ArrayBuffer.transfer strawman

ArrayBuffer.transfer

Proposal
--------
 
The proposal is to add a static ArrayBuffer.transfer(oldBuffer [, newByteLength]). This
method returns a new ArrayBuffer whose contents are taken from oldBuffer.[[ArrayBufferData]]
and then either truncated or zero-extended to be newByteLength. This operation leaves
oldBuffer in a detached state. If newByteLength is undefined, oldBuffer.byteLength is
used.
 
var buf1 = new ArrayBuffer(40);
new Int32Array(buf1)[0] = 42;
 
var buf2 = ArrayBuffer.transfer(buf1, 80);
assert(buf1.byteLength == 0);
assert(buf2.byteLength == 80);
assert(new Int32Array(buf2)[0] == 42);
 
var buf3 = ArrayBuffer.transfer(buf2, 0);
assert(buf2.byteLength == 0);
assert(buf3.byteLength == 0);
 
Motivation
----------
 
This proposal gives JS developers several new memory management powers:
 
+ The ability to detach an ArrayBuffer (with the assumption that the implementation will
release the backing memory). Currently, JS programs have to drop all references and wait
for finalization. In a tight 32-bit address space (or even 64-bit on mobile without paging)
this explicit control can be used to avoid accidental (and non-deterministic, because of
finalization) out-of-memory conditions.
 
+ The ability to grow an ArrayBuffer without copying, in the same manner as
realloc. The advantage over copying is:
- realloc can be two orders of magnitude faster for large buffers (because realloc doesn't
need to copy, it can remap pages)
- realloc can avoid out-of-memory in a fragmented 32-bit address space since realloc is
 
Any JS program that needs to manipulate large ArrayBuffers efficiently and/or work reliably
within a 32-bit address space (particularly if the address space is shared with other
libraries, iframes or tabs) could benefit from these added capabilities. Emscripten-ported
applications are a concrete example of this.
 
Implementation
--------------
 
Since this function is semantically just copying and detaching, it should be fairly easy to
implement without any broader impact on the engine.
 
(Credit goes to Dmitry Lomov for proposing ArrayBuffer.transfer as an alternative to 
ArrayBuffer.prototype.resize)

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