RFR: 8292285: C2: remove unreachable block after NeverBranch-to-Goto conversion
Tobias Hartmann
thartmann at openjdk.org
Mon Aug 15 11:11:29 UTC 2022
On Fri, 12 Aug 2022 12:42:29 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:
> 
> `PhaseCFG::convert_NeverBranch_to_Goto()` transforms such nodes into simple `Goto` nodes, removing the fake branch and thus making its destination block unreachable:
> 
> This changeset removes the unreachable destination block created by `PhaseCFG::convert_NeverBranch_to_Goto()` from the list of basic blocks:
> 
> 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).
Looks good to me.
-------------
Marked as reviewed by thartmann (Reviewer).
PR: https://git.openjdk.org/jdk/pull/9853
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