On Wed, 15 Nov 2023 22:46:03 GMT, Jorn Vernee <jver...@openjdk.org> wrote:
> See the JBS issue for an extended problem description. > > This patch changes the specification and implementation of > `MethodHandles::byteArrayViewVarHandle`, > `MethodHandles::byteBufferViewVarHandle`, `ByteBuffer::alignedSlice`, and > `ByteBuffer::alignmentOffset` to weaken the guarantees they make about the > alignment of Java array elements, in order to bring them in line with the > guarantees made by an arbitrary JVM implementation (which makes no guarantees > about array element alignment beyond them being aligned to their natural > alignment). > > - `MethodHandles::byteArrayViewVarHandle`: we can not guarantee any alignment > for the accesses. Which means we can only reliably support plain get and set > access modes. The javadoc text explaining which other access modes are > supported, or how to compute aligned offsets into the array is dropped, as it > is not guaranteed to be correct on all JVM implementations. The > implementation of the returned VarHandle is changed to throw an > `UnsupportedOperationException` for the unsupported access modes, as mandated > by the spec of `VarHandle` [1]. > > - `MethodHandles::byteBufferViewVarHandle`: the implementation/spec is > incorrect when accessing a heap buffer (wrapping a byte[]), for the same > reasons as `byteArrayViewVarHandle`. The spec is changed to specify that when > accessing a _heap buffer_, only plain get and set access modes are supported. > The implementation of the returned var handle is changed to throw an > `IllegalStateException` when an access is attempted on a heap buffer using an > access mode other than plain get or set. Note that we don't throw an outright > `UnsupportedOperationException` for this case, since whether the access modes > are supported depends on the byte buffer instance being used. > > - `ByteBuffer::alignedSlice` and `ByteBuffer::alignmentOffset`: The former > method depends directly on the latter for all its alignment computations. We > change the implementation of the latter method to throw an > `UnsupportedOperationException` for all unit sizes greater than 1, when the > buffer is non-direct. This change is largely covered by the existing > specification: > > > * @throws UnsupportedOperationException > * If the native platform does not guarantee stable alignment > offset > * values for the given unit size when managing the memory regions > * of buffers of the same kind as this buffer (direct or > * non-direct). For example, if garbage collection would result > * in the mo... This pull request has now been integrated. Changeset: 9c852df6 Author: Jorn Vernee <jver...@openjdk.org> URL: https://git.openjdk.org/jdk/commit/9c852df6aa019f63d6fae733d7a73521b7151dd0 Stats: 5133 lines in 19 files changed: 612 ins; 3440 del; 1081 mod 8318966: Some methods make promises about Java array element alignment that are too strong Reviewed-by: psandoz, mcimadamore ------------- PR: https://git.openjdk.org/jdk/pull/16681
