Re: Example of the optimiser recognising a pattern

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.