RFR: 8261731: shallow copy the internal buffer of a scalar-replaced java.lang.String object [v4]

[Xin Liu]

> There are 3 nodes involving in the construction of a java.lang.String object.
> 1. Allocate of itself, aka. alloc
> 2. AllocateArray of a byte array, which is value:byte[], aka. aa
> 3. ArrayCopyNode which copys in the contents of value, aka. ac
> 
> Lemma
> When a String object `alloc` is scalar replaced, C2 can eliminate `aa` and `ac`.
> 
> Because `alloc` is scalar replaced, it must be non-escaped. The field value:byte[] of j.l.String cannot be seen by external world, therefore it must not be global escaped. Because the buffer is marked as stable, it is safe to assume its contents are whatever ac copies in. Because all public java.lang.String constructors clone the incoming array, the source of `ac` is stable as well.
> 
> It is possible to rewire `aa` to the source of ac with the correct offset. That is to say, we can replace both `aa` and `ac` with a “shallow copy” of the source of `ac`. It’s safe if C2 keeps a reference of the source oop for all safepoints.

Xin Liu has updated the pull request incrementally with one additional commit since the last revision:

  fix a typo introduced in prior change.
  
  This fixes the crashing case reported on github's PR.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2570/files
  - new: https://git.openjdk.java.net/jdk/pull/2570/files/ea660ee6..672bf3c1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2570&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2570&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2570.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2570/head:pull/2570

PR: https://git.openjdk.java.net/jdk/pull/2570