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Re: Optimization of conditional access to globals: thread-unsafe?
On Sun, Oct 28, 2007 at 09:47:36 -0400, Robert Dewar wrote:
> Bart Van Assche wrote:
> >Requiring that all thread-shared variables should be declared
> >volatile is completely unacceptable.
> Why is this unacceptable .. seems much better to me than writing
> undefined stuff.
There's a parallel thread in the Linux Kernel Mailing List. Everyone
is advised to read it, if not already. There are several good points
- the problem is not limited to multithreaded domain: the page with
the object could be made read-only during execution, thus
if (! page_is_read_only)
v = 1;
would SIGSEGV for no apparent reason.
- making things volatile is unacceptable from performance POV.
- optimization in question might well turn out to be misoptimization
for anything but microbenchmarks (read LKML for cache flush/dirty
- "people knowledgeable in POSIX say that this optimization is
bogus". I would add that though we may say that Standard C is not
aware of threads, POSIX _is_ aware of Standard C. While POSIX
failed to solve the issue by formal word, its intent is clear: to
make POSIX Threads usable. The compiler that claims to be POSIX
compatible should take this into account.
- there's also a good talk on lawyer-ish vs attached-to-reality
approach. I personally doubt those who continue to advise to use
volatile are actually writing such multithreaded programs. Most
argue just for the fun of it.
> Well Hans is talking about C/C++, you are talking about some other
> language in which programs which do not have well defined semantics
> in C or C++ do have well defined semantics in your language.
Good thing we have this _bug_ in languages that define memory
semantics (Ada, Java), and no one yet argues that GCC should be fixed
wrt to only those languages.