eal: fix large multiple calculation in reciprocal division
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Commit Message
From: Pavan Nikhilesh <pbhagavatula@marvell.com>
Fix large multiple calculation in 64 bit reciprocal division.
Fixes: 6d45659eacb8 ("eal: add u64-bit variant for reciprocal divide")
Cc: stable@dpdk.org
Reported-by: Stefan Kanthak <stefan.kanthak@nexgo.de>
Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
---
lib/librte_eal/common/rte_reciprocal.c | 76 ++++++--------------------
1 file changed, 16 insertions(+), 60 deletions(-)
Comments
On Sun, 14 Apr 2019 10:52:59 +0530
<pbhagavatula@marvell.com> wrote:
> + int64_t i;
Since i only does 1..64 it can be int.
> + 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;
Pavan Nikhilesh <pbhagavatula@marvell.com> wrote Sunday, April 14, 2019 7:22 AM:
> From: Pavan Nikhilesh <pbhagavatula@marvell.com>
>
> Fix large multiple calculation in 64 bit reciprocal division.
This "fix" has a SERIOUS bug, it needs yet another fix.
I recommend to understand code first before you copy it!
[...]
> + int64_t i;
uint i = 64;
> + uint64_t t;
> +
> + for (i = 1; i <= 64; i++) {
do {
> + t = x >> 63;
t = (int64_t) x >> 63; // t is either 0ULL or ~0ULL
> + x = (x << 1) | (y >> 63);
> + y = y << 1;
> + if ((x | t) >= z) {
> + x = x - z;
> + y = y + 1;
> + }
} while (--i > 0);
Stefan Kanthak
@@ -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;