, c, sexp_unbox_fx_abs(b));
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sexp_gc_release1(ctx);
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return c;
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}
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@ -599,7 +599,7 @@ sexp sexp_bignum_remainder (sexp ctx, sexp a, sexp b) {
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}
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sexp sexp_bignum_expt (sexp ctx, sexp a, sexp b) {
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sexp_sint_t e = sexp_unbox_fixnum(sexp_fx_abs(b));
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sexp_sint_t e = sexp_unbox_fx_abs(b);
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sexp_gc_var2(res, acc);
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sexp_gc_preserve2(ctx, res, acc);
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res = sexp_fixnum_to_bignum(ctx, SEXP_ONE);
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@ -1390,7 +1390,7 @@ sexp sexp_mul (sexp ctx, sexp a, sexp b) {
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r = (a==SEXP_ZERO ? a : sexp_make_flonum(ctx, sexp_fixnum_to_double(a)*sexp_flonum_value(b)));
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break;
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case SEXP_NUM_FIX_BIG:
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r = sexp_bignum_fxmul(ctx, NULL, b, sexp_unbox_fixnum(sexp_fx_abs(a)), 0);
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r = sexp_bignum_fxmul(ctx, NULL, b, sexp_unbox_fx_abs(a), 0);
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sexp_bignum_sign(r) = sexp_fx_sign(a) * sexp_bignum_sign(b);
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r = sexp_bignum_normalize(r);
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break;
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@ -1473,7 +1473,7 @@ sexp sexp_div (sexp ctx, sexp a, sexp b) {
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tmp = sexp_make_ratio(ctx, a, b);
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r = sexp_ratio_normalize(ctx, tmp, SEXP_FALSE);
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#else
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r = sexp_make_flonum(ctx, sexp_fixnum_to_double(a)/sexp_bignum_to_double(b));
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r = sexp_make_flonum(ctx, sexp_fixnum_to_double(a)/sexp_bignum_to_double(b));
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#endif
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break;
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case SEXP_NUM_FLO_FIX:
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@ -1182,6 +1182,8 @@ SEXP_API sexp sexp_symbol_table[SEXP_SYMBOL_TABLE_SIZE];
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#define sexp_fx_neg(a) (sexp_make_fixnum(-(sexp_unbox_fixnum(a))))
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#define sexp_fx_abs(a) ((((sexp_sint_t)a) < 0) ? sexp_fx_neg(a) : a)
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#define sexp_unbox_fx_abs(a) ((((sexp_sint_t)a) < 0) ? -sexp_unbox_fixnum(a) : sexp_unbox_fixnum(a))
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#define sexp_fp_add(x,a,b) (sexp_make_flonum(x, sexp_flonum_value(a) + sexp_flonum_value(b)))
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#define sexp_fp_sub(x,a,b) (sexp_make_flonum(x, sexp_flonum_value(a) - sexp_flonum_value(b)))
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#define sexp_fp_mul(x,a,b) (sexp_make_flonum(x, sexp_flonum_value(a) * sexp_flonum_value(b)))
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@ -143,6 +143,68 @@
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-9223372036854775808))
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(sign-combinations M7 (+ 1 (expt 2 64))))
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;; fixnum-bignum boundaries (machine word - 1 bit for sign - 2 bits for tag)
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(test 8191 (- -8191))
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(test 8192 (- -8192))
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(test 8193 (- -8193))
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(test 536870911 (- -536870911))
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(test 536870912 (- -536870912))
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(test 536870913 (- -536870913))
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(test 2305843009213693951 (- -2305843009213693951))
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(test 2305843009213693952 (- -2305843009213693952))
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(test 2305843009213693953 (- -2305843009213693953))
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(test 42535295865117307932921825928971026431 (- -42535295865117307932921825928971026431))
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(test 42535295865117307932921825928971026432 (- -42535295865117307932921825928971026432))
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(test 42535295865117307932921825928971026433 (- -42535295865117307932921825928971026433))
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(test '((536879104 -536862720 4398046511104 0 8192)
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(536862720 -536879104 -4398046511104 0 -8192)
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(-536862720 536879104 -4398046511104 0 8192)
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(-536879104 536862720 4398046511104 0 -8192))
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(sign-combinations (expt 2 13) (expt 2 29)))
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(test '((536879104 536862720 4398046511104 65536 0)
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(-536862720 -536879104 -4398046511104 -65536 0)
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(536862720 536879104 -4398046511104 -65536 0)
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(-536879104 -536862720 4398046511104 65536 0))
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(sign-combinations (expt 2 29) (expt 2 13)))
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(test '((2305843009750564864 -2305843008676823040 1237940039285380274899124224 0 536870912)
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(2305843008676823040 -2305843009750564864 -1237940039285380274899124224 0 -536870912)
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(-2305843008676823040 2305843009750564864 -1237940039285380274899124224 0 536870912)
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(-2305843009750564864 2305843008676823040 1237940039285380274899124224 0 -536870912))
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(sign-combinations (expt 2 29) (expt 2 61)))
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(test '((2305843009750564864 2305843008676823040 1237940039285380274899124224 4294967296 0)
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(-2305843008676823040 -2305843009750564864 -1237940039285380274899124224 -4294967296 0)
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(2305843008676823040 2305843009750564864 -1237940039285380274899124224 -4294967296 0)
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(-2305843009750564864 -2305843008676823040 1237940039285380274899124224 4294967296 0))
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(sign-combinations (expt 2 61) (expt 2 29)))
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(test '((42535295865117307935227668938184720384 -42535295865117307930615982919757332480
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98079714615416886934934209737619787751599303819750539264 0 2305843009213693952)
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(42535295865117307930615982919757332480 -42535295865117307935227668938184720384
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-98079714615416886934934209737619787751599303819750539264 0 -2305843009213693952)
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(-42535295865117307930615982919757332480 42535295865117307935227668938184720384
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-98079714615416886934934209737619787751599303819750539264 0 2305843009213693952)
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(-42535295865117307935227668938184720384 42535295865117307930615982919757332480
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98079714615416886934934209737619787751599303819750539264 0 -2305843009213693952))
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(sign-combinations (expt 2 61) (expt 2 125)))
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(test '((42535295865117307935227668938184720384 42535295865117307930615982919757332480
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98079714615416886934934209737619787751599303819750539264 18446744073709551616 0)
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(-42535295865117307930615982919757332480 -42535295865117307935227668938184720384
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-98079714615416886934934209737619787751599303819750539264 -18446744073709551616 0)
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(42535295865117307930615982919757332480 42535295865117307935227668938184720384
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-98079714615416886934934209737619787751599303819750539264 -18446744073709551616 0)
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(-42535295865117307935227668938184720384 -42535295865117307930615982919757332480
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98079714615416886934934209737619787751599303819750539264 18446744073709551616 0))
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(sign-combinations (expt 2 125) (expt 2 61)))
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(test #f (< +nan.0 +nan.0))
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(test #f (<= +nan.0 +nan.0))
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(test #f (= +nan.0 +nan.0))
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