linux-kernel - Discrepancy between comments for sched_find_first

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Message-ID: <b4198de61003282037p502e9fbfi62123088b9d73923@mail.gmail.com>
Date:	Sun, 28 Mar 2010 23:37:21 -0400
From:	Matt Turner <mattst88@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>, linux-alpha@...r.kernel.org
Subject: Discrepancy between comments for sched_find_first_bit

include/asm-generic/bitops/sched.h says
/*
 * Every architecture must define this function. It's the fastest
 * way of searching a 100-bit bitmap.  It's guaranteed that at least
 * one of the 100 bits is cleared.
 */

arch/alpha/include/asm/bitops.h says
/*
 * Every architecture must define this function. It's the fastest
 * way of searching a 140-bit bitmap where the first 100 bits are
 * unlikely to be set. It's guaranteed that at least one of the 140
 * bits is set.
 */

Is the guarantee that one of the first 100-bits set (and that the last
40 are useless?), or 140-bits? If it's just the first 100 bits we care
about, then the alpha version needs to be fixed.

I'm just checking this out, because gcc produces horrendous code for
sched_find_first_bit on alpha. I rewrote it in assembly and it's
better than 4 times faster.

Also, is it even worth optimizing that function? It looks like it's
only used in kernel/sched_rt.c.

Thanks,
Matt
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