Summary
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In JDK-8309204, we would like to remove all functionality of DoReserveCopyInSuperWord, after which this product flag becomes useless.

I already have a [PR for JDK-8309204][1].

Problem
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Currently, the flag DoReserveCopyInSuperWord (on by default) creates a copy of the loop at the beginning of SuperWord::output, so that if anything goes wrong while modifying the graph, we can revert to a unmodified copy. If a user explicitly disables the flag, we would instead hit a ShouldNotReachHere and crash.

But these are absolute edge cases: only when we hit asserts in debug mode. Hence, it is not justified to have the extra computational overhead of cloning/copying the loop, and the code complexity of CountedLoopReserveKit.

Solution
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Instead of copy / revert, we simply bail out and recompile without SuperWord.

Remove CountedLoopReserveKit.

Obsolete DoReserveCopyInSuperWord. Simply deprecating it would not allow for the removal of the functionality, hence we want to go for obsoletion directly.

Specification
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The changes to argument handling:

    --- a/src/hotspot/share/opto/c2_globals.hpp
    +++ b/src/hotspot/share/opto/c2_globals.hpp
    @@ -341,9 +341,6 @@
       product(bool, UseCMoveUnconditionally, false,                             \
               "Use CMove (scalar and vector) ignoring profitability test.")     \
                                                                                 \
    -  product(bool, DoReserveCopyInSuperWord, true,                             \
    -          "Create reserve copy of graph in SuperWord.")                     \
    -                                                                            \
       notproduct(bool, TraceSuperWord, false,                                   \
               "Trace superword transforms")                                     \
                                                                                 \

    --- a/src/hotspot/share/runtime/arguments.cpp
    +++ b/src/hotspot/share/runtime/arguments.cpp
    @@ -524,6 +524,7 @@ static SpecialFlag const special_jvm_flags[] = {
       { "G1ConcRSHotCardLimit",         JDK_Version::undefined(), JDK_Version::jdk(21), JDK_Version::undefined() },
       { "RefDiscoveryPolicy",           JDK_Version::undefined(), JDK_Version::jdk(21), JDK_Version::undefined() },
       { "MetaspaceReclaimPolicy",       JDK_Version::undefined(), JDK_Version::jdk(21), JDK_Version::undefined() },
    +  { "DoReserveCopyInSuperWord",     JDK_Version::undefined(), JDK_Version::jdk(22), JDK_Version::jdk(23) },
     
     #ifdef ASSERT
       { "DummyObsoleteTestFlag",        JDK_Version::undefined(), JDK_Version::jdk(18), JDK_Version::undefined() },

Further, all uses of DoReserveCopyInSuperWord are removed.

  [1]: https://github.com/openjdk/jdk/pull/16022