diff options
| -rw-r--r-- | miscutils/bc.c | 31 |
1 files changed, 23 insertions, 8 deletions
diff --git a/miscutils/bc.c b/miscutils/bc.c index dd9f4f8..930baaa 100644 --- a/miscutils/bc.c +++ b/miscutils/bc.c @@ -2152,6 +2152,7 @@ static FAST_FUNC BC_STATUS zbc_num_p(BcNum *a, BcNum *b, BcNum *restrict c, size BcNum copy; unsigned long pow; size_t i, powrdx, resrdx; + size_t a_rdx; bool neg; // GNU bc does not allow 2^2.0 - we do @@ -2182,16 +2183,30 @@ static FAST_FUNC BC_STATUS zbc_num_p(BcNum *a, BcNum *b, BcNum *restrict c, size bc_num_init(©, a->len); bc_num_copy(©, a); + a_rdx = a->rdx; // pull it into a CPU register (hopefully) + // a is not used beyond this point if (!neg) { - if (a->rdx > scale) - scale = a->rdx; - if (a->rdx * pow < scale) - scale = a->rdx * pow; - } - - - for (powrdx = a->rdx; !(pow & 1); pow >>= 1) { + unsigned long new_scale; + if (a_rdx > scale) + scale = a_rdx; + new_scale = a_rdx * pow; + // Don't fall for multiplication overflow. Example: + // 0.01^2147483648 a_rdx:2 pow:0x80000000, 32bit mul is 0. +//not that it matters with current algorithm, it would OOM on such large powers, +//but it can be improved to detect zero results etc. Example: with scale=0, +//result of 0.01^N for any N>1 is 0: 0.01^2 = 0.0001 ~= 0.00 (trunc to scale) +//then this would matter: + // if (new_scale >= pow) is false, we had overflow, correct + // "new_scale" value is larger than ULONG_MAX, thus larger + // than any possible current value of "scale", + // thus "scale = new_scale" should not be done: + if (new_scale >= pow) + if (new_scale < scale) + scale = new_scale; + } + + for (powrdx = a_rdx; !(pow & 1); pow >>= 1) { powrdx <<= 1; s = zbc_num_mul(©, ©, ©, powrdx); if (s) goto err; |