WG: [atomic] can it bechecked that boost::atomic<T> *v = reinterpret_cast<boost::atomic<T>*>(var); is safe?
On 15 Sep 2014, at 18:00, boost-users-request@lists.boost.orgmailto:boost-users-request@lists.boost.org wrote: On Monday, September 15, 2014 07:31 PM, Tim Odenthal wrote: Dear experts, essentially, I use this type of reinterpret_cast for doubles, and it __seems__ to work fine. I compile-time check that the sizeof(boost::atomic<T>) == sizeof( T ), but is that enough to ensure that this will always work? Should/could I check alignment issues as well? Even if the "atomic_cast" function has a (small) run-time overhead, it would be very much worth it in my setting, where I actually want to safely add to a double at some location in memory. Obviously, I want to use boost::atomic to be system/compiler agnostic, since it's supposed to eventually run with clang, gcc, (linux, OS X) as well as MS compilers, and so far we didn't see the need to move to C++11... I'm not sure I can figure out what problem you're trying to solve. Well, essentially, I have an array of doubles that at exactly one point in the code needs to be added to where I cannot ensure that each thread only adds to its own part of the array, so I want to ensure that each addition there is executed atomically. I think the cast is probably "safe", but far less can be said of it when it's dereferenced. That would depend entirely on the type of var. Is *var actually a boost::atomic<T> in the first place? If it is, why are you erasing its type? No, as I was saying, it’s a double *. How can you be sure there is nobody else aliasing var or *var and then reading or writing it? That is actually a problem, but since I completely control that part of the code, I hope to not make that mistake ;-). If *var is not a boost::atomic<T>, reinterpret_cast isn't going to magically convert it, you have undefined behaviour. Of course, one effect of UB is that it __seems__ to work, until, presumably, it doesn’t. But what, then, is the difference between an atomic<double>* and a normal double*, if the sizes (and possibly alignments) are the same? Hmmm, I just read up on what a reinterpret_cast<> is supposed to be good for, which is very little indeed, so it seems that the behaviour is undefined - but then there is no way to solve that problem? (So far I only ever used it to print the address of a pointer...) Since openMP allows for a pragma atomic, somehow, it seems that there should be a way. Using a simple spin_lock is way too slow (tested), since it doesn’t lock a specific memory location, but essentially everything, and I suspect that more complicated locking schemes will be also more expensive, just for different reasons. Ben Thanks for your insight! Tim
On Mon, Sep 15, 2014 at 10:33 AM, Tim Odenthal wrote: On 15 Sep 2014, at 18:00, boost-users-request@lists.boost.org wrote: On Monday, September 15, 2014 07:31 PM, Tim Odenthal wrote: Dear experts, essentially, I use this type of reinterpret_cast for doubles, and it
__seems__ to work fine. I compile-time check that the
sizeof(boost::atomic<T>) == sizeof( T ), but is that enough to ensure that
this will always work? Should/could I check alignment issues as well? Even
if the "atomic_cast" function has a (small) run-time overhead, it would be
very much worth it in my setting, where I actually want to safely add to a
double at some location in memory. Obviously, I want to use boost::atomic
to be system/compiler agnostic, since it's supposed to eventually run with
clang, gcc, (linux, OS X) as well as MS compilers, and so far we didn't see
the need to move to C++11... I'm not sure I can figure out what problem you're trying to solve. Well, essentially, I have an array of doubles that at exactly one point in
the code needs to be added to where I cannot ensure that each thread only
adds to its own part of the array, so I want to ensure that each addition
there is executed atomically. I think the cast is probably "safe", but far less can be said of it when
it's dereferenced. That would depend entirely on the type of var. Is *var actually a boost::atomic<T> in the first place? If it is, why
are you erasing its type? No, as I was saying, it’s a double *. How can you be sure there is nobody else aliasing var or *var and then
reading or writing it? That is actually a problem, but since I completely control that part of
the code, I hope to not make that mistake ;-). If *var is not a boost::atomic<T>, reinterpret_cast isn't going to
magically convert it, you have undefined behaviour. Of course, one
effect of UB is that it __seems__ to work, until, presumably, it doesn’t. But what, then, is the difference between an atomic<double>* and a normal
double*, if the sizes (and possibly alignments) are the same? Hmmm, I just read up on what a reinterpret_cast<> is supposed to be good
for, which is very little indeed, so it seems that the behaviour is
undefined - but then there is no way to solve that problem? (So far I only
ever used it to print the address of a pointer...)
Since openMP allows for a pragma atomic, somehow, it seems that there
should be a way. Using a simple spin_lock is way too slow (tested), since
it doesn’t lock a specific memory location, but essentially everything, and
I suspect that more complicated locking schemes will be also more
expensive, just for different reasons. These functions are probably close to what you want:
http://en.cppreference.com/w/cpp/atomic
Note, however, that atomic_is_lock_free<double> is unlikely to be true for
most platforms; You can try to alias your memory with 64-bit integers, and
use compare_exchange to ensure atomicity (as opposed to fetch_add, which
could be used if you natively had integers).
Brian
participants (2)
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Brian Budge -
Tim Odenthal