Why not volatile on System.Double and System.Long?

Question

A question like mine has been asked, but mine is a bit different. The question is, "Why is the volatile keyword not allowed in C# on types System.Double and System.Int64, etc.?"

On first blush, I answered my colleague, "Well, on a 32-bit machine, those types take at least two ticks to even enter the processor, and the .Net framework has the intention of abstracting away processor-specific details like that." To which he responds, "It's not abstracting anything if it's preventing you from using a feature because of a processor-specific problem!"

He's implying that a processor-specific detail should not show up to a person using a framework that "abstracts" details like that away from the programmer. So, the framework (or C#) should abstract away those and do what it needs to do to offer the same guarantees for System.Double, etc. (whether that's a Semaphore, memory barrier, or whatever). I argued that the framework shouldn't add the overhead of a Semaphore on volatile, because the programmer isn't expecting such overhead with such a keyword, because a Semaphore isn't necessary for the 32-bit types. The greater overhead for the 64-bit types might come as a surprise, so, better for the .Net framework to just not allow it, and make you do your own Semaphore on larger types if the overhead is acceptable.

That led to our investigating what the volatile keyword is all about. (see this page). That page states, in the notes:

In C#, using the volatile modifier on a field guarantees that all access to that field uses VolatileRead or VolatileWrite.

Hmmm.....VolatileRead and VolatileWrite both support our 64-bit types!! My question, then, is,

"Why is the volatile keyword not allowed in C# on types System.Double and System.Int64, etc.?"

Answer 1

Not really an answer to your question, but...

I'm pretty sure that the MSDN documentation you've referenced is incorrect when it states that "using the volatile modifier on a field guarantees that all access to that field uses VolatileRead or VolatileWrite".

Directly reading or writing to a volatile field only generates a half-fence (an acquire-fence when reading and a release-fence when writing).

The VolatileRead and VolatileWrite methods use MemoryBarrier internally, which generates a full-fence.

Joe Duffy knows a thing or two about concurrent programming; this is what he has to say about volatile:

(As an aside, many people wonder about the difference between loads and stores of variables marked as volatile and calls to Thread.VolatileRead and Thread.VolatileWrite. The difference is that the former APIs are implemented stronger than the jitted code: they achieve acquire/release semantics by emitting full fences on the right side. The APIs are more expensive to call too, but at least allow you to decide on a callsite-by-callsite basis which individual loads and stores need the MM guarantees.)

Answer 2

He's implying that a processor-specific detail should not show up to a person using a framework that "abstracts" details like that away from the programmer.

If you are using low-lock techniques like volatile fields, explicit memory barriers, and the like, then you are entirely in the world of processor-specific details. You need to understand at a deep level precisely what the processor is and is not allowed to do as far as reordering, consistency, and so on, in order to write correct, portable, robust programs that use low-lock techniques.

The point of this feature is to say "I am abandoning the convenient abstractions guaranteed by single-threaded programming and embracing the performance gains possible by having a deep implementation-specific knowledge of my processor." You should expect less abstractions at your disposal when you start using low-lock techniques, not more abstractions.

You're going "down to the metal" for a reason, presumably; the price you pay is having to deal with the quirks of said metal.

Answer 3

Yes. Reason is that you even can't read double or long in one operation. I agree that it is poor abstraction. I have a feeling that reason was that reading them atomically requires effort and it would be too smart for compiler. So they let you choose the best solution: locking, Interlocked, etc.

Interesting thing is that they can actually be read atomically on 32 bit using MMX registers. This is what java JIT compiler does. And they can be read atomically on 64 bit machine. So I think it is serious flaw in design.

Answer 4

It's a simple explanation of legacy. If you read this article - http://msdn.microsoft.com/en-au/magazine/cc163715.aspx, you'll find that the only implementation of the .NET Framework 1.x runtime was on x86 machines, so it makes sense for Microsoft to implement it against the x86 memory model. x64 and IA64 were added later. So the base memory model was always one of x86.

Could it have been implemented for x86? I'm actually not sure it can be fully implemented - a ref of a double returned from native code could be aligned to 4 bytes instead of 8. In which case, all your guarantees of atomic reads/writes no longer hold true.

Answer 5

Starting from .NET Framework 4.5, it is now possible to perform a volatile read or write on long or double variables by using the Volatile.Read and Volatile.Write methods. Although it's not documented, these methods perform atomic reads and writes on the long/double variables, as it's evident from their implementation:

private struct VolatileIntPtr { public volatile IntPtr Value; }

[Intrinsic]
[NonVersionable]
public static long Read(ref long location) =>
#if TARGET_64BIT
    (long)Unsafe.As<long, VolatileIntPtr>(ref location).Value;
#else
    // On 32-bit machines, we use Interlocked, since an ordinary volatile read would not be atomic.
    Interlocked.CompareExchange(ref location, 0, 0);
#endif

Using these two methods is not as convenient as the volatile keyword though. Attention is required to not forget wrapping every read/write access of the volatile field in Volatile.Read or Volatile.Write respectively.