From: Paolo Bonzini <pbonzini@redhat.com>
To: Aurelien Jarno <aurelien@aurel32.net>
Cc: qemu-devel@nongnu.org, Corentin Chary <corentin.chary@gmail.com>,
Richard Henderson <rth@twiddle.net>
Subject: Re: [Qemu-devel] [PATCH] bitops: provide an inline implementation of find_first_bit
Date: Fri, 20 Jun 2014 10:58:31 +0200 [thread overview]
Message-ID: <53A3F7B7.2030206@redhat.com> (raw)
In-Reply-To: <20140620084806.GB13901@ohm.rr44.fr>
Il 20/06/2014 10:48, Aurelien Jarno ha scritto:
> In practice on x86_64, this function takes 27 instructions in the
> general case, and 18 instructions in the fixed case, even for big
> sizes. I therefore think that checking if the size is constant is a good
> idea, but we should not make any test on the size itself and trust the
> compiler to correctly decide if the loop should be unrolled or not.
But if the size is large enough that the compiler will (likely) not
unroll the function, then it should pay off to use the more optimized
code in find_next_bit.
This of course is unless you expect find_first_bit to return a small
value and not be used in a loop; and dually expect find_next_bit's usage
to be more like walking sparser bitmaps in a loop.
This actually makes sense, and then there's no need to change anything.
Paolo
next prev parent reply other threads:[~2014-06-20 8:58 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-22 11:32 [Qemu-devel] [PATCH] bitops: provide an inline implementation of find_first_bit Aurelien Jarno
2013-12-22 17:04 ` Richard Henderson
2014-06-19 8:36 ` Paolo Bonzini
2014-06-20 8:48 ` Aurelien Jarno
2014-06-20 8:58 ` Paolo Bonzini [this message]
2014-06-20 9:43 ` Aurelien Jarno
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