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

From: Dave Hansen
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. 

   switch (ioport)
    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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