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

[Magnus Ihse Bursie]

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.

Configure files need a bit more work.

make/autoconf/jvm-features.m4 line 47:

> 45:     ifdef([custom_jvm_features_valid], custom_jvm_features_valid) \
> 46:     \
> 47:     cds compiler1 compiler2 dtrace epsilongc g1gc hardlse jfr jni-check \

The feature should be named `hard-lse` to match the `HARD_LSE` define in Hotspot code. (Also, it improves readability.)

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.

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

Changes requested by ihse (Reviewer).

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