RFR: 8358329: AArch64: emit direct branches in static stubs for small code caches

[Andrew Haley]

On Mon, 9 Jun 2025 19:17:53 GMT, Mikhail Ablakatov <mablakatov at openjdk.org> wrote:

> In the A64 ISA, the B (direct branch) instruction can encode a target within a ±128MB range relative to the instruction. Due to this limitation, when generating static stubs, HotSpot conservatively emits indirect branches for calls to c2i interface stubs. These indirect branches are implemented using a four-instruction sequence: three instructions to materialize the target address in a register, followed by a BR instruction to perform the jump.
> 
> This patch optimizes static stub generation when the code cache is small enough to guarantee that the target entry point of the c2i interface stub lies within the direct branch range. In such cases, a single direct B instruction can be used instead of the indirect sequence, saving 3 instructions (12 bytes) per static stub.
> 
> Below is an example of the optimization's impact, measured using the movie-lens benchmark from the Renaissance benchmark suite:
> 
> | Metric      | Before        | After         | Difference |
> |-------------|---------------|---------------|------------|
> | totalInHeap | Avg: 1883.875 | Avg: 1871.667 | -0.65%     |
> |             | Sum: 6653848  | Sum: 6616344  | -0.56%     |
> | stubCode    | Avg: 103.164  | Avg: 87.285   | -15.38%    |
> |             | Sum: 364376   | Sum: 308552   | -15.33%    |
> 
> Full jtreg passed on AArch64.

src/hotspot/cpu/aarch64/compiledIC_aarch64.cpp line 106:

> 104:   } else {
> 105:     NativeJump::insert(method_holder->next_instruction_address(), entry);
> 106:   }

We're also calling `ICache::invalidate_range` twice, which is kinda lame. That perhaps doesn't matter because calls to `CompiledDirectCall::set_to_interpreted()` are fairly rare.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25702#discussion_r2137520422