[Bug 1001344] Allow nc_test_slave for lwIP to compile for targets with 128KiB RAM.

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--- Comment #29 from Jonathan Larmour <jifl@ecoscentric.com> 2012-05-18 16:27:46 BST ---
(In reply to comment #28)
> 
> > I think that "volatile" is strong enough so compiler wouldn't dare to drop
> > it even for an automatic variable.

I know that isn't always true. For example, if you call
do_some_random_computation with a constant, gcc will optimise away the
calculation. And with some code, gcc may be able to optimise expressions into
constants that you may not expect it to be able to - it doesn't have to be a
literal constant, but an expression gcc has worked out can only have one value.

For the case of nc_test_slave.c it does appear to be fine for that particular
case - it's not called with a constant. But my point is that your statement is
not generally true that volatile is guaranteed to be respected. And if it isn't
generally true, what guarantee do we have that it will remain true in the case
of nc_test_slave.c's use of it as well.

Anyway...

> > I wonder if you can use both USED and UNUSED at the same time. Given their
> > intended functions, you should be able to, despite the English meanings
> > implying they are in some way opposites to each other. That would deal with the
> > uncertainty.
> 
> Then we get ...warning: ‘used’ attribute ignored... as per Comment #26.

In that case we'll have to trust the GCC people will indeed continue to respect
the volatile because we don't seem to have any alternative.

Jifl

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