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It facilitates the cooperative interruption of Cranelift-compiled code, triggered by a background thread's twiddling of MMU protection bits on a polled page of memory. This is the first step toward implementing MMU-triggered interruption of Wasm (bytecodealliance#1749). Saúl did the aarch64 implementation. Co-authored-by: Saúl Cabrera <saulecabrera@gmail.com>
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This is the first step toward implementing MMU-triggered interruption of Wasm (originally suggested in #1749). It's the first little PR broken off the large #12990 for ease of review.
This PR introduces a Cranelift instruction that facilitates resumeable preemptions: in this case, with MMU-based Wasm interruption in mind. However, the same technique is used by HotSpot for stop-the-world GC, so there is potential for other applications.
The original name for this instruction was
dead_load_with_context. At the time, that was sufficiently descriptive. However, since then, we've added…Thus, it's become specialized enough to the task of implementing virtual-memory-triggered interruption mechanisms that it makes sense to rename it with that in mind. Other names I considered include "preempt_poll" (but it ultimately functions cooperatively, so nah), "yield_poll" (too tightly coupled to the one use case I'm chasing right now), and "probe" instead of "poll". HotSpot uses "poll" for their mechanism, so that'll be familiar to some, and it usefully suggests that it will have to be done repeatedly in the usual case.
I recommend reviewing this commit-by-commit.