linux-kernel - Re: [patch 2/7] lib/hashmod: Add modulo based hash mechanism

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Date:	Fri, 29 Apr 2016 18:12:29 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	George Spelvin <linux@...izon.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 2/7] lib/hashmod: Add modulo based hash mechanism

On Fri, Apr 29, 2016 at 5:32 PM, George Spelvin <linux@...izon.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