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... src/java.base/share/classes/java/lang/invoke/MethodHandles.java line 4518: > 4516: * Only plain {@linkplain VarHandle.AccessMode#GET get} and > {@linkplain VarHandle.AccessMode#SET set} > 4517: * access modes are supported by the returned var handle. For all > other access modes, an > 4518: * {@link UnsupportedOperationException} will be thrown. I recommend adding an api note explaining that native memory segments, direct byte buffers, or heap memory segments backed by long[] should be used if support for other access modes are required. src/java.base/share/classes/java/lang/invoke/MethodHandles.java line 4610: > 4608: * {@link Double#doubleToRawLongBits}, respectively). > 4609: * <p> > 4610: * Access to heap byte buffers is always unaligned. I recommend merging that sentence into the paragraph on heap byte buffers e.g.: > For direct buffers, access of the bytes at an index is always misaligned. As > a result only the plain... src/java.base/share/classes/java/nio/X-Buffer.java.template line 2218: > 2216: * @implNote > 2217: * This implementation throws {@code UnsupportedOperationException} > for > 2218: * non-direct buffers when the given unit size is greater then > {@code 1}. This is no longer an implementation note, its now part of the specified API. So i think we can simplify the text of the `@throws UOE ...` to just say: @throws UOE if the buffer is non-direct and the unit size > 1 Same for the other method. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/16681#discussion_r1396097577 PR Review Comment: https://git.openjdk.org/jdk/pull/16681#discussion_r1396101675 PR Review Comment: https://git.openjdk.org/jdk/pull/16681#discussion_r1396109655