RFR: 8321387: SegmentAllocator:allocateFrom(AddressLayout, MemorySegment) does not throw stated UnsupportedOperationException

[Maurizio Cimadamore]

On Wed, 6 Dec 2023 14:06:00 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> This PR proposes to change the exception type for exceptions thrown for certain methods with a parameter of type `MemorySegment` when it is `MemorySegment::isReadOnly`. Previously an `UnsupportedOperationException` was specified but in some cases, in reality, an `IllegalArgumentException` was thrown. 
>> 
>> The principle used in this PR is that operations acting on an MS where the MS is `this` should throw an `UnsupportedOperationException` whereas in cases where the MS is a *parameter* an `IllegalArgumentException` should be thrown.
>> 
>> It should be noted that this PR retains the previous behavior for MS VarHandle access (even though the MS is a parameter to the accessor methods, the first parameter can be said to represent `this`).
>
> src/java.base/share/classes/java/lang/invoke/X-VarHandleSegmentView.java.template line 153:
> 
>> 151:     static void set(VarHandle ob, Object obb, long base, $type$ value) {
>> 152:         VarHandleSegmentViewBase handle = (VarHandleSegmentViewBase)ob;
>> 153:         AbstractMemorySegmentImpl bb = checkAddress(obb, base, handle.length, ON_WRITE_UOE);
> 
> I don't think this should throw UOE. It should throw IAE, because the bad segment is passed to the var handle. I also realize that this exception is not captured in the documentation of MemoryLayout::varHandle. So that should be fixed as well.

This is actually tricky: for var handles it seems like the right exception is IAE - but then for MemorySegment::get the right exception is UOE (because the receiver itself is read-only). We might need to give up consistency a bit, as otherwise I don't think memory segment getter/setter can reuse the VarHandle implementation.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16993#discussion_r1417386458