From: Dave Hansen <dave.hansen@intel.com>
To: Zhou Chengming <zhouchengming1@huawei.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Cc: akpm@linux-foundation.org, tj@kernel.org, guohanjun@huawei.com,
huawei.libin@huawei.com
Subject: Re: [PATCH] make __section_nr more efficient
Date: Wed, 20 Jul 2016 14:36:04 -0700 [thread overview]
Message-ID: <578FEEC4.9060209@intel.com> (raw)
In-Reply-To: <1468988310-11560-1-git-send-email-zhouchengming1@huawei.com>
On 07/19/2016 09:18 PM, Zhou Chengming wrote:
> When CONFIG_SPARSEMEM_EXTREME is disabled, __section_nr can get
> the section number with a subtraction directly.
Does this actually *do* anything?
It was a long time ago, but if I remember correctly, the entire loop in
__section_nr() goes away because root_nr==NR_SECTION_ROOTS, so
root_nr=1, and the compiler optimizes away the entire subtraction.
So this basically adds an #ifdef and gets us nothing, although it makes
the situation much more explicit. Perhaps the comment should say that
this works *and* is efficient because the compiler can optimize all the
extreme complexity away.
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WARNING: multiple messages have this Message-ID (diff)
From: Dave Hansen <dave.hansen@intel.com>
To: Zhou Chengming <zhouchengming1@huawei.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Cc: akpm@linux-foundation.org, tj@kernel.org, guohanjun@huawei.com,
huawei.libin@huawei.com
Subject: Re: [PATCH] make __section_nr more efficient
Date: Wed, 20 Jul 2016 14:36:04 -0700 [thread overview]
Message-ID: <578FEEC4.9060209@intel.com> (raw)
In-Reply-To: <1468988310-11560-1-git-send-email-zhouchengming1@huawei.com>
On 07/19/2016 09:18 PM, Zhou Chengming wrote:
> When CONFIG_SPARSEMEM_EXTREME is disabled, __section_nr can get
> the section number with a subtraction directly.
Does this actually *do* anything?
It was a long time ago, but if I remember correctly, the entire loop in
__section_nr() goes away because root_nr==NR_SECTION_ROOTS, so
root_nr=1, and the compiler optimizes away the entire subtraction.
So this basically adds an #ifdef and gets us nothing, although it makes
the situation much more explicit. Perhaps the comment should say that
this works *and* is efficient because the compiler can optimize all the
extreme complexity away.
next prev parent reply other threads:[~2016-07-20 21:36 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-20 4:18 [PATCH] make __section_nr more efficient Zhou Chengming
2016-07-20 4:18 ` Zhou Chengming
2016-07-20 21:36 ` Dave Hansen [this message]
2016-07-20 21:36 ` Dave Hansen
2016-07-21 1:55 ` zhouchengming
2016-07-21 1:55 ` zhouchengming
2016-07-21 14:38 ` Dave Hansen
2016-07-21 14:38 ` Dave Hansen
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