RFR: 8329998: Remove double initialization for parts of small TypeArrays in ZObjArrayAllocator

[Stefan Karlsson]

On Sun, 5 May 2024 05:07:47 GMT, Guoxiong Li <gli at openjdk.org> wrote:

>> [JDK-8139457](https://bugs.openjdk.org/browse/JDK-8139457) introduced non-word aligned payloads for TypeArrays. This required adaptation in `ZObjArrayAllocator` segmented array allocator. But the implementation does this adaptation even when the size is small enough and dispatches the allocation to `ObjArrayAllocator`, which already handles clearing this memory.
>> 
>> Instead move the initialization of any unaligned payload start to after the segmented allocation decision.
>
> src/hotspot/share/gc/z/zObjArrayAllocator.cpp line 86:
> 
>> 84:     // fill the first 4 bytes here.
>> 85:     assert(process_start_offset_in_bytes - base_offset_in_bytes == 4, "Must be 4-byte aligned");
>> 86:     assert(!is_reference_type(element_type), "Only TypeArrays can be 4-byte aligned");
> 
> Actually, only the type arrays whose element type is not double word can be 4-byte aligned. So this assertion could be `!is_reference_type(element_type) && !is_double_word_type(element_type)`.

I think the point is that when it is not a reference type, it is OK to initialize with a plain `0`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18721#discussion_r1590689185