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WebAssembly.Memory: length, initial, maximum #1027

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WebAssembly.Memory: length, initial, maximum
jfbastien Mar 30, 2017
267e27a
Initialize slots
jfbastien Mar 30, 2017
621e946
s/length/byteLength/ and feature-test
jfbastien Mar 31, 2017
5289756
Update per comments
jfbastien May 12, 2017
37949fd
Back to .byteLength as asked by @domenic
jfbastien May 12, 2017
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41 changes: 38 additions & 3 deletions JS.md
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Expand Up @@ -7,6 +7,14 @@ be loaded and run directly from an HTML `<script type='module'>` tag—and
any other Web API that loads ES6 modules via URL—as part of
[ES6 Module integration](Modules.md#integration-with-es6-modules).)

## Post-MVP JavaScript API

Some JavaScript APIs were added post-MVP and can be
[feature-tested](FeatureTest.md) together. These are grouped together,
and each group is denoted with one of the following symbols:

* ⚜

## Traps

Whenever WebAssembly semantics specify a [trap](Semantics.md#traps),
Expand Down Expand Up @@ -510,11 +518,12 @@ call one with the `new` operator.

A `WebAssembly.Memory` object contains a single [linear memory](Semantics.md#linear-memory)
which can be simultaneously referenced by multiple `Instance` objects. Each
`Memory` object has two internal slots:
`Memory` object has three internal slots:

* [[Memory]] : a [`Memory.memory`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/memory.mli)
* [[BufferObject]] : the current `ArrayBuffer` whose [[ArrayBufferByteLength]]
matches the current byte length of [[Memory]]
* [[Maximum]] : the `maximum` number of WebAssembly pages, or `undefined` if `maximum` is `None`

### `WebAssembly.Memory` Constructor

Expand Down Expand Up @@ -559,8 +568,10 @@ Any attempts to [`detach`](http://tc39.github.io/ecma262/#sec-detacharraybuffer)
the detachment performed by [`m.grow`](#webassemblymemoryprototypegrow) shall throw a
[`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)

Return a new `WebAssembly.Memory` instance with [[Memory]] set to `m` and
[[BufferObject]] set to `buffer`.
Return a new `WebAssembly.Memory` instance with
[[Memory]] set to `m`,
[[BufferObject]] set to `buffer`, and
[[Maximum]] set to `undefined` if `maximum` is `None` or `maximum` otherwise.
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@littledan littledan Oct 26, 2017

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What's the motivation for storing the maximum here, rather than looking it up in the store?

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@jfbastien jfbastien Oct 27, 2017

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What do you mean "in the store"?

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@rossberg rossberg Oct 27, 2017

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He means that the memory instance m (living in the Wasm abstract store) already has this information, so the internal slot is redundant.


### `WebAssembly.Memory.prototype [ @@toStringTag ]` Property

Expand Down Expand Up @@ -608,6 +619,30 @@ accessor function performs the following steps:
If `this` is not a `WebAssembly.Memory`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown. Otherwise return `M.[[BufferObject]]`.

### `WebAssembly.Memory.prototype.byteLength`

[⚜](#Post-MVP-JavaScript-API)

This is an accessor property whose [[Set]] is Undefined and whose [[Get]]
accessor function performs the following steps:

Let `T` be the `this` value. If `T` is not a `WebAssembly.Memory`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

Return `T.[[BufferObject]].[[ArrayBufferByteLength]]`.
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@littledan littledan Oct 26, 2017

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What is the motivation for this feature, when a user can do memory.buffer.byteLength? Is it more optimizable if you never access the ArrayBuffer, or something like that?

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@jfbastien jfbastien Oct 27, 2017

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Yeah creating the array buffer is expensive-ish. We can optimize it away, but that's being extra silly IMO.


### `WebAssembly.Memory.prototype.maximum`

[⚜](#Post-MVP-JavaScript-API)

This is an accessor property whose [[Set]] is Undefined and whose [[Get]]
accessor function performs the following steps:

Let `T` be the `this` value. If `T` is not a `WebAssembly.Memory`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

Return `T.[[Maximum]]`.

## `WebAssembly.Table` Objects

A `WebAssembly.Table` object contains a single [table](Semantics.md#table)
Expand Down