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SubjectRe: [linux-usb-devel] Re: [BUG/PATCH/RFC] Oops while completing async USB via usbdevio
Date
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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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