Reducing byte[] copies of MemorySegment.getString

[Philippe Marschall]

Hello

I had a look at the implementation of MemorySegment.getString and it 
seems to allocate every byte[] once in StringSupport and then a copy in 
the String constructor. StringSupport already has access to 
JavaLangAccess and therefore uncheckedNewStringNoRepl which can allocate 
a String without copying the given byte[].

I did an experiment and changed the StringSupport implementation to call 
uncheckedNewStringNoRepl instead [1]. uncheckedNewStringNoRepl behaves 
different from new String() and throws CharacterCodingException on 
malformed input. The API contract of MemorySegment#getString specifies 
replacement characters in case of malformed input. The easiest way I 
found to support this is simply catch and call the String constructor 
for malformed input.

There is already an existing JMH microbenchmark for 
MemorySegment#getString in ToJavaStringTest, I ran it with the GC 
profiler (MICRO="OPTIONS=-prof gc"). And the results are as follows:

- the average time per op goes down by about 25% to 30% depending on the 
String size
- allocations per op goes down by 33% to 50% depending on the String 
size, the the relative savings increasing with the String size

I ran the tier1 test suite and it passed.

  [1] 
https://github.com/marschall/jdk/commit/b43920dd7b666c5d09c088a9918c3d65a5a32e8c

Cheers
Philippe