From: David Howells <dhowells@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: dhowells@redhat.com, David Miller <davem@davemloft.net>,
James.Bottomley@hansenpartnership.com, geert@linux-m68k.org,
linux-arch@vger.kernel.org
Subject: Re: Arch maintainers Ahoy!
Date: Thu, 24 May 2012 17:45:29 +0100 [thread overview]
Message-ID: <14266.1337877929@redhat.com> (raw)
In-Reply-To: <CA+55aFzwZ3mCeALQx7fXX_HO4GZF03q4gtZOFGmdWkJrvwjrJg@mail.gmail.com>
Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > Could you use cpu_to_be32/64() and then ffs()? That ought to work for both
> > variants of endianness. The cpu_to_beXX() should be a noop on BE and is
> > likely to be a single instruction on LE. The meat of ffs() is usually a
> > single instruction, though it may have to have zero-detect logic added.
>
> First off, the *last* thing you want to do is go to big-endian mode.
> All the bit counting gets *much* more complicated, and your argument
> that it's "free" on some architectures is pointless, since it is only
> free on the architectures that have the *least* users.
I didn't suggest it was free, but it might be cheaper. Besides x86/x86_64 has
BSF/BSR instructions - though having played with Dave's algorithm some, I
don't think they're usable for this.
My suggestion assumes that there would be zeros beyond the terminal NUL, which
isn't something you can rely on.
David
next prev parent reply other threads:[~2012-05-24 16:45 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-05-21 16:50 Arch maintainers Ahoy! (was Re: x86: faster strncpy_from_user()) Linus Torvalds
2012-05-23 5:46 ` Arch maintainers Ahoy! David Miller
2012-05-23 8:02 ` Geert Uytterhoeven
2012-05-23 9:40 ` James Bottomley
2012-05-23 15:15 ` Linus Torvalds
2012-05-23 17:21 ` David Miller
2012-05-23 17:32 ` Linus Torvalds
2012-05-23 18:16 ` David Miller
2012-05-23 18:27 ` Linus Torvalds
2012-05-23 18:35 ` David Miller
2012-05-23 18:44 ` Linus Torvalds
2012-05-23 18:46 ` Linus Torvalds
2012-05-23 20:36 ` David Miller
2012-05-23 21:01 ` Linus Torvalds
2012-05-24 2:11 ` David Miller
2012-05-24 5:25 ` Paul Mackerras
2012-05-24 5:56 ` David Miller
2012-05-24 9:40 ` David Howells
2012-05-24 15:53 ` Linus Torvalds
2012-06-13 11:08 ` Michael Cree
2012-06-13 14:51 ` Linus Torvalds
2012-05-24 16:45 ` David Howells [this message]
2012-05-24 16:56 ` Linus Torvalds
2012-05-24 17:16 ` David Howells
2012-05-23 17:19 ` David Miller
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