|Subject:||RE: [avr-gcc-list] Tip: handling volatile operands|
|Date:||Fri, 11 Jan 2008 10:40:44 -0500|
> From: address@hidden
> 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.
In cases such as these, a copy to a temporary would be required -- the value of the port could change between tests. If you fail to use a temporary, not only are you generating multiple reads to the port, but you are also opening a window where all the tests could fail when at least one of them should succeed.
Another solution in this particular case would be to replace the if-else cascade with a switch.
case 1: ...
case 2: ...
case 99: ...
This is (IMHO) a closer abstraction of what you actually want done. Though you have to be clever if you're going to mask bits like the original example. In which case an if-else cascade using a temporary would be better.
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