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Subject: Re: [linux-usb-devel] Re: [BUG/PATCH/RFC] Oops while completing async USB via usbdevio
Date: Tue, 11 Oct 2005 13:58:25 -0400
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Thread-Topic: [linux-usb-devel] Re: [BUG/PATCH/RFC] Oops while completing async USB via usbdevio
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From: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
To: "Paul Jackson" <pj@sgi.com>
Cc: "Alan Stern" <stern@rowland.harvard.edu>, <laforge@gnumonks.org>,
       <torvalds@osdl.org>, <chrisw@osdl.org>, <vsu@altlinux.ru>,
       <linux-usb-devel@lists.sourceforge.net>, <linux-kernel@vger.kernel.org>,
       <security@linux.kernel.org>, <vendor-sec@lst.de>
Reply-To: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
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On Tue, 11 Oct 2005, Paul Jackson wrote:

> Alan asked:
>> But why do people go to the
>> effort of confusing readers by using "^" instead of "!="?
>
> My guess - eor (^) was quicker than not-equal (!=) on a PDP-11.
>
> That code fragment for checking uid's has been around a -long-
> time, if my memory serves me.
>
> It's gotten to be like the infamous "!!" boolean conversion
> operator, a bit of vernacular that would be harder to read if
> recoded using modern coding style.
>
> --
>                  I won't rest till it's the best ...
>                  Programmer, Linux Scalability
>                  Paul Jackson <pj@sgi.com> 1.925.600.0401

Also, at one time, people used to spend a lot of time
minimizing the number of CPU cycles used in the code.

For instance, when it's appropriate, using XOR makes the
resulting generated code simpler and usually faster:

int funct0(int i)
{
     return i ^ 1;
}
int funct1(int i)
{
     return (i != 1);
}

int main()
{
     int i;
     for(i=0; i< 100; i++)
     {
         if(funct0(i))
             printf("Yep %d\n", i);
         if(funct1(i))
             printf("Yep %d\n", i);
     }
     return 0;
}

gcc -O2 -fomit-frame-pointer ...

Here, funct0 clearly uses less code.

Disassembly of section .text:

00000000 <funct0>:
    0:	8b 44 24 04          	mov    0x4(%esp),%eax
    4:	83 f0 01             	xor    $0x1,%eax
    7:	c3                   	ret

00000008 <funct1>:
    8:	31 c0                	xor    %eax,%eax
    a:	83 7c 24 04 01       	cmpl   $0x1,0x4(%esp)
    f:	0f 95 c0             	setne  %al
   12:	c3                   	ret
   13:	90                   	nop

00000014 <main>:
[SNIPPED...]




Cheers,
Dick Johnson
Penguin : Linux version 2.6.13 on an i686 machine (5589.44 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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