RFR: 8352316: More MergeStoreBench

[Vladimir Ivanov]

On Wed, 19 Mar 2025 03:28:59 GMT, Shaojin Wen <swen at openjdk.org> wrote:

> Added performance tests related to String.getBytes/String.getChars/StringBuilder.append/System.arraycopy in constant scenarios to verify whether MergeStore works

On naming:
 * `null` usages confuse me (`NULL_STR` et al, `putNull`). Why "null" is special? Can you just use an arbitrary 4-byte string?
 * PR proposes a mix of snake & camel case while the code around uses camel case. Worth considering grouping similar benchmarks into an inner class.

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PR Review: https://git.openjdk.org/jdk/pull/24108#pullrequestreview-2726955147