RFR: 8261027: AArch64: Support for LSE atomics C++ HotSpot code [v2]

[Andrew Haley]

On Tue, 9 Feb 2021 17:05:01 GMT, Volker Simonis <simonis at openjdk.org> wrote:

> But nevertheless I think implementing the default versions in native assembly isn't really simple and putting that Linux/gcc specific assembly code into the generic aarch64 directory `src/hotspot/cpu/aarch64` will break other aarch64 platforms like Windows and Mac.

OK. We can do Windows another way, and I will move the assembler stubs to Linux. 

> Why don't you leave the default implementation as simple wrappers for the C++ compiler intrinsics

Because the atomic stubs use a non-standard calling convention that only clobbers a few registers, so they can't be written in C++ because we can't control which registers the C++ compiler uses. If we were to use the native calling convention to call stubs we'd need to save and restore a ton of registers somehow - and not just the integer registers but also the vectors. It wouldn't be any simpler.

I do intend to provide lower-overhead versions of the Atomic functions in a later patch. This one does the LSE/non-LSE split without changing anything else.

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PR: https://git.openjdk.java.net/jdk/pull/2434