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[GR-81562] Guarantee that hybrid objects like pods are not allocated in unaligned chunks. #15048

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[GR-81562] Guarantee that hybrid objects like pods are not allocated in unaligned chunks. #15048

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12 changes: 9 additions & 3 deletions ...racle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/ThreadLocalAllocation.java
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Original file line number Diff line number Diff line change
Expand Up @@ -323,11 +323,17 @@ private static Object slowPathNewArrayLikeObjectWithoutAllocation0(DynamicHub hu
Object array;
if (!GenScavengeAllocationSupport.shouldAllocateInAlignedChunk(size)) {
/*
* Large arrays go into their own unaligned chunk. Only arrays and stored continuations
* may be allocated in an unaligned chunk.
* Large arrays go into their own unaligned chunk.
*
* Instance objects with object reference fields, including hybrid objects such as pods,
* must not be allocated in unaligned chunks because field write barriers assume an
* aligned chunk.
*/
int layoutEncoding = hub.getLayoutEncoding();
assert LayoutEncoding.isArray(layoutEncoding) || StoredContinuation.class.isAssignableFrom(DynamicHub.toClass(hub));
if (!LayoutEncoding.isArray(layoutEncoding) && !StoredContinuation.class.isAssignableFrom(DynamicHub.toClass(hub))) {
throw VMError.shouldNotReachHere("Only arrays and StoredContinuation may be allocated in an unaligned chunk. " +
"Hybrid objects such as pods must be small enough to be allocated in an aligned chunk.");
}

boolean needsZeroing = !HeapChunkProvider.areUnalignedChunksZeroed();
UnalignedHeapChunk.UnalignedHeader newTlabChunk = HeapImpl.getChunkProvider().produceUnalignedChunk(size);
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7 changes: 6 additions & 1 deletion substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/heap/StoredContinuation.java
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Expand Up @@ -36,7 +36,12 @@
*
* Stored continuations are {@link Hybrid} objects where the array part contains the raw stack data.
* After writing the stack data into the object, we manually emit the correct GC write barriers for
* all the references (see {@link Heap#dirtyAllReferencesOf}).
* all the references (see {@link Heap#dirtyAllReferencesOf}). Following initialization, an object
* is considered effectively immutable, and only GC is expected to write the references it contains.
* Therefore, implicit write barriers intentionally do not support stored continuations, and they
* cannot have declared object reference fields.
*
* Objects of this type can reside in unaligned chunks.
*/
@Hybrid(componentType = Word.class)
public final class StoredContinuation {
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