linux-kernel - Re: [PATCH 01/18] flag parameters: helper function

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Message-Id: <20080505195638.4c4bdc40.akpm@linux-foundation.org>
Date:	Mon, 5 May 2008 19:56:38 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	davidel@...ilserver.org, mtk.manpages@...il.com,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH 01/18] flag parameters: helper function

On Mon, 05 May 2008 19:39:34 -0700 Ulrich Drepper <drepper@...hat.com> wrote:

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> Hash: SHA1
> 
> Andrew Morton wrote:
> > hm, that looks expensive.  The compiler will need to generate a deref of m
> > and rf multiple times around the loop.  Copying them into locals does
> > improve that a lot.
> 
> There really is no problem.  The value is in L1d when it is reused.
> This is the generated code (%rdi is m):
> 
>    f:   85 17                   test   %edx,(%rdi)
>   11:   74 0b                   je     1e <flags_remap+0x1e>
>   13:   8b 47 04                mov    0x4(%rdi),%eax
>   16:   09 01                   or     %eax,(%rcx)

the deref of %rcx can be avoided.

>   18:   8b 07                   mov    (%rdi),%eax
>   1a:   f7 d0                   not    %eax
>   1c:   21 c2                   and    %eax,%edx
> 
> At address 18 the load will be satisfied from L1d.  If you would want to
> cache the value at address f you'd have to create one more instruction.
> 
> This really is the best code sequence.  The compiler could have chosen
> to move the value into a register because the array is const.  But it
> didn't.
> 
> 
> > Also: sorry, but ugh-at-the-naming.  We don't *gain* anything from having
> > idenitifers called f, of, m, n and rf.  And we lose quite a lot in
> > readability and understandability.  It would be much nicer to invest a
> > little bit more typing-time here, IMO.
> 
> That's Davide's code and I didn't change it because it doesn't really
> matter.  This is a trivial function which doesn't need more than 10
> seconds to be understood.  If you insist I'll rename the variables and
> elements but I consider this just busy work.

Well if the objective is saving work then why write any code at all?
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