That comment is not the reason for the function’s existence, but the details of how the function interacts with GC.
The reason for the function’s existence is that it allows typed arrays to dynamically switch between a fast/compact representation for the case that the JSVM owns the data, and a slightly slower and slightly less compact version for when the JSVM allows native code to share ownership of the data.
This function, slowDownAndWasteMemory, switches to the less efficient version that allows aliasing with native code.
Of course the name is sarcastic. The actual effect of having this is that JSC can handle both the owned case and the aliased case, and get a small will if you’re in the owned case while being able to switch to the aliased case at any time. Since there’s no way to support the aliased case without some slightly higher memory/time usage, we are sort of literally slowing down and wasting memory when we go aliased.
Source: I’m pretty sure I wrote most of this code.