RFR: 8256827: C2: Avoid reallocations by pre-sizing lists in post_allocate_copy_removal

[Claes Redestad]

On Sun, 22 Nov 2020 17:06:13 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> By pre-sizing the Node_Lists created in PhaseChaitin::post_allocate_copy_removal we avoid all re-allocations. 
>> 
>> As lists are allocated to the size we need we might also reduce memory waste, which reduce memory used by these structures by up to 50%
>> 
>> Throughput wise this is also a gain and saves about 10,000 instructions per C2 compilation in SimpleRepeatCompilation.trivialMath. By using bulk clearing, avoiding redundant clearing of newly created lists and adding a shallow copy routine to Node_List we save another ~9,500 instructions per compilation in the same benchmark. This corresponds with a small but statistically significant gain:
>> 
>> Benchmark                                      Mode  Cnt    Score   Error  Units
>> SimpleRepeatCompilation.trivialMath_repeat_c2    ss   50  355.342 ± 2.069  ms/op # baseline
>> SimpleRepeatCompilation.trivialMath_repeat_c2    ss   50  350.705 ± 1.577  ms/op # patch
>> 
>> Testing: tier1-4
>
> Nice.

@vnkozlov @TobiHartmann - thank you for reviewing!

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PR: https://git.openjdk.java.net/jdk/pull/1370