[dpdk-dev] [PATCH] eal: fix large multiple calculation in reciprocal division

[pbhagavatula]

From: Pavan Nikhilesh <pbhagavat...@marvell.com>

Fix large multiple calculation in 64 bit reciprocal division.

Fixes: 6d45659eacb8 ("eal: add u64-bit variant for reciprocal divide")
Cc: sta...@dpdk.org

Reported-by: Stefan Kanthak <stefan.kant...@nexgo.de>
Signed-off-by: Pavan Nikhilesh <pbhagavat...@marvell.com>
---
 lib/librte_eal/common/rte_reciprocal.c | 76 ++++++--------------------
 1 file changed, 16 insertions(+), 60 deletions(-)

diff --git a/lib/librte_eal/common/rte_reciprocal.c 
b/lib/librte_eal/common/rte_reciprocal.c
index f017d0c28..920fbcdbe 100644
--- a/lib/librte_eal/common/rte_reciprocal.c
+++ b/lib/librte_eal/common/rte_reciprocal.c
@@ -60,72 +60,28 @@ struct rte_reciprocal rte_reciprocal_value(uint32_t d)
 }
 
 /*
- * Code taken from Hacker's Delight:
- * http://www.hackersdelight.org/hdcodetxt/divlu.c.txt
+ * Code based on from Hacker's Delight:
+ * http://www.hackersdelight.org/hdcodetxt/divluh.c.txt
  * License permits inclusion here per:
  * http://www.hackersdelight.org/permissions.htm
  */
 static uint64_t
-divide_128_div_64_to_64(uint64_t u1, uint64_t u0, uint64_t v, uint64_t *r)
+divlluh(uint64_t x, uint64_t y, uint64_t z)
 {
-       const uint64_t b = (1ULL << 32); /* Number base (16 bits). */
-       uint64_t un1, un0,           /* Norm. dividend LSD's. */
-                vn1, vn0,           /* Norm. divisor digits. */
-                q1, q0,             /* Quotient digits. */
-                un64, un21, un10,   /* Dividend digit pairs. */
-                rhat;               /* A remainder. */
-       int s;                       /* Shift amount for norm. */
-
-       /* If overflow, set rem. to an impossible value. */
-       if (u1 >= v) {
-               if (r != NULL)
-                       *r = (uint64_t) -1;
-               return (uint64_t) -1;
-       }
-
-       /* Count leading zeros. */
-       s = __builtin_clzll(v);
-       if (s > 0) {
-               v = v << s;
-               un64 = (u1 << s) | ((u0 >> (64 - s)) & (-s >> 31));
-               un10 = u0 << s;
-       } else {
-
-               un64 = u1 | u0;
-               un10 = u0;
-       }
-
-       vn1 = v >> 32;
-       vn0 = v & 0xFFFFFFFF;
-
-       un1 = un10 >> 32;
-       un0 = un10 & 0xFFFFFFFF;
-
-       q1 = un64/vn1;
-       rhat = un64 - q1*vn1;
-again1:
-       if (q1 >= b || q1*vn0 > b*rhat + un1) {
-               q1 = q1 - 1;
-               rhat = rhat + vn1;
-               if (rhat < b)
-                       goto again1;
-       }
-
-       un21 = un64*b + un1 - q1*v;
-
-       q0 = un21/vn1;
-       rhat = un21 - q0*vn1;
-again2:
-       if (q0 >= b || q0*vn0 > b*rhat + un0) {
-               q0 = q0 - 1;
-               rhat = rhat + vn1;
-               if (rhat < b)
-                       goto again2;
+       int64_t i;
+       uint64_t t;
+
+       for (i = 1; i <= 64; i++) {
+               t = x >> 63;
+               x = (x << 1) | (y >> 63);
+               y = y << 1;
+               if ((x | t) >= z) {
+                       x = x - z;
+                       y = y + 1;
+               }
        }
 
-       if (r != NULL)
-               *r = (un21*b + un0 - q0*v) >> s;
-       return q1*b + q0;
+       return y;
 }
 
 struct rte_reciprocal_u64
@@ -137,7 +93,7 @@ rte_reciprocal_value_u64(uint64_t d)
 
        l = 63 - __builtin_clzll(d);
 
-       m = divide_128_div_64_to_64((1ULL << l), 0, d, NULL) << 1;
+       m = divlluh((1ULL << l), 0, d) << 1;
        m = (1ULL << l) - d ? m + 2 : 1;
        R.m = m;
 
-- 
2.21.0