From: Bill Wendling <morbo@google.com>
To: dwarves@vger.kernel.org, bpf@vger.kernel.org
Cc: arnaldo.melo@gmail.com, Bill Wendling <morbo@google.com>
Subject: [PATCH] dwarf_loader: use a better hashing function
Date: Wed, 10 Feb 2021 15:23:27 -0800 [thread overview]
Message-ID: <20210210232327.1965876-1-morbo@google.com> (raw)
This hashing function[1] produces better hash table bucket
distributions. The original hashing function always produced zeros in
the three least significant bits.
The new hashing funciton gives a modest performance boost.
Original New
0:11.41 0:11.38
0:11.36 0:11.34
0:11.35 0:11.26
-----------------------
Avg: 0:11.373 0:11.327
for a performance improvement of 0.4%.
[1] From Numerical Recipes, 3rd Ed. 7.1.4 Random Hashes and Random Bytes
Signed-off-by: Bill Wendling <morbo@google.com>
---
hash.h | 25 ++++++++++---------------
1 file changed, 10 insertions(+), 15 deletions(-)
diff --git a/hash.h b/hash.h
index d3aa416..ea201ab 100644
--- a/hash.h
+++ b/hash.h
@@ -33,22 +33,17 @@
static inline uint64_t hash_64(const uint64_t val, const unsigned int bits)
{
- uint64_t hash = val;
+ uint64_t hash = val * 0x369DEA0F31A53F85UL + 0x255992D382208B61UL;
- /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
- uint64_t n = hash;
- n <<= 18;
- hash -= n;
- n <<= 33;
- hash -= n;
- n <<= 3;
- hash += n;
- n <<= 3;
- hash -= n;
- n <<= 4;
- hash += n;
- n <<= 2;
- hash += n;
+ hash ^= hash >> 21;
+ hash ^= hash << 37;
+ hash ^= hash >> 4;
+
+ hash *= 0x422E19E1D95D2F0DUL;
+
+ hash ^= hash << 20;
+ hash ^= hash >> 41;
+ hash ^= hash << 5;
/* High bits are more random, so use them. */
return hash >> (64 - bits);
--
2.30.0.478.g8a0d178c01-goog
next reply other threads:[~2021-02-10 23:24 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-10 23:23 Bill Wendling [this message]
2021-02-10 23:59 ` [PATCH] dwarf_loader: use a better hashing function Andrii Nakryiko
2021-02-11 1:24 ` Bill Wendling
2021-02-11 1:31 ` Andrii Nakryiko
2021-02-11 13:01 ` Arnaldo Carvalho de Melo
2021-02-12 6:55 ` Bill Wendling
2021-02-12 12:35 ` Arnaldo Carvalho de Melo
2021-02-12 8:01 ` [PATCH v2] " Bill Wendling
2021-02-12 12:37 ` Arnaldo Carvalho de Melo
2021-02-12 12:39 ` Arnaldo Carvalho de Melo
2021-02-12 20:14 ` Bill Wendling
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210210232327.1965876-1-morbo@google.com \
--to=morbo@google.com \
--cc=arnaldo.melo@gmail.com \
--cc=bpf@vger.kernel.org \
--cc=dwarves@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).