RFR: 8348855: G1: Implement G1BarrierSetC2::estimate_stub_size

[Andrew Dinn]

On Wed, 29 Jan 2025 10:08:06 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> src/hotspot/share/gc/g1/c2/g1BarrierSetC2.cpp line 550:
>> 
>>> 548: 
>>> 549:   // Add slop to avoid expansion during emit_stubs.
>>> 550:   return size + PhaseOutput::MAX_inst_size;
>> 
>> Is this needed? `init_buffer` already bumps `code_req` up by `MAX_inst_size` immediately after calling `bs->estimate_stub_size()` and that headroom is maintained at each instruction emit.
>
> Yes, I think so. Leyden experiments show this is needed. AFAICS, the `MAX_inst_size` added in `init_buffer` is going to be taken by normal code itself, so then when we'll go to final stub emit with `G1BarrierSetC2::emit_stubs`, we will encounter `maybe_expand_to_ensure_remaining(MAX_inst_size)`, but the part of `MAX_inst_size` would already be eaten, and this will force expansion.
> 
> In other words, if we maintain `MAX_inst_size` window when emitting stubs in `G1BarrierSetC2::emit_stubs`, we need to include that in estimate.

Ok, good to go then.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23333#discussion_r1933749743