RFR: 8032218: Emit single post-constructor barrier for chain of superclass constructors [v2]

[Joshua Cao]

On Wed, 24 Apr 2024 17:14:51 GMT, Joshua Cao <duke at openjdk.org> wrote:

>> Opening this PR on top of https://github.com/openjdk/jdk/pull/18505. This PR is only valid if we agree it is sufficient to use `StoreStore` barriers at the end of constructors instead of `Release` barriers.
>> 
>> Currently on master, [C2 emits a Release barrier for each constructor call](https://github.com/openjdk/jdk/blob/32946e1882e9b22c983cbba3c6bda3cc7295946a/src/hotspot/share/opto/parse1.cpp#L1019) in a chain of superclass constructor calls. After https://github.com/openjdk/jdk/pull/18505 is merged, it is the same except that the barrier is a `StoreStore`. It is unnecessary. We only need to emit a single barrier for each object allocation / each pair of `Allocation/InitializeNode`. 
>> 
>> [Macro expansion emits a trailing StoreStore after an InitializeNode](https://github.com/openjdk/jdk/blob/32946e1882e9b22c983cbba3c6bda3cc7295946a/src/hotspot/share/opto/macro.cpp#L1610-L1628). This `StoreStore` is sufficient as the post-constructor barrier. From the [InitializeNode definition](https://github.com/openjdk/jdk/blob/32946e1882e9b22c983cbba3c6bda3cc7295946a/src/hotspot/share/opto/memnode.cpp#L3639-L3642):
>> 
>>> // An InitializeNode collects and isolates object initialization after
>> // an AllocateNode and before the next possible safepoint.  As a
>> // memory barrier (MemBarNode), it keeps critical stores from drifting
>> // down past any safepoint or any publication of the allocation.
>> 
>> All the writes that occur in the constructor will come before the `InitializeNode/StoreStore`. This PR removes the emitting of `StoreStore` barriers in `Parse::do_exits()`. This PR subsumes https://github.com/openjdk/jdk/pull/18505.
>> 
>> Passes hotspot tier1 locally on x86 linux machine. New tests make sure that there is a single `StoreStore` for chained constructors.
>
> Joshua Cao has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 22 commits:
> 
>  - 8032218: Emit single post-constructor barrier for chain of superclass constructors
>  - Add riscv64 to test
>  - Merge branch 'master' into storestore
>  - Merge branch 'master' into storestore
>  - Apply suggestions from code review
>    
>    some formatting suggestions from @shipilev
>    
>    Co-authored-by: Aleksey Shipilëv <shipilev at amazon.de>
>  - Guard everything by feature flag
>  - Revert "Statistics for barriers generated/eliminated"
>    
>    This reverts commit 33d23635048afd3a1b40ae91e6fadf577742fa4f.
>  - Make flag product diagnostic and guard string concat storestore by flag
>  - Statistics for barriers generated/eliminated
>  - global flag to turn on storestore barrier emission and membar acquires
>    IR tests
>  - ... and 12 more: https://git.openjdk.org/jdk/compare/235ba9a7...717fe65b

Had to step away for a while but still plan on working on this. Comment to keep alive.

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PR Comment: https://git.openjdk.org/jdk/pull/18870#issuecomment-2174225955