I've been working to port our WebGL game engine (www.construct.net) to WebGPU and got as far as running some performance benchmarks. I found one which is ~10% slower with WebGPU than WebGL, and a notable difference in the profile is createBufferMapped() taking ~10% of CPU time. Where WebGL does gl.bufferSubData(), in WebGPU we use the pattern of: createBufferMapped, copy data, unmap, copyBufferToBuffer(), then destroy that buffer after the next submit().

I can see a lot of buffer upload discussion has already happened in #138, so I apologise if this ground has been covered. But it seems that createBufferMapped() adds CPU overhead in the JS environment relative to WebGL's gl.bufferSubData() in that it must both create and zero the contents of a new ArrayBuffer, and also later GC that.

Would it be possible to add a way to re-use an existing ArrayBuffer to eliminate this overhead? Instead of createBufferMapped() creating a new ArrayBuffer, perhaps you could pass one created in JS. Or more strictly, have a way to map the buffer for writing again, but with exactly the same ArrayBuffer you got from the original call to createBufferMapped(). The aim would essentially be to achieve the same buffer upload process, but without the WebGPU API calls having to create their own ArrayBuffer.