From b2e96b3086cc3033ddc31cc981b601ff5b9b5f08 Mon Sep 17 00:00:00 2001
From: Vitaly Chikunov <vt@altlinux.org>
Date: Thu, 30 Jan 2020 04:05:10 +0300
Subject: [PATCH] gosthash2012: Simpler version of add512
Similar to my commit into adegtyarev/streebog@432d5de.
---
gosthash2012.c | 46 +++++++++++++++++++++-------------------------
1 file 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;
--
2.39.5