linux-kernel - Re: [PATCH] linux/string.h: Introduce streq macro.

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Message-ID: <1303931761.18763.101.camel@gandalf.stny.rr.com>
Date:	Wed, 27 Apr 2011 15:16:01 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thiago Farina <tfransosi@...il.com>,
	linux-kernel@...r.kernel.org,
	Alexey Dobriyan <adobriyan@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ingo Molnar <mingo@...e.hu>,
	"David S. Miller" <davem@...emloft.net>,
	Al Viro <viro@...iv.linux.org.uk>, Ted Ts'o <tytso@....edu>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] linux/string.h: Introduce streq macro.

On Wed, 2011-04-27 at 21:51 +0300, Pekka Enberg wrote:

> It's the same kind of API extension kstrdup(), for example, is.
> Whether or not we should it do it is a separate matter and I think the
> only reasonable argument for and against is whether it (a) reduces the
> number of bugs,

I did a quick search through the git logs, and found no bug fixes due to
the semantics. At least by the time it got to mainline, they are fixed
(which is a good thing).


>  (b) improves code readability significantly,

This is a matter of preference. I think I would prefer it, but obviously
others do not.


>  or (c)
> generates better code.

If we implement streq() separately from strcmp() it gets slightly
better:

00000000000001be <strcmp>:
 1be:   55                      push   %rbp
 1bf:   48 89 e5                mov    %rsp,%rbp
 1c2:   8a 07                   mov    (%rdi),%al
 1c4:   8a 16                   mov    (%rsi),%dl
 1c6:   48 ff c7                inc    %rdi
 1c9:   48 ff c6                inc    %rsi
 1cc:   38 d0                   cmp    %dl,%al
 1ce:   74 07                   je     1d7 <strcmp+0x19>
 1d0:   19 c0                   sbb    %eax,%eax
 1d2:   83 c8 01                or     $0x1,%eax
 1d5:   eb 06                   jmp    1dd <strcmp+0x1f>
 1d7:   84 c0                   test   %al,%al
 1d9:   75 e7                   jne    1c2 <strcmp+0x4>
 1db:   31 c0                   xor    %eax,%eax
 1dd:   c9                      leaveq 
 1de:   c3                      retq   

00000000000001df <streq>:
 1df:   55                      push   %rbp
 1e0:   48 89 e5                mov    %rsp,%rbp
 1e3:   8a 07                   mov    (%rdi),%al
 1e5:   8a 16                   mov    (%rsi),%dl
 1e7:   48 ff c7                inc    %rdi
 1ea:   48 ff c6                inc    %rsi
 1ed:   38 d0                   cmp    %dl,%al
 1ef:   75 0b                   jne    1fc <streq+0x1d>
 1f1:   84 c0                   test   %al,%al
 1f3:   75 ee                   jne    1e3 <streq+0x4>
 1f5:   b8 01 00 00 00          mov    $0x1,%eax
 1fa:   eb 02                   jmp    1fe <streq+0x1f>
 1fc:   31 c0                   xor    %eax,%eax
 1fe:   c9                      leaveq 
 1ff:   c3                      retq   


-- Steve


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