RFR: 8139457: Array bases are aligned at HeapWord granularity

[Stefan Karlsson]

On Tue, 8 Nov 2022 20:18:09 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> See [JDK-8139457](https://bugs.openjdk.org/browse/JDK-8139457) for details.
> 
> Basically, when running with -XX:-UseCompressedClassPointers, arrays will have a gap between the length field and the first array element, because array elements will only start at word-aligned offsets. This is not necessary for smaller-than-word elements.
> 
> Also, while it is not very important now, it will become very important with Lilliput, which eliminates the Klass field and would always put the length field at offset 8, and leave a gap between offset 12 and 16.
> 
> Testing:
>  - [x] runtime/FieldLayout/ArrayBaseOffsets.java (x86_64, x86_32, aarch64, arm, riscv, s390)
>  - [x] bootcycle (x86_64, x86_32, aarch64, arm, riscv, s390)
>  - [x] tier1 (x86_64, x86_32, aarch64, riscv)
>  - [x] tier2 (x86_64, aarch64, riscv)
>  - [x] tier3 (x86_64, riscv)

Changes requested by stefank (Reviewer).

src/hotspot/share/gc/shared/collectedHeap.cpp line 435:

> 433: 
> 434: void CollectedHeap::zap_filler_array_with(HeapWord* start, size_t words, juint value) {
> 435:   int payload_start = align_up(arrayOopDesc::base_offset_in_bytes(T_INT), HeapWordSize) / HeapWordSize;

I see this in a number of places in the patch. Could you explain why this is correct? If base offset is not HeapWordSize aligned, then it looks like payload_start * HeapWordSize would be larger than the base offset.

src/hotspot/share/gc/z/zObjArrayAllocator.cpp line 55:

> 53:   const size_t header = arrayOopDesc::base_offset_in_bytes(element_type);
> 54:   size_t byte_size = _word_size * BytesPerWord;
> 55:   const size_t payload_size = byte_size - header;

Generational ZGC needs it to be able to write colored NULLs in object arrays, so a byte-centric approach will not work for us. Could you rewrite the code to take care of the "unaligned" 32 bits at the start of the array, and leave the word-centric segmented-clearing loop intact?

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PR: https://git.openjdk.org/jdk/pull/11044