8325730: StringBuilder.toString allocation for the empty String

[cl4es]

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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Issue

• JDK-8325730: StringBuilder.toString allocation for the empty String (Bug - P4)

Reviewers

• Jim Laskey (@JimLaskey - Reviewer) ⚠️ Review applies to 7f9566b8
• Aleksey Shipilev (@shipilev - Reviewer)

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• 02: Full - Incremental (1846746b)
• 01: Full - Incremental (40cca3e3)
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[JimLaskey]

LGTM

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@cl4es This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8325730: StringBuilder.toString allocation for the empty String

Reviewed-by: jlaskey, shade

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At the time when this comment was updated there had been 3 new commits pushed to the master branch:

• b419e95: 8294148: Support JSplitPane for instructions and test UI
• 9538f5d: 8319651: Several network tests ignore vm flags when start java process
• 4957d6a: 8325441: IGV: update pom.xml such that IntelliJ can import as maven project

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[shipilev]

OK, so before JDK-8282429 this was handled by String{Latin1|UTF16}.newString, gotcha.

[cl4es]

Thanks!

/integrate

[openjdk]

Going to push as commit d2590c6.
Since your change was applied there have been 5 commits pushed to the master branch:

• aa792ea: 8164094: javadoc allows to create a @link to a non-existent method
• a2d027e: 8326089: Text incorrectly placed in breadcrumbs list in generated docs
• b419e95: 8294148: Support JSplitPane for instructions and test UI
• 9538f5d: 8319651: Several network tests ignore vm flags when start java process
• 4957d6a: 8325441: IGV: update pom.xml such that IntelliJ can import as maven project

Your commit was automatically rebased without conflicts.

[openjdk]

@cl4es Pushed as commit d2590c6.

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