Re: GCC warnings for unused global variables

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

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.

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

[...]

| ignored the intent of volatile.  I don't think the committee ever
| intended to allow implementations to optimize away volatile variables,

Why not?  The C standard clearly says:

       [#3] In the abstract machine, all expressions are  evaluated
       as  specified  by  the  semantics.  An actual implementation
       need not evaluate part of an expression  if  it  can  deduce
       that  its  value is not used and that no needed side effects
       are produced (including any caused by calling a function  or
       accessing a volatile object).

That is clear.

| and so the lack of clear wording in the current standard explicitly
| prohibiting this may be just oversight.

or not.  See above.

-- Gaby