Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Alexey Dobriyan <adobriyan@gmail.com>
Cc:     Kaitao Cheng <pilgrimtao@gmail.com>, christian@brauner.io,
        akpm@linux-foundation.org, gladkov.alexey@gmail.com, guro@fb.com,
        walken@google.com, avagin@gmail.com, khlebnikov@yandex-team.ru,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
References: <20200527141155.47554-1-pilgrimtao@gmail.com>
        <87k10x5tji.fsf@x220.int.ebiederm.org>
        <20200527152340.GA19985@localhost.localdomain>
Date:   Wed, 27 May 2020 11:36:48 -0500
In-Reply-To: <20200527152340.GA19985@localhost.localdomain> (Alexey Dobriyan's
        message of "Wed, 27 May 2020 18:23:40 +0300")
Message-ID: <87k10x49nj.fsf@x220.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [PATCH] proc/base: Skip assignment to len when there is no error on d_path in do_proc_readlink.
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Alexey Dobriyan <adobriyan@gmail.com> writes:

> On Wed, May 27, 2020 at 09:41:53AM -0500, Eric W. Biederman wrote:
>> Kaitao Cheng <pilgrimtao@gmail.com> writes:
>> 
>> > we don't need {len = PTR_ERR(pathname)} when IS_ERR(pathname) is false,
>> > it's better to move it into if(IS_ERR(pathname)){}.
>> 
>> Please look at the generated code.
>> 
>> I believe you will find that your change will generate worse assembly.
>
> I think patch is good.
>
> Super duper CPUs which speculate thousands instructions forward won't
> care but more embedded ones do. Or in other words 1 unnecessary instruction
> on common path is more important for slow CPUs than for fast CPUs.

No.  This adds an entire extra basic block, with an extra jump.

A good compiler should not even generate an extra instruction for this
case.  A good compiler will just let len and pathname share the same
register.

So I think this will hurt your slow cpu case two as it winds up just
plain being more assembly code, which stress the size of the slow cpus
caches.


I do admit a good compiler should be able to hoist the assignment above
the branch (as we have today) it gets tricky to tell if hoisting the
assignment is safe.

> This style separates common path from error path more cleanly.

Very arguable.

[snip a completely different case]

Yes larger cases can have different solutions.

Eric