[M3devel] FW: optimizing range checks?

[hendrik at topoi.pooq.com]

On Tue, Mar 02, 2010 at 01:16:03PM +0000, Jay K wrote:
> 
> Hey that's pretty clever.
> 
> It costs a register, but given:
> 
>  
> 
>     if (b >= constantX|| b <= -constantY)
>         a = 0;
> 
>  
> 
> The C compiler instead does something like:
> 
>   if ((b + constantY - 1) > (constantX + constantY - 1))
> 
>     a = 0;
> 
>  
> 
> This is something the front end could do in many cases.?
> 
> Adding a constant to eliminate one of the compares and branches is a win.
> 
> If an x86 compiler will give up a register for this, then it is probably a win everywhere.
> 
>  
> 
> Granted, it probably requires silent overflow. Oh well.

It requires unsigned arithmetic; in particular the final compare has to 
be unsigned.

-- hendrik