From: Linus Torvalds <torvalds@linux-foundation.org>
To: George Spelvin <linux@horizon.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [patch 2/7] lib/hashmod: Add modulo based hash mechanism
Date: Fri, 29 Apr 2016 18:12:29 -0700 [thread overview]
Message-ID: <CA+55aFzcKdp3o=JTS=jESD4sXK2Ee+Z_=aG1vGf4Y0EvZQSbVw@mail.gmail.com> (raw)
In-Reply-To: <20160430003206.25509.qmail@ns.horizon.com>
On Fri, Apr 29, 2016 at 5:32 PM, George Spelvin <linux@horizon.com> wrote:
>
> Odd is important. If the multiplier is even, the msbit of the input
> doesn't affect the hash result at all.
Fair enough. My test-set was incomplete.
>> Yeah. gcc will actually do the clever stuff for the 32-bit case, afaik.
>
> It's not as clever as it could be; it just does the same Booth
> recoding thing, a simple series of shifts with add/subtract.
Ahh. I thought gcc did the Bernstein's algorithm thing, which is
exponential in the bit size. That would have explained why it only
does it for 32-bit constants.
Not doing it for 64-bit constants makes no sense if it just uses the
trivial Booth's algorithm version.
So the odd "we don't do it for 64-bit" is apparently just an
oversight, not because gcc does something clever.
Oh well.
Linus
next prev parent reply other threads:[~2016-04-30 1:12 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-29 2:57 [patch 2/7] lib/hashmod: Add modulo based hash mechanism George Spelvin
2016-04-29 3:16 ` Linus Torvalds
2016-04-29 4:12 ` George Spelvin
2016-04-29 23:31 ` George Spelvin
2016-04-30 0:05 ` Linus Torvalds
2016-04-30 0:32 ` George Spelvin
2016-04-30 1:12 ` Linus Torvalds [this message]
2016-04-30 3:04 ` George Spelvin
[not found] <CA+55aFxBWfAHQNAdBbdVr+z8ror4GVteyce3D3=vwDWxhu5KqQ@mail.gmail.com>
2016-04-30 20:52 ` George Spelvin
2016-05-01 8:35 ` Thomas Gleixner
2016-05-01 9:43 ` George Spelvin
2016-05-01 16:51 ` Linus Torvalds
2016-05-14 3:54 ` George Spelvin
2016-05-14 18:35 ` Linus Torvalds
2016-05-02 7:11 ` Thomas Gleixner
-- strict thread matches above, loose matches on Subject: below --
2016-04-28 16:42 [patch 0/7] futex: Add support for process private hashing Thomas Gleixner
2016-04-28 16:42 ` [patch 2/7] lib/hashmod: Add modulo based hash mechanism Thomas Gleixner
2016-04-28 18:32 ` Linus Torvalds
2016-04-28 23:26 ` Thomas Gleixner
2016-04-29 2:25 ` Linus Torvalds
2016-04-30 13:02 ` Thomas Gleixner
2016-04-30 16:45 ` Eric Dumazet
2016-04-30 17:12 ` Linus Torvalds
2016-04-30 17:37 ` Eric Dumazet
2016-06-12 12:18 ` Sandy Harris
2016-04-29 21:10 ` Linus Torvalds
2016-04-29 23:51 ` Linus Torvalds
2016-04-30 1:34 ` Rik van Riel
2016-05-02 9:39 ` Torvald Riegel
2016-04-30 15:22 ` Thomas Gleixner
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