RFR: 8363620: AArch64: reimplement emit_static_call_stub()

[Andrew Haley]

On Tue, 5 Aug 2025 10:30:13 GMT, Fei Gao <fgao at openjdk.org> wrote:

> In the existing implementation, the static call stub typically emits a sequence like:
> `isb; movk; movz; movz; movk; movz; movz; br`.
> 
> This patch reimplements it using a more compact and patch-friendly sequence:
> 
> ldr x12, Label_data
> ldr x8, Label_entry
> br x8
> Label_data:
>   0x00000000
>   0x00000000
> Label_entry:
>   0x00000000
>   0x00000000
> 
> The new approach places the target addresses adjacent to the code and loads them dynamically. This allows us to update the call target by modifying only the data in memory, without changing any instructions. This avoids the need for I-cache flushes or issuing an `isb`[1], which are both relatively expensive operations.
> 
> While emitting direct branches in static stubs for small code caches can save 2 instructions compared to the new implementation, modifying those branches still requires I-cache flushes or an `isb`. This patch unifies the code generation by emitting the same static stubs for both small and large code caches.
> 
> A microbenchmark (StaticCallStub.java) demonstrates a performance uplift of approximately 43%.
> 
> 
> Benchmark                       (length)   Mode   Cnt Master     Patch      Units
> StaticCallStubFar.callCompiled    1000     avgt   5   39.346     22.474     us/op
> StaticCallStubFar.callCompiled    10000    avgt   5   390.05     218.478    us/op
> StaticCallStubFar.callCompiled    100000   avgt   5   3869.264   2174.001   us/op
> StaticCallStubNear.callCompiled   1000     avgt   5   39.093     22.582     us/op
> StaticCallStubNear.callCompiled   10000    avgt   5   387.319    217.398    us/op
> StaticCallStubNear.callCompiled   100000   avgt   5   3855.825   2206.923   us/op
> 
> 
> All tests in Tier1 to Tier3, under both release and debug builds, have passed.
> 
> [1] https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/caches-self-modifying-code-working-with-threads

There may still be a race in `set_to_clean()`, which doesn't zero out the [method, entry} fields in the stub. I'd fix that, to be safe. It'd be tricky to test.

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

PR Comment: https://git.openjdk.org/jdk/pull/26638#issuecomment-3172045863