Subject: Re: Bitfields and kernel
To: Matt Thomas <>
From: Mike Cheponis <mac@Wireless.Com>
List: tech-kern
Date: 09/30/1999 20:17:23
On Thu, 30 Sep 1999, Matt Thomas wrote:

> At 09:57 PM 9/30/99 , Mike Cheponis wrote:
> >I like:  if ((msr&POW_BIT) && !(msr&ILE_BIT)) or
> >
> >          if (msr.pow && !msr.ile)
> >Let the compiler smash the logicals & bits together efficiently.
> But it can't.  The C language specifies that msr.pow if evaluates to
> false then !msr.ile must not be evaluated.  This is so you can do
>          if (foo != NULL && foo->bar)
> Things get even nastier if you through volatile into the mess.

The point is that the C compiler can often optimize, and it should.  In 
this case, you are exactly right, there must be a couple of compares,
bitfields or no.

Imagine  if(msr.pow ||

could be easily coded as a single constant test against msr.

-Mike Cheponis <mac@Wireless.Com>