RFR: 8270446: Remove the MemBarNode that has been optimized from the current block

[Vladimir Kozlov]

On Wed, 14 Jul 2021 07:35:25 GMT, SUN Guoyun <github.com+40024232+sunny868 at openjdk.org> wrote:

> Hi all,
> aarch64 and mips64 also had an optimization to merge two memory barrier instructions. But when I use the args -XX:+UnlockDiagnosticVMOptions -XX:+PrintOptoAssembly, I see some information like the following 
> 
> 1a0 membar_release 
>             dmb ish 
> 1a4 membar_release 
>             dmb ish 
> 1a4 spill R19 -> R0 # spill size = 64 
> 
> 
> Here, "1a4 membar_release" is actually optimized out . so I think it should be better to display it like this 
> 
> 
> 1a0 membar_release 
>             dmb ish 
> 1a4 spill R19 -> R0 # spill size = 64
> 
> Please review this trivial change.
> 
> Thanks,
> Sun Guoyun

Modifying block during emit phase could be dangerous. These empty membars still used for instructions ordering.

And your case should be handled already:
https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/aarch64.ad#L8517

`-XX:+PrintOptoAssembly` output should be `"membar_release (elided)"`
Please, look why it is not the case.

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

Changes requested by kvn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/4774