Re: GCC warnings for unused global variables

[Paul Koning <pkoning at equallogic dot com>]

>>>>> "Gabriel" == Gabriel Dos Reis <gdr@integrable-solutions.net> writes:

 Gabriel> Fergus Henderson <fjh@cs.mu.OZ.AU> writes: [...]

 >> > C++98 1.9/1:
 >> > 
 >> > The semantic descriptions in this International Standard define
 >> a > parameterized nondeterministic abstract machine. This
 >> International > Standard places no requirement on the structure of
 >> conforming > implementa-tions. In particular, they need not copy
 >> or emulate the > structure of the abstract machine. Rather,
 >> conform-ing > implementations are required to emulate (only) the
 >> observable > behavior of the abstract machine as explained
 >> below.5)
 >> >
 >> > There is nothing there that prevents the compiler to apply the
 >> "as-if" > rule to to volatile objects.  The only thinr that is
 >> required is to > emulate the observable behavuour".  The
 >> implementation is free to > implement whatever it likes as far as
 >> the observable behaviour is > preserved.
 >> 
 >> But reads and writes to volatile variables are defined to be *part
 >> of* "the observable behaviour" (C++98 1.9 paragraph 6).  So they
 >> can't be optimized away under the as-if rule.

 Gabriel> They can be optimized out if the program cannot tell the
 Gabriel> difference.

I can't tell if you're trying to say something that goes directly
against the intent of volatile or not, so let's talk about an example:

static volatile int foo;

void test (void)
{
	foo = 1;
	foo = 1;
}

Does your statement mean that you believe the compiler is allowed to
optimize this into:
void test (void)
{
	foo = 1;
}
?

The answer clearly *must* be NO, otherwise you've just defined
volatile into total uselessness.

	 paul