Re: [avr-gcc-list] Tip: handling volatile operands

[Andrew Hutchinson]

Agreed. But my example was purposely "if", as "while" would indeed 
indicate that its volatile qualities were required.

The point I was trying to make (poorly)  was that io (and other 
variables) that may universally declared volatile,  may in fact have 
values that need to be used in a "non-volatile" fashion.

You have same problem with unrollable operations such as:

if (ioport == 1)
else if (ioport == 2)
else if (ioport == 99)

So, copy to temporary seems a more general solution - if "non volatile" 
access is required.

(I suppose casts would work but that might be a risky general 
assumption for all compilers)


David Kelly wrote:
> On Thu, Jan 10, 2008 at 02:24:46PM -0500, address@hidden wrote:
>   
> > I have come across a few instances where very inefficient c code is
> > created by volatile operands - often accidentally and thru no fault of
> > the compiler(s)
> >     
>
> ...
>
>   
> > For example:
> >
> >  while (ioport != 0)
> > {
> >
> > }
> >     
>
> ...
>
>   
> > If ioport were a "normal" variable, there would be one read and one
> > test!
> >
> > So, if you don't want the absolute latest value of a volatile,  use a
> > temporary!
> >
> > unsigned char a = ioport;
> >
> > if ((a& 1) ||  (a& 2) || (a& 0x0c))
> > {
> > }
> >
> > Will produce much better code!
> >     
>
> You changed horses in the middle of the stream. Replace "if" above with
> "while" that you started with and your non-volatile may loop forever
> based on one read.
>
> The solution is to roll your conditions into one test:
>
> while( ioport & ( 1 | 2 | 0x0c ) )
> {
> }
>
>