diff options
-rw-r--r-- | gosthash2012.c | 46 |
1 files changed, 21 insertions, 25 deletions
diff --git a/gosthash2012.c b/gosthash2012.c index 201fe31..3dacc35 100644 --- a/gosthash2012.c +++ b/gosthash2012.c @@ -57,34 +57,30 @@ static INLINE void add512(const union uint512_u *x, const union uint512_u *y, union uint512_u *r) { #ifndef __GOST3411_BIG_ENDIAN__ - unsigned int CF, OF; - unsigned long long tmp; + unsigned int CF; unsigned int i; CF = 0; - for (i = 0; i < 8; i++) - { - /* Detecting integer overflow condition for three numbers - * in a portable way is tricky a little. */ - - /* Step 1: numbers cause overflow */ - tmp = x->QWORD[i] + y->QWORD[i]; - - /* Compare with any of two summands, no need to check both */ - if (tmp < x->QWORD[i]) - OF = 1; - else - OF = 0; - - /* Step 2: carry bit causes overflow */ - tmp += CF; - - if (CF > 0 && tmp == 0) - OF = 1; - - CF = OF; - - r->QWORD[i] = tmp; + for (i = 0; i < 8; i++) { + const unsigned long long left = x->QWORD[i]; + unsigned long long sum; + + sum = left + y->QWORD[i] + CF; + /* + * (sum == left): is noop, because it's possible only + * when `left' is added with `0 + 0' or with `ULLONG_MAX + 1', + * in that case `CF' (carry) retain previous value, which is correct, + * because when `left + 0 + 0' there was no overflow (thus no carry), + * and when `left + ULLONG_MAX + 1' value is wrapped back to + * itself with overflow, thus creating carry. + * + * (sum != left): + * if `sum' is not wrapped (sum > left) there should not be carry, + * if `sum' is wrapped (sum < left) there should be carry. + */ + if (sum != left) + CF = (sum < left); + r->QWORD[i] = sum; } #else const unsigned char *xp, *yp; |