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

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Message-ID: <1303932382.18763.104.camel@gandalf.stny.rr.com>
Date:	Wed, 27 Apr 2011 15:26:22 -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 15:16 -0400, Steven Rostedt wrote:
> On Wed, 2011-04-27 at 21:51 +0300, Pekka Enberg wrote:

> 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   

I just noticed that 5 byte move instruction. So changing the function to
bool, makes it a little nicer:

bool streq(const char *cs, const char *ct)
{
        unsigned char c1, c2;

        while (1) {
                c1 = *cs++;
                c2 = *ct++;
                if (c1 != c2)
                        return false;
                if (!c1)
                        break;
        }
        return true;
}

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 08                   jne    1f9 <streq+0x1a>
 1f1:   84 c0                   test   %al,%al
 1f3:   75 ee                   jne    1e3 <streq+0x4>
 1f5:   b0 01                   mov    $0x1,%al
 1f7:   eb 02                   jmp    1fb <streq+0x1c>
 1f9:   31 c0                   xor    %eax,%eax
 1fb:   c9                      leaveq 
 1fc:   c3                      retq   

-- Steve


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