From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dario Faggioli <dario.faggioli@citrix.com>
Subject: Re: [PATCH 0/5] use mask operations instead of
	test_bit()
Date: Fri, 2 Oct 2015 11:03:29 +0200
Message-ID: <1443776609.14525.41.camel@citrix.com>
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To: Juergen Gross <jgross@suse.com>
Cc: george.dunlap@eu.citrix.com, andrew.cooper3@citrix.com, keir@xen.org, jbeulich@suse.com, xen-devel <xen-devel@lists.xen.org>
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On Fri, 2015-10-02 at 06:40 +0200, Juergen Gross wrote:
> Instead of using test_bit() which is an atomic operation and limits
> the compiler's choices to do optimization, use logical ANDs with
> bitmasks where possible.
>=20
That's a good idea, I think.

> The possible candidates have been detected by searching definitions
> of bitmasks in the form:
>=20
> #define MASK  1 << _MASK
>=20
Right.

> On x86 the resulting code is slightly smaller (about 2 bytes for each
> case, checked via disassembly in few examples).
>=20
> I'm quite sure I didn't replace a test_bit() call required to be
> atomic, but I'd be grateful for a thorough review especially in the
> scheduler.
>=20
I'll have a deep look.

One question, can we introduce a __test_bit() macro/inline function,
like Jan did with __set_bit?

I've quickly-&-dirtily tested this:

#define __test_bit(nr, addr) ({  \
    unsigned _flags =3D 1 << nr;   \
    addr & _flags; \
})

and the result (I've checked a couple of cases) seems the same to me.

Regards,
Dario
--=20
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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