linux-kernel - Re: [PATCH] blkdev: use an efficient way to check merge flags

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Date:	Thu, 3 Apr 2014 16:00:44 +0800
From:	Zhan Jianyu <nasa4836@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	axboe@...nel.dk, hch@....de, viro@...iv.linux.org.uk,
	kmo@...erainc.com, namjae.jeon@...sung.com,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] blkdev: use an efficient way to check merge flags

On Thu, Apr 3, 2014 at 2:13 AM, Jan Kara <jack@...e.cz> wrote:
>   OK, but have you checked the generated code is actually any better? This
> is something I'd expect a compiler might be able to optimize anyway. And the
> original code looks more readable to me.

Hi, Jan,

I've disassemble the code on my x86_64 box
(it's inline though, I just look at its call site),
and found that this patch DOES make it more efficient.

Orig asm snippt                                               with
patch asm snippt
============                                              ================

mov    %edx,%ecx                                          mov    %rdx,%r9
xor    %r8d,%ecx                                             xor    %r8d,%r8d
test   $0x80,%cl                                              and    $0x380,%r9d
jne    14c5 <blk_rq_merge_ok+0x15>                test   $0x380,%ecx
and    $0x3,%ch                                              sete   %r8b
jne    14c5 <blk_rq_merge_ok+0x15>                cmp    %r8,%r9

je     14b5 <blk_rq_merge_ok+0x15>

This saves a branch.

Furthermore,  I found that gcc is smart enough to try to optimize the
code, so if we do
like this, it will generate the most optimal and smallest code :


static inline bool blk_check_merge_flags(unsigned int flags1,
                                        ¦unsigned int flags2)
{
        return ((flags1 ^ flags2) &
                (REQ_DISCARD | REQ_SECURE | REQ_WRITE_SAME))
                == 0;
}

this gives out  :

mov    %edx,%r8d
xor    %ecx,%r8d
and    $0x380,%r8d
jne    14a5 <blk_rq_merge_ok+0x15>

But yes, it compromises readibility.



Regards,
Jianyu Zhan
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