Re: GCC warnings for unused global variables

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

<tm_gccmail@mail.kloo.net> writes:

| On 2 May 2003, Gabriel Dos Reis wrote:
| 
| > Fergus Henderson <fjh@cs.mu.OZ.AU> writes:
| > 
| > | On 02-May-2003, Gabriel Dos Reis <gdr@integrable-solutions.net> wrote:
| > | > prj@po.cwru.edu (Paul Jarc) writes:
| > | > 
| > | > | Gabriel Dos Reis <gdr@integrable-solutions.net> wrote:
| > | > | > Fergus Henderson <fjh@cs.mu.OZ.AU> writes:
| > | > | >| 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).
| >                ^^^^^^^^^^^^^^^^^^^^^^^^^^^ 
| > 
| > | > | 
| > | > | Accessing a volatile object includes writing, and writing includes
| > | > | initializing.  So the initialization cannot be optimized out.
| > | > 
| > | > Why? the last part of the paragraph quoted above does specifically
| > | > permit that optimization where applicable.
| > | 
| > | In the case of accesses to volatile objects, it is never applicable.
| > 
| > Even when the standard writes it?  Hugh. 
| 
| Seemingly meaningless read/writes from/to volatile variables usually have

The discussion was not about "seemingly meaningless read/writes".
Please to not confuse the matter further.

-- Gaby