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, [EMAIL PROTECTED] 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 ) )
{
}
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