RFR: 8324186: AARCH64: Use "dmb.ishst+dmb.ishld" for release barrier [v3]

[Andrew Haley]

On Wed, 24 Jan 2024 13:48:21 GMT, Nick Gasson <ngasson at openjdk.org> wrote:

>> kuaiwei has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add AlwaysMergeDMB option
>
> src/hotspot/cpu/aarch64/globals_aarch64.hpp line 130:
> 
>> 128:   product(ccstr, UseBranchProtection, "none",                           \
>> 129:           "Branch Protection to use: none, standard, pac-ret")          \
>> 130:   product(bool, AlwaysMergeDMB, false,                                  \
> 
> It should really be a diagnostic option if we're going to have it configurable at all, as a product option needs a CSR and it's not something end users would want to fiddle with.

Exactly so, yes. I think we should simply stop merging DMBs where the end result of doing so is to strengthen the barrier beyond what was requested.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17511#discussion_r1465279207