On Tue, 21 May 2024 10:20:27 GMT, Maurizio Cimadamore <[email protected]>
wrote:
>> When creating a nested memory access var handle, we ensure that the segment
>> is accessed at the correct alignment for the root layout being accessed. But
>> we do not ensure that the segment has at least the size of the accessed root
>> layout. Example:
>>
>>
>> MemoryLayout LAYOUT = sequenceLayout(2, structLayout(JAVA_INT.withName("x"),
>> JAVA_INT.withName("y")));
>> VarHandle X_HANDLE = LAYOUT.varHandle(PathElement.sequenceElement(),
>> PathElement.groupElement("x"));
>>
>>
>> If I have a memory segment whose size is 12, I can successfully call
>> X_HANDLE on it with index 1, like so:
>>
>>
>> MemorySegment segment = Arena.ofAuto().allocate(12);
>> int x = (int)X_HANDLE.get(segment, 0, 1);
>>
>>
>> This seems incorrect: after all, the provided segment doesn't have enough
>> space to fit _two_ elements of the nested structs.
>>
>> This PR adds checks to detect this kind of scenario earlier, thus improving
>> usability. To achieve this we could, in principle, just tweak
>> `LayoutPath::checkEnclosingLayout` and add the required size check.
>>
>> But doing so will add yet another redundant check on top of the other
>> already added by [pull/19124](https://git.openjdk.org/jdk/pull/19124).
>> Instead, this PR rethinks how memory segment var handles are created, in
>> order to eliminate redundant checks.
>>
>> The main observation is that size and alignment checks depend on an
>> _enclosing_ layout. This layout *might* be the very same value layout being
>> accessed (this is the case when e.g. using `JAVA_INT.varHandle()`). But, in
>> the general case, the accessed value layout and the enclosing layout might
>> differ (e.g. think about accessing a struct field).
>>
>> Furthermore, the enclosing layout check depends on the _base_ offset at
>> which the segment is accessed, _prior_ to any index computation that occurs
>> if the accessed layout path has any open elements. It is important to notice
>> that this base offset is only available when looking at the var handle that
>> is returned to the user. For instance, an indexed var handle with
>> coordinates:
>>
>>
>> (MemorySegment, long /* base */, long /* index 1 */, long /* index 2 */,
>> long /* index 3 */)
>>
>>
>> Is obtained through adaptation, by taking a more basic var handle of the
>> form:
>>
>>
>> (MemorySegment, long /* offset */)
>>
>>
>> And then injecting the result of the index multiplication into `offset`. As
>> such, we can't add an enclosing layout check inside the var handle returned
>> by `VarHandles::memorySegmentViewHandle`, as doing so will end up seeing the
>> *wrong* off...
>
> Maurizio Cimadamore has updated the pull request incrementally with one
> additional commit since the last revision:
>
> Fix typo in javadoc
Marked as reviewed by jvernee (Reviewer).
src/java.base/share/classes/java/lang/invoke/X-VarHandleSegmentView.java.template
line 123:
> 121: static $type$ get(VarHandle ob, Object obb, long base) {
> 122: VarHandleSegmentViewBase handle = (VarHandleSegmentViewBase)ob;
> 123: AbstractMemorySegmentImpl bb = checkReadOnly(obb, true);
For getter methods, which pass a constant `true` here, `checkReadOnly`
essentially just does a null check and cast on the segment. Not sure if it's
worth simplifying... (I'm happy if you want to leave it like this as well)
-------------
PR Review: https://git.openjdk.org/jdk/pull/19251#pullrequestreview-2072366007
PR Review Comment: https://git.openjdk.org/jdk/pull/19251#discussion_r1610647133
