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Programming > C > Re: Example of ...
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Re: Example of the optimiser recognising a pattern

by Ian Collins <ian-news@[EMAIL PROTECTED] > May 3, 2008 at 11:23 AM

Tomás Ó hÉilidhe wrote:
> I'm working with a microcontroller at the moment that has a single
> instruction for clearing a bit in a byte.
> 
> I started off with the following line of code:
> 
>   x &= ~0x8u;    /* Clear the 4th bit */
> 
> But then I changed it to the following because I thought I might get
> more efficient assembler out of it:
> 
>    x &= 0xF7u;   /* Clear the 4th bit */
> 
> Suprisingly, the compiler produced more efficient code for the latter,
> presumably because it recognises the pattern of " x &= ~y" for
> clearing a single bit.
> 
Odd, is x an unsigned 8 bit type?  If so, the two expressions should
generate identical code.

-- 
Ian Collins.
 




 9 Posts in Topic:
Example of the optimiser recognising a pattern
=?ISO-8859-1?Q?Tom=E1s_=D  2008-05-02 14:59:33 
Re: Example of the optimiser recognising a pattern
=?ISO-8859-1?Q?Tom=E1s_=D  2008-05-02 15:20:39 
Re: Example of the optimiser recognising a pattern
Ian Collins <ian-news@  2008-05-03 11:23:54 
Re: Example of the optimiser recognising a pattern
=?ISO-8859-1?Q?Tom=E1s_=D  2008-05-02 16:55:43 
Re: Example of the optimiser recognising a pattern
"Bartc" <bc@  2008-05-03 08:12:42 
Re: Example of the optimiser recognising a pattern
vippstar@[EMAIL PROTECTED  2008-05-03 07:16:16 
Re: Example of the optimiser recognising a pattern
Andrey Tarasevich <and  2008-05-03 09:45:25 
Re: Example of the optimiser recognising a pattern
vippstar@[EMAIL PROTECTED  2008-05-03 09:48:17 
Re: Example of the optimiser recognising a pattern
Andrey Tarasevich <and  2008-05-03 10:09:19 

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