[lworld] RFR: 8267763: [lworld][lw3] Change "non-tearable" nomenclature to "access atomic"
thartmann at openjdk.java.net
Thu May 27 07:38:29 UTC 2021
On Wed, 26 May 2021 07:39:17 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:
> Current Valhalla code has the experimental marker interface `java.lang.NonTearable`, which is actually about access atomicity. It makes weird claims about word tearing and out-of-thin air values.
> First, this is not word tearing. Word tearing, as defined by JLS 17.6 is: _"This problem is sometimes known as word tearing, and on processors that cannot easily update a single byte in isolation some other approach will be required"._ That is, word tearing is when we cannot update the _narrow_ member without doing a _wider_ access, thus necessarily affecting the adjacent members. In Valhalla case, what we are dealing with is access atomicity: we sometimes cannot access the _wide_ member without doing a set of _narrower_ accesses. This is why JLS 17.7 says "non-atomic treatment of double and longs", not "word-tearing of double and longs".
> Second, the docs for `j.l.NonTearable` mention "out-of-thin-air" (OOTA) values, which are not related here at all. OOTA are the beasts from the causality loops: those are values that were never written by normal execution of the program (i.e. speculative values). In Valhalla case, the writes that produce the broken hybrid are known and expected writes from the conflicting writers.
> This nomenclature percolates to Valhalla VM code, so some change is needed there as well.
> Additional testing:
> - [x] `runtime/valhalla` tests
The corresponding tests in `compiler/valhalla/inlinetypes` should be renamed as well (see `TestBufferTearing.java` and `TestBufferTearingC1.java`).
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