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    SubjectRe: Is gcc thread-unsafe?


    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
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