RFR: 8275874: [JVMCI] use volatile accessors for aligned reads in c2v_readFieldValue

[Andrew Dinn]

On Mon, 25 Oct 2021 14:33:27 GMT, Doug Simon <dnsimon at openjdk.org> wrote:

> [JDK-8275645](https://bugs.openjdk.java.net/browse/JDK-8275645) resulted in loosing single-copy atomicity for reads in `c2v_readFieldValue`. This PR fixes that by using `<type>_field_acquire` accessors for all aligned reads and only using `<type>_field` accessors for unaligned reads.

Indeed, the problem here is not that the hardware is disallowing a volatile read at an unaligned address but that the compiler is being *asked* to generate a volatile read at an unaligned address. That is a symptom of the calling code either using an invalid layout, or otherwise computing an invalid offset.

It's no good trying to fix that symptom because, as Aleksey says, there is no fix that will preserve both volatility and single-copy atomicity. The fix needs to be made to to the root problem i.e. correct whatever code determined that a volatile field could legitimately reside at an unaligned address.

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PR: https://git.openjdk.java.net/jdk/pull/6109