From: Alexey Dobriyan <adobriyan@gmail.com>
To: David Laight <David.Laight@ACULAB.COM>
Cc: "steffen.klassert@secunet.com" <steffen.klassert@secunet.com>,
"herbert@gondor.apana.org.au" <herbert@gondor.apana.org.au>,
"davem@davemloft.net" <davem@davemloft.net>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>
Subject: Re: [PATCH] xfrm: branchless addr4_match() on 64-bit
Date: Sat, 25 Mar 2017 19:37:12 +0300 [thread overview]
Message-ID: <20170325163712.GA4950@avx2> (raw)
In-Reply-To: <063D6719AE5E284EB5DD2968C1650D6DCFFB93A8@AcuExch.aculab.com>
On Fri, Mar 24, 2017 at 05:16:44PM +0000, David Laight wrote:
> From: Alexey Dobriyan
> > Sent: 23 March 2017 23:33
> > Current addr4_match() code has special test for /0 prefixes because of
> > standard required undefined behaviour. However, it is possible to omit
> > it on 64-bit because shifting can be done in a 64-bit register and then
> > truncated to the expected value (which is 0 mask).
> >
> > Implicit truncation by htonl() fits nicely into R32-within-R64 model
> > on x86-64.
> ...
> > static inline bool addr4_match(__be32 a1, __be32 a2, u8 prefixlen)
> > {
> > +#ifdef CONFIG_64BIT
> > + /* L in UL is not a typo. */
> > + return !((a1 ^ a2) & htonl(~0UL << (32 - prefixlen)));
> > +#else
> > /* C99 6.5.7 (3): u32 << 32 is undefined behaviour */
> > if (prefixlen == 0)
> > return true;
> > return !((a1 ^ a2) & htonl(0xFFFFFFFFu << (32 - prefixlen)));
> > +#endif
>
> Can't this just be written:
>
> if (sizeof (long) == 4 && prefixlen == 0)
Indeed.
> return true;
> return !((a1 ^ a2) & htonl(0xFFFFFFFFUL << (32 - prefixlen)));
0xFFFFFFFFUL is really long movabs, ~0UL is better.
next prev parent reply other threads:[~2017-03-25 16:37 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-23 23:32 [PATCH] xfrm: branchless addr4_match() on 64-bit Alexey Dobriyan
2017-03-24 17:16 ` David Laight
2017-03-25 16:37 ` Alexey Dobriyan [this message]
2017-03-25 16:41 ` [PATCH v2] " Alexey Dobriyan
2017-03-27 10:37 ` Steffen Klassert
2017-03-27 13:11 ` Alexey Dobriyan
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=20170325163712.GA4950@avx2 \
--to=adobriyan@gmail.com \
--cc=David.Laight@ACULAB.COM \
--cc=davem@davemloft.net \
--cc=herbert@gondor.apana.org.au \
--cc=netdev@vger.kernel.org \
--cc=steffen.klassert@secunet.com \
/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 an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.