Specify default offset and size for getMappedRange

[kunalmohan]

Defaults are specified w.r.t buffer mapping_range.
While setting the default size, offset is checked with only lower bound inclusion to prevent zero-sized array buffers to be returned.

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Preview | Diff

[kunalmohan]

mapAsync offset should be a multiple of 4 whereas getMappedRange offset should be a multiple of 8. IIUC, the user will never be able to access the first 4 bytes of the mapping_range in case mapAsync offset is multiple of 4 but not 8. (maybe I'm wrong here)
Also if we keep the mapAsync offset to be multiple of 4, we should change the getMappedRange rangeOffset to-

If offset is unspecified, let rangeOffset be smallest multiple of 8 greater than or equal to this.[[mapping_range]][0].

for validation rules to apply.

I have added an issue for the same at the moment.

[kainino0x]

For the meeting, here's a concrete version of the proposal that I made above:

Two overloads:

ArrayBuffer getMappedRange();
ArrayBuffer getMappedRange(GPUSize64 offset, GPUSize64 size);

Eliminates ambiguities about what size=0 means, etc. and eliminates an (honestly useless) overload of this function.

[kainino0x]

Resolution: getMappedRange(optional GPUSize64 offset = 0, optional GPUSize64 size), with:

• offset is relative to whole buffer
• offset defaults to 0
• size defaults to buffer.size - offset

[kunalmohan]

So in case offset isn't specified and this.[[mapping_range]][0] > 0, we throw Operation Error?

[kainino0x]

Correct!

[kainino0x]

Small unresolved thing (didn't realize before): we should figure out whether it's

• optional GPUSize64 size, undefined means buffer.size, 0 means 0?
• optional GPUSize64 size = 0, 0 means buffer.size - offset

We can talk about this at the editors meeting in an hour.

[kainino0x]

• optional GPUSize64 size, undefined means buffer.size, 0 means 0?

This one, to keep the signalling out of band.

[kainino0x]

@kunalmohan Do you want to take care of this revision work, or would you like someone else to take care of it? Editors would be happy to take it.

[kunalmohan]

I would like to take a stab at it :)

[kainino0x]

You can view my nits commit without whitespace changes: 369b83a?diff=split&w=1

[kunalmohan]

This means that if size is specified, but offset is >= this.[[size]], then we use this.[[size]].

Not sure what your concern here was. I would interpret such a case to be using |rangeSize| = |size| (rather than this.[[size]]) which then throws validation Error because |offset| + |rangeSize| > this.[[mapping_range]][1]. But it certainly sounds more clear with your changes :)

Also, not completely sure but we might consider changing the language in mapAsync() as well if this is the case. (when the size is 0 there)

[kainino0x]

This means that if size is specified, but offset is >= this.[[size]], then we use this.[[size]].

Not sure what your concern here was.

As written:

If |size| is unspecified and |offset| is less than this.{{[[size]]}},
let |rangeSize| be this.{{[[size]]}} - |offset|. Otherwise, let |rangeSize| be |size|.

it would mean:

int rangeSize;
if (size == undefined && offset < this.size) {
  rangeSize = this.size - offset;
} else {
  rangeSize = size; // <- size could be undefined here
}

but the "offset > this.size" case has to be an error.

[kainino0x]

You're right about mapAsync needing a fix too.

[kunalmohan]

This means that if size is specified, but offset is >= this.[[size]], then we use this.[[size]].

Not sure what your concern here was.

As written:

If |size| is unspecified and |offset| is less than this.{{[[size]]}},
let |rangeSize| be this.{{[[size]]}} - |offset|. Otherwise, let |rangeSize| be |size|.

it would mean:

int rangeSize;
if (size == undefined && offset < this.size) {
  rangeSize = this.size - offset;
} else {
  rangeSize = size; // <- size could be undefined here
}

but the "offset > this.size" case has to be an error.

Ohh..I see. Thank you for clarifynig!

[kainino0x]

I would interpret such a case to be using |rangeSize| = |size| (rather than this.[[size]]) which then throws validation Error because |offset| + |rangeSize| > this.[[mapping_range]][1].

Oops, I misread this - I think you're right the original case I pointed out would have been caught correctly. But the opposite issue (size undefined and offset ≥ this.size) was still there.

[kvark]

One last thing from me