Re: GCC warnings for unused global variables

[Bernd Jendrissek <berndj at prism dot co dot za>]

On Fri, May 02, 2003 at 09:42:23AM -0700, Joe Buck wrote:
> On Fri, May 02, 2003 at 06:00:30PM +0200, Gabriel Dos Reis wrote:
> > | Because, since the object is volatile, it may affect the program in
> > | ways unknown to the compiler.
> > 
> > This is my favorite peeve against the definition of "volatile".
> > Does an alpha ray fired from Centauri count as a "way unknown to the
> > compiler"?
> 
> This is a much better example than you thought it was.  The answer is
> "yes".  Alpha particles are a major source of soft errors; they can
> spontaneously change the value that is stored in RAM.  Duplicated
> reads can't be eliminated because they might miss the change caused
> by the alpha particle.  A memory test might be written in C with
> a big volatile array, and one of the things it would be looking for
> is effects caused by alpha particles.

Thanks!  I hadn't thought of this kind of "unknown to the compiler" effect
until now.  What I was missing was that the "unknown effect" needn't
necessarily be deterministic, IOW just because the compiler can *prove*
that a variable isn't deterministically accessible doesn't mean that no
"unknown" accesses happen.  So now I'm convinced that in

void foo(void)
{
	int volatile bar = 42;
	int baz;
	baz = bar - bar;
}

there is no guarantee that baz == 0.  And that GCC may not play smarty-pants
even here.

Hmm, I seem to recall a similar thread last year or so...

http://gcc.gnu.org/ml/gcc/1998-06/msg00098.html (Hmm, no, too old.)

Here we go: [PATCH] C undefined behavior fix

http://gcc.gnu.org/ml/gcc/2002-01/msg00808.html is part of
http://gcc.gnu.org/ml/gcc/2002-01/msg00031.html

> (Of course, an alpha particle can't make it through the atmosphere,
> so you'll need to pick some other particle or photon type).

But IIRC alpha particles do (or did) come out of weird isotopes in the
ceramic casing around DRAM chips.

bernd