RFR: 8255339: CodeBuffer::_last_insn is only used on AArch64

[Aleksey Shipilev]

On Thu, 7 Jan 2021 08:07:16 GMT, Eric Liu <github.com+10482586+theRealELiu at openjdk.org> wrote:

> CodeBuffer::_last_insn was used to merge consecutive memory barriers and
> adjacent load/store on AArch64.
> 
> This patch marks it as AArch64 only so that 8 bytes could be dropped
> from CodeBuffer on other architectures.

I understand the desire to do this, but I really doubt that introducing arch-specific behaviors in shared code buys us enough. I believe it saves 8 bytes for a transient code buffer, so ballparking 10K methods are compiled, it saves 80K of temporary allocations? If this was saving more and for non-transient footprint, it would be interesting.

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PR: https://git.openjdk.java.net/jdk/pull/1971