Clarify default context behavior

[gmlueck]

Clarify that the queue constructors that do not take a context parameter use the platform's default context.

The WG decided to make this as a clarification to the SYCL 2020 specification because:

• DPC++ has always behaved this way, and AdaptiveCpp mostly behaves this way now.

• It would be difficult to enable this as an extension because it doesn't add any new API. Therefore, an application could end up relying on the "default context" behavior without the author of the application even being aware that the application is using extended behavior.

[gmlueck]

NOTE This PR is "stacked" on top of #606. Once #606 is merged, I'll retarget this PR to the "main" branch.

EDIT #606 has been merged now, so I retargeted this PR to the "main" branch.

[TApplencourt]

Potential rephrasing (which was a little simpler to read for me)

All [code]#queue# objects have an associated device, platform, and context which
are determined by the constructor.

The description of each constructor below tells how the associated device is chosen.
The associated platform is always the platform that contains this device.
When the queue constructor does not take a context object as a parameter, the queue is associated to the <<platform's default context>>.

And move <<platform's default context>>. to the platform section?

## Platform's default context

Each platform has a single context object which is used as its default context.
That context object contains all of the devices in the platform.

We repeat the The queue has the default context of the platform that contains this device. all the time.

So IMO we can remove the When the queue constructor does not take a context object as a parameter, the queue is associated to the <<platform's default context>>. then.

Or is this a tradeoff between duplicating the information and convenience/clarity for reader of the spec?

[gmlueck]

OK. How about this:

• Remove the big paragraph entirely (the one starting with "All queue objects have an associated device..."

• Add a short paragraph to the "Platform class" section defining the default context as you suggest.

• Change the descriptions of the constructors like:

  Effects: Constructs a queue object using the device selected by default_selector_v. The queue's platform is
  the platform that contains this device. The queue's context is this platform's default context as described in section X.Y.

[TApplencourt]

I like it!

[keryell]

Thanks.

[tomdeakin]

@illuhad Can you confirm if a single object is OK? Intent should be compare equal.

[tomdeakin]

@gmlueck to update!

[gmlueck]

I updated the wording in 2d63525.

Note that I also changed the wording to say that the default context has no async error handler. I presume that is our intent.

[TApplencourt]

I don't like the Each platform has a single context . A platform has no context; a context has a platform, not the other way around.

I would prefer the term "is associated to".

Applications can retrieve a copy of this default context object, for example,
by constructing a queue.

Maybe something like "A copy of 'default context' will be used when a context is implicitly created (for example by a the queue constructor (1))." ?

[gmlueck]

I don't like the Each platform has a single context . A platform has no context; a context has a platform, not the other way around.

The text you quote was removed in 2d63525. The new text is:

Each platform has a default context which contains all of the devices in the platform and has no associated asynchronous error handler.

Do you object to that text?

Maybe something like "A copy of 'default context' will be used when a context is implicitly created (for example by a the queue constructor (1))." ?

How about this:

Certain operations require the platform's default context to be created. For example, this happens for some of the queue constructors. When a platform's default context is created, it behaves as though it is a copy of a single internal context object that is associated with each platform.

[TApplencourt]

Sorry, my bad.

Do you object to that text?

Yes, I don't like the has. It sounds like an implementation leak to my ear. For me as == member variable

It's not like each platform has a default context.

It's more that when a queue is created, we will deduce a platform (following the rule in the table). Then, from this platform, we instantiate a context. This context is (copy?) an the instance of a singleton who have all the device of the platform.

So I will prefer is associated with.

When a platform's default context is created, it behaves as though it is a copy of a single internal context object that is associated with each platform.

What about:

Certain operations require a context to the implicitly created. For example, this happens for some of the queue constructors. The context associated with this queue is a copy of the default context associated with the platform of this queue.

Sorry for so much nitpicking... If I'm still not clear,I can accept your change, no problem. It's already better than before!

[gmlueck]

Note to reviewers: I made a slight change in 3a69290 to clarify that the default context contains only the platform's root devices. I think this is what we intended all along, but we should say that explicitly.