RFR: 8292285: C2: remove unreachable block after NeverBranch-to-Goto conversion [v2]

[Roberto Castañeda Lozano]

On Tue, 16 Aug 2022 09:04:38 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

>> `NeverBranch` nodes introduce fake branches out of infinite loops to simplify intermediate optimizations that assume full reachability:
>> ![before](https://user-images.githubusercontent.com/8792647/184355521-8b54725e-f7d2-4d37-a7ba-e0f7d1086421.png)
>> `PhaseCFG::convert_NeverBranch_to_Goto()` transforms such nodes into simple `Goto` nodes, removing the fake branch and thus making its destination block unreachable:
>> ![after](https://user-images.githubusercontent.com/8792647/184355553-3b862832-7302-4de2-aa29-86d7da0d985a.png)
>> This changeset removes the unreachable destination block created by `PhaseCFG::convert_NeverBranch_to_Goto()` from the list of basic blocks:
>> ![after-removal](https://user-images.githubusercontent.com/8792647/184355581-82038683-3f4f-4f4f-a8d7-16e507bfdf0c.png)
>> Removing unreachable blocks created by `PhaseCFG::convert_NeverBranch_to_Goto()` simplifies late control-flow analysis (for example, recomputing dominators during the output phase as required for late barrier analysis in Generational ZGC) and slightly reduces the size of the generated code.
>> 
>> #### Testing
>> 
>> - hs-tier1-3 (windows-x64, linux-x64, linux-aarch64, and macosx-x64; release and debug mode). The changeset does not introduce new test cases because it is already exercised by at least two existing ones (`compiler/inlining/StringConcatInfiniteLoop.java`, `compiler/loopopts/TestInfiniteLoopNotInnerMost.java`).
>> - fuzzing (~30 min. on each platform).
>
> Roberto Castañeda Lozano has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Apply style guidelines to surrounding code

Thanks for reviewing, Vladimir!

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PR: https://git.openjdk.org/jdk/pull/9853