Withdrawn: 8032218: Emit single post-constructor barrier for chain of superclass constructors

[duke]

On Fri, 19 Apr 2024 22:31:10 GMT, Joshua Cao <duke at openjdk.org> wrote:

> [C2 emits a StoreStore barrier for each constructor call](https://github.com/openjdk/jdk/blob/72ca7bafcd49a98c1fe09da72e4e47683f052e9d/src/hotspot/share/opto/parse1.cpp#L1016) in a chain of superclass constructor calls. 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.
> 
> This PR modifies `Parse::do_exits()` such that it only emits a barrier for a constructor if we find that the constructed object does not have an `InitializeNode`. It is possible that we cannot find an `InitializeNode` i.e. if the outermost method of the compilation unit is the constructor. We still need to emit a barrier in these cases.
> 
> Passes hotspot tier1 locally on x86 linux machine. New tests make sure that there is a single `StoreStore` for chained constructors.

This pull request has been closed without being integrated.

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PR: https://git.openjdk.org/jdk/pull/18870