src,buffer: allow heap-allocated typed arrays #26301

This was added in 16f86d6, based on the assumption that otherwise, the memory behind `ArrayBuffer` instances could be moved around on the heap while native code holds references to it. This does not match what V8 actually does (and also did at the time): - The option/build variable was about always only about TypedArrays, not ArrayBuffers. Calls like `new ArrayBuffer(4)` call into C++ regardless of the option value, but calls like `new Uint8Array(4)` would not call into C++ under V8 defaults. - When first accessing a heap-allocated TypedArray’s `ArrayBuffer`, whether that is through the JS `.buffer` getter or the C++ `ArrayBufferView::Buffer()` function, a copy of the contents is created using the ArrayBuffer allocator and stored as the (permanent, unmovable) backing store. As a consequence, the memory returned by `ArrayBuffer::GetContents()` is not moved around, because it is fixed once the `ArrayBuffer` object itself first comes into explicit existence in any way. Removing this build option significantly speeds up creation of typed arrays from JS: $ ./node benchmark/compare.js --new ./node --old ./node-master --runs 10 --filter buffer-creation.js buffers | Rscript benchmark/compare.R confidence improvement accuracy (*) (**) (***) buffers/buffer-creation.js n=1024 len=10 type='buffer()' *** 593.66 % ±28.64% ±41.10% ±60.36% buffers/buffer-creation.js n=1024 len=10 type='fast-alloc-fill' *** 675.42 % ±90.67% ±130.24% ±191.54% buffers/buffer-creation.js n=1024 len=10 type='fast-alloc' *** 663.55 % ±58.41% ±83.87% ±123.29% buffers/buffer-creation.js n=1024 len=10 type='fast-allocUnsafe' 3.10 % ±9.63% ±13.22% ±18.07% buffers/buffer-creation.js n=1024 len=10 type='slow-allocUnsafe' 4.67 % ±5.55% ±7.77% ±10.97% buffers/buffer-creation.js n=1024 len=10 type='slow' -2.48 % ±4.47% ±6.12% ±8.34% buffers/buffer-creation.js n=1024 len=1024 type='buffer()' -1.91 % ±4.71% ±6.45% ±8.79% buffers/buffer-creation.js n=1024 len=1024 type='fast-alloc-fill' -1.34 % ±7.53% ±10.33% ±14.10% buffers/buffer-creation.js n=1024 len=1024 type='fast-alloc' 0.52 % ±5.00% ±6.87% ±9.40% buffers/buffer-creation.js n=1024 len=1024 type='fast-allocUnsafe' 0.39 % ±5.65% ±7.78% ±10.67% buffers/buffer-creation.js n=1024 len=1024 type='slow-allocUnsafe' -0.13 % ±5.68% ±7.83% ±10.77% buffers/buffer-creation.js n=1024 len=1024 type='slow' -5.07 % ±7.15% ±9.80% ±13.35% buffers/buffer-creation.js n=1024 len=2048 type='buffer()' 0.57 % ±2.70% ±3.74% ±5.16% buffers/buffer-creation.js n=1024 len=2048 type='fast-alloc-fill' -1.60 % ±4.96% ±6.79% ±9.25% buffers/buffer-creation.js n=1024 len=2048 type='fast-alloc' 1.29 % ±3.79% ±5.20% ±7.09% buffers/buffer-creation.js n=1024 len=2048 type='fast-allocUnsafe' 2.73 % ±8.79% ±12.05% ±16.41% buffers/buffer-creation.js n=1024 len=2048 type='slow-allocUnsafe' -0.99 % ±6.27% ±8.65% ±11.91% buffers/buffer-creation.js n=1024 len=2048 type='slow' -5.98 % ±6.24% ±8.71% ±12.20% buffers/buffer-creation.js n=1024 len=4096 type='buffer()' -1.75 % ±3.48% ±4.78% ±6.56% buffers/buffer-creation.js n=1024 len=4096 type='fast-alloc-fill' -3.18 % ±3.97% ±5.45% ±7.45% buffers/buffer-creation.js n=1024 len=4096 type='fast-alloc' 2.05 % ±4.05% ±5.58% ±7.65% buffers/buffer-creation.js n=1024 len=4096 type='fast-allocUnsafe' 1.44 % ±5.51% ±7.63% ±10.57% buffers/buffer-creation.js n=1024 len=4096 type='slow-allocUnsafe' * -4.77 % ±4.30% ±5.90% ±8.06% buffers/buffer-creation.js n=1024 len=4096 type='slow' -3.31 % ±6.38% ±8.86% ±12.34% buffers/buffer-creation.js n=1024 len=8192 type='buffer()' 0.06 % ±2.70% ±3.77% ±5.31% buffers/buffer-creation.js n=1024 len=8192 type='fast-alloc-fill' -1.20 % ±3.30% ±4.53% ±6.17% buffers/buffer-creation.js n=1024 len=8192 type='fast-alloc' -1.46 % ±2.75% ±3.84% ±5.38% buffers/buffer-creation.js n=1024 len=8192 type='fast-allocUnsafe' 1.27 % ±4.69% ±6.49% ±8.98% buffers/buffer-creation.js n=1024 len=8192 type='slow-allocUnsafe' -1.68 % ±3.30% ±4.62% ±6.49% buffers/buffer-creation.js n=1024 len=8192 type='slow' -2.49 % ±3.24% ±4.44% ±6.07% (Re-running the outlier with 30 runs instead of 10:) buffers/buffer-creation.js n=1024 len=4096 type='slow-allocUnsafe' 2.06 % ±2.39% ±3.19% ±4.15% The performance gains effect are undone once native code accesses the underlying ArrayBuffer, but then again that a) does not happen for all TypedArrays, and b) it should also make sense to look into using `ArrayBufferView::CopyContents()` in some places, which is made specifically to avoid such a performance impact and allows us to use the benefits of heap-allocated typed arrays. Refs: nodejs@16f86d6 Refs: nodejs#2893 Refs: nodejs@74178a5#commitcomment-13250880 Refs: http://logs.libuv.org/node-dev/2015-09-15

Where appropriate, use a helper that wraps around `ArrayBufferView::Buffer()` or `ArrayBufferView::CopyContents()` rather than `Buffer::Data()`, as that may help to avoid materializing the underlying `ArrayBuffer` when reading small typed arrays from C++. This allows keeping the performance benefits of the faster creation of heap-allocated small typed arrays in many cases.

Do not create an `ArrayBuffer` if the engine’s settings avoid it and we don’t need it.

Check that small typed arrays, including `Buffer`s (unless allocated by `Buffer.allocUnsafe()`), are indeed heap-allocated.

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src,buffer: allow heap-allocated typed arrays #26301

src,buffer: allow heap-allocated typed arrays #26301

Commits on Feb 25, 2019