RFR: 8325730: StringBuilder.toString allocation for the empty String

[Claes Redestad]

JDK-8282429 accidentally removed an optimization (JDK-8240094) that ensured StringBuilder/StringBuffer::toString returns `""` when the builders are empty.


Name                         Cnt     Base      Error   Test   Error   Unit  Change
StringBuffers.emptyToString    5   12,289 ±    0,384  9,883 ± 0,721  ns/op   1,24x (p = 0,000*)
  :gc.alloc.rate                 1862,398 ±   57,647  0,007 ± 0,000 MB/sec   0,00x (p = 0,000*)
  :gc.alloc.rate.norm              24,000 ±    0,000  0,000 ± 0,000   B/op   0,00x (p = 0,000*)
  :gc.count                        31,000             0,000         counts
  :gc.time                         21,000                               ms
StringBuilders.emptyToString   5    4,146 ±    0,567  0,646 ± 0,003  ns/op   6,42x (p = 0,000*)
  :gc.alloc.rate                 9208,656 ± 1234,399  0,007 ± 0,000 MB/sec   0,00x (p = 0,000*)
  :gc.alloc.rate.norm              40,000 ±    0,000  0,000 ± 0,000   B/op   0,00x (p = 0,000*)
  :gc.count                        96,000             0,000         counts
  :gc.time                         64,000                               ms
  * = significant

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Commit messages:
 - Copyrights
 - 8325730: StringBuilder.toString allocation for the empty String

Changes: https://git.openjdk.org/jdk/pull/17931/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17931&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325730
  Stats: 53 lines in 5 files changed: 14 ins; 31 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/17931.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17931/head:pull/17931

PR: https://git.openjdk.org/jdk/pull/17931