On Wed, 15 Jun 2022 22:43:55 GMT, Paul Sandoz <psan...@openjdk.org> wrote:
>> src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java line
>> 729:
>>
>>> 727: MethodHandle base = PREPEND_BASE;
>>> 728: if (base == null) {
>>> 729: base = PREPEND_BASE = MethodHandles.dropArguments(
>>
>> I'm curious about this idiom - couldn't this result in the `PREPEND_BASE`
>> field being assigned twice, with objects that are morally identical, but
>> could have different identities? (thus violating `@Stable` contract)
>
> It's not a violation, it's relying on behavior not discussed by the `@Stable`
> doc, which refers to such behavior as "undefined".
>
> It's a carefully correlated dance between the code, interpreter, and C2
> relying on:
> 1. safe publication of equivalent immutable structures with no dependence on
> their identity when operating on them;
> 2. the field being set in the interpreter; and
> 3. by the time C2 inlines `prependBase` the field is no longer be updated,
> thus C2 can replace the field access with its value.
Thanks, I think I was missing (2)
-------------
PR: https://git.openjdk.org/jdk/pull/9154
