[XviD-devel] Don't do " signed /= unsigned "

[Christoph Lampert]

On Sun, 12 Jan 2003, Radek Czyz wrote:

> Hi,
> 
> gruel wrote
> 
> > In branch motion_comp.c there is a line
> 
> > dx /= (1+quarterpel)
> 
> > and this gives me wrong results if dx is signed int (which it is)
> > and quarterpel is unsigned (which I declared that way, since it's a 0 or
> > 1 flag). Division is then done unsigned!
> 
> now you know why I made this signed ;)

Yes, I found it out the hard way... 
And btw. I just changed 'quarterpel' in decoder-struct to signed, too. 

> I think that division is faster than if - am I correct? 

I'm not so sure. In theory, division is better than branch, but 
since qpel is a constant for the whole sequence, branch prediction might
kick in and the if be faster in the end. We can simply measure it and
decide then. 

> However, I'm
> sure that some kind of bitshift would be even faster... do you know a
> way to make it a bitshift, but dividing negative values correctly?

Not really, maybe with an extra if ;-)

if (dx<0) dx++;
dx >>= qpel; 

;-p 


> PS: @gruel, how's your GME going? :)

Will be commited any minute. 

gruel