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

[Dmitry Chuyko]

On Fri, 20 May 2022 08:36:28 GMT, Magnus Ihse Bursie <ihse 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.
>
> make/autoconf/jvm-features.m4 line 442:
> 
>> 440:     JVM_FEATURES_VARIANT_FILTER="link-time-opt opt-size"
>> 441:   fi
>> 442:   # Filter out hardlse feature by default
> 
> You should not set up a PLATFORM filter in `JVM_FEATURES_PREPARE_VARIANT`. 
> 
> In fact, you will need a `JVM_FEATURES_CHECK_HARD_LSE`, which verifies that it can only be enabled on aarch64. See e.g. `JVM_FEATURES_CHECK_JVMCI` foor inspiration on how to write this and where to call it. I suggest you set up `JVM_FEATURES_PLATFORM_FILTER` in that function as well.

Magnus, thank you for the comments. I'm taking a direction suggested by Andrew and Nick -- linux-aarch64 only solution without build configuration feature, determined by actual compiler flags. Will update the PR shortly after some final testing.

-------------

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