RFR: 8329331: Intrinsify Unsafe::setMemory [v6]

[Maurizio Cimadamore]

On Tue, 9 Apr 2024 23:45:39 GMT, Scott Gibbons <sgibbons at openjdk.org> wrote:

> Is there any way to disable some of the optimizations C2 will attempt on the IR? We need to maintain atomicity, so vectorization shouldn't occur, for instance. This seems like a rat-hole that would need constant maintenance as C2 optimizations get better.

Sorry, I do not know that (I'm not a C2 engineer :-) ). One small observation: how important is atomicity in the "full off-heap case" ? E.g. if a `setMemory` is occurring at a location that is provably off-heap (and we should have ways to detect that, we do that also for other unsafe memory access routines), then perhaps the atomicity requirement can go (as I suppose that requirement is there for the Java Memory Model) ?

So, perhaps, while we might not be able to fully optimize for on-heap access, we might be able to do so for off-heap access (which is an important case for FFM).

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PR Comment: https://git.openjdk.org/jdk/pull/18555#issuecomment-2047288498