RFR (S) 8150180: String.value contents should be trusted

[Aleksey Shipilev]

Hi,

Please review a simple performance improvement in Strings:
  https://bugs.openjdk.java.net/browse/JDK-8150180

In short, we want VM to trust constant String contents, so that
"Foo".charAt(0) is folded. As far as current Hotspot goes, this is only
achievable with @Stable -- we need to trust the array contents:
  http://cr.openjdk.java.net/~shade/8150180/webrev.jdk.01/

This, however, steps into the compiler bug caused by StringUTF16.getChar
intrinsic producing a mismatched load, that is then folded incorrectly.
So we instead rely on Java level folding in cases like these:
  http://cr.openjdk.java.net/~shade/8150180/webrev.hs.01/

...and it works:
  http://cr.openjdk.java.net/~shade/8150180/notes.txt

While VM change looks like a workaround, Vladimir I. and me concluded
that @Stable folding code should just reject folding mismatched loads to
begin with. So, getChar intrinsic change is the actual fix. Vladimir I.
ought to add more assertions in @Stable folding code separately:
  https://bugs.openjdk.java.net/browse/JDK-8150186

Since this issue might trigger compiler bugs, I would like to push
through hs-comp to pass compiler nightlies.

Cheers,
-Aleksey

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