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

[Tobias Hartmann]

On Sun, 22 Nov 2020 01:05:28 GMT, Claes Redestad <redestad 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

Looks good to me.

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Marked as reviewed by thartmann (Reviewer).

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