Re: Is gcc thread-unsafe?

Linus Torvalds
2007-10-26T17:25:57Z

On Fri, 26 Oct 2007, Andrew Haley wrote:
>
> Bart Van Assche writes:
>
> > Andrew, do you know whether gcc currently contains any optimization
> > that interchanges the order of accesses to non-volatile variables
> > and function calls ?
>
> It sure does.

Note that doing so is perfectly fine.

But only for local variables that haven't had their addresses taken.

The fact is, those kinds of variables really *are* special. They are
provably not accessible from any other context, and re-ordering them (or
doing anything AT ALL to them - the most basic and very important
optimization is caching them in registers, of course) is always purely an
internal compiler issue.

But if gcc re-orders functions calls with *other* memory accesses, gcc is
totally broken. I doubt it does that. It would break on all but the most
trivial programs, and it would be a clear violation of even standard C.

HOWEVER: the bug that started this thread isn't even "reordering
accesses", it's *adding* accesses that weren't there (and please don't mix
this up with "volatile", since volatile is a totally unrelated issue and
has nothing what-so-ever to do with anything).

Linus