linux-kernel - Re: [PATCH] make __section

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Message-ID: <578FEEC4.9060209@intel.com>
Date:	Wed, 20 Jul 2016 14:36:04 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Zhou Chengming <zhouchengming1@...wei.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:	akpm@...ux-foundation.org, tj@...nel.org, guohanjun@...wei.com,
	huawei.libin@...wei.com
Subject: Re: [PATCH] make __section_nr more efficient

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.