RFR: 8282322: AArch64: Provide a means to eliminate all STREX family of instructions

[Andrew Haley]

On Wed, 18 May 2022 19:05:03 GMT, Dmitry Chuyko <dchuyko at openjdk.org> wrote:

> On AArch64 it is sometimes convenient to have LSE atomics right from the start. Currently they are enabled after feature detection and RR reverse debugger works incorrectly.
> 
> New build configuration feature 'hardlse' is added. If it is enabled for aarch64 type of build, then statically compiled stubs replace the initial pessimistic implementation and dynamically generated replacements (when LSE support is detected). The feature works for builds of all debug levels.
> 
> New file atomic_linux_aarch64_lse.S is derived from atomic_linux_aarch64.S and inherits its copyright. This alternative static implementation corresponds to the dynamically generated code.
> 
> Note, this configuration part is necessary but not sufficient to fully avoid strex instructions for practical purposes. Other parts are:
> 
> * Run on the OS built without strex family instructions. E.g. Amazon Linux 2022.
> * Compile with outline atomics enabled and the configuration flag enabled. E.g. configure with
> --with-extra-cflags='-march=armv8.3-a+crc+crypto -moutline-atomics' --with-extra-cxxflags='-march=armv8.3-a+crc+crypto -moutline-atomics' --with-extra-ldflags='-Wl,--allow-multiple-definition' --with-jvm-features=hardlse
> 
> Testing: tier1, tier2 on linux-aarch64 release builds with feature off and feature on.

This looks reasonable enough, but I take it that this would create an OpenJDK build that would not run on AArch64 systems without LSE instructions. Might it not be a better idea to build with outline-atomics, and use them in OpenJDK? That would also solve your problem, and we wouldn't have AArch64 Linux OpenJDK binaries which don't work on all systems.

Note that atomic_bsd_aarch64.hpp already does this.

I'm also concerned about code rot with options like this one. atomic_linux_aarch64_lse.S would not be tested in any standard configuration.

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PR: https://git.openjdk.java.net/jdk/pull/8779