[jdk16] RFR: 8260927: StringBuilder::insert is incorrect without Compact Strings [v2]

[Aleksey Shipilev]

> Discovered it with ARM32 tier1 tests, which runs with -CompactStrings by default. But the bug is actually generic:
> 
> $ CONF=linux-x86_64-server-fastdebug make run-test TEST=java/lang/StringBuilder/Insert.java TEST_VM_OPTS="-XX:-CompactStrings"
> 
> test Insert.insertOffset(): failure
> java.lang.AssertionError: expected [??abc] but found [efabc]
> 	at org.testng.Assert.fail(Assert.java:94)
> 	at org.testng.Assert.failNotEquals(Assert.java:496)
> 	at org.testng.Assert.assertEquals(Assert.java:125)
> 	at org.testng.Assert.assertEquals(Assert.java:178)
> 	at org.testng.Assert.assertEquals(Assert.java:188)
> 	at Insert.insertOffset(Insert.java:45)
> 	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
> 
> I believe this is a regression from [JDK-8254082](https://bugs.openjdk.java.net/browse/JDK-8254082). 
> 
>     void getBytes(byte[] dst, int srcPos, int dstBegin, byte coder, int length) {
>         if (coder() == coder) {
>             System.arraycopy(value, srcPos, dst, dstBegin << coder, length << coder());
>         } else {    // this.coder == LATIN && coder == UTF16
>             StringLatin1.inflate(value, srcPos, dst, dstBegin, length);
>         }
>     }
> 
> When coder is `UTF16` (which it guaranteed to be without `CompactStrings`), then `srcPos` in `byte[]` array has to be adjusted by `coder` as well.
> 
> Additional testing:
>  - [x] Linux ARM32, affected test now passes
>  - [x] Linux x86_64, affected test now passes
>  - [x] Linux x86_64 `tier1` default, passes
>  - [x] Linux x86_64 `tier1`, `-XX:-CompactStrings`, passes modulo two testbugs ([JDK-8260933](https://bugs.openjdk.java.net/browse/JDK-8260933), [JDK-8260934](https://bugs.openjdk.java.net/browse/JDK-8260934))

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Add the regression test case

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk16/pull/143/files
  - new: https://git.openjdk.java.net/jdk16/pull/143/files/e3c4da2b..0aa73e5f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk16&pr=143&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk16&pr=143&range=00-01

  Stats: 3 lines in 1 file changed: 2 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/143.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/143/head:pull/143

PR: https://git.openjdk.java.net/jdk16/pull/143