Vincent Lefevre <[email protected]> writes:
> On 2023-07-19 21:24:03 +0200, Niels Möller wrote:
>> I think that's needed, to be able to support any size of
>> MINI_GMP_LIMB_TYPE. Something like
>>
>> #define umullo_limb(u, v) \
>> (sizeof(mp_limb_t) >= sizeof(int)) ? (u)*(v) : (unsigned int)(u) * (v))
>>
>> If I understand you correctly, the two multiplies in
>> gmp_udiv_qrnnd_preinv and gmp_udiv_qr_3by2 are the only places where we
>> need a mullo operation, i.e., producing the low limb of a limb product?
>
> These are the only ones that affect MPFR, but I haven't looked
> at the whole code.
Can you try out the attached patch?
Regards,
/Niels
diff -r 73d9ef70d14f mini-gmp/mini-gmp.c
--- a/mini-gmp/mini-gmp.c Wed Jul 19 11:44:37 2023 +0200
+++ b/mini-gmp/mini-gmp.c Wed Jul 19 21:54:11 2023 +0200
@@ -172,12 +172,19 @@
} \
} while (0)
+/* If mp_limb_t is of size smaller than int, plain u*v implies
+ automatic promotion to *signed* int, and then multiply may overflow
+ and cause undefined behavior. Explicitly cast to unsigned int for
+ that case. */
+#define gmp_umullo_limb(u, v) \
+ ((sizeof(mp_limb_t) >= sizeof(int)) ? (u)*(v) : (unsigned int)(u) * (v))
+
#define gmp_udiv_qrnnd_preinv(q, r, nh, nl, d, di) \
do { \
mp_limb_t _qh, _ql, _r, _mask; \
gmp_umul_ppmm (_qh, _ql, (nh), (di)); \
gmp_add_ssaaaa (_qh, _ql, _qh, _ql, (nh) + 1, (nl)); \
- _r = (nl) - _qh * (d); \
+ _r = (nl) - gmp_umullo_limb (_qh, (d)); \
_mask = -(mp_limb_t) (_r > _ql); /* both > and >= are OK */ \
_qh += _mask; \
_r += _mask & (d); \
@@ -198,7 +205,7 @@
gmp_add_ssaaaa ((q), _q0, (q), _q0, (n2), (n1)); \
\
/* Compute the two most significant limbs of n - q'd */ \
- (r1) = (n1) - (d1) * (q); \
+ (r1) = (n1) - gmp_umullo_limb ((d1), (q)); \
gmp_sub_ddmmss ((r1), (r0), (r1), (n0), (d1), (d0)); \
gmp_umul_ppmm (_t1, _t0, (d0), (q)); \
gmp_sub_ddmmss ((r1), (r0), (r1), (r0), _t1, _t0); \
--
Niels Möller. PGP key CB4962D070D77D7FCB8BA36271D8F1FF368C6677.
Internet email is subject to wholesale government surveillance.
_______________________________________________
gmp-bugs mailing list
[email protected]
https://gmplib.org/mailman/listinfo/gmp-bugs
