RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v26]

[Sandhya Viswanathan]

On Fri, 4 Oct 2024 10:41:46 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> @rkennke The small loop looks to me that it will run over the end of the array.
>> Say the haystack_len is 7, the index below would be 0 after the shrq instruction, and the  movq(XMM_TMP1, Address(haystack, index, Address::times_8)) in the loop will read 8 bytes i.e. one byte past the end of the array:
>>        // num_words (zero-based) = (haystack_len - 1) / 8;
>>       __ movq(index, haystack_len);
>>       __ subq(index, 1);
>>       __ shrq(index, LogBytesPerWord);
>> 
>>       __ bind(L_loop);
>>       __ movq(XMM_TMP1, Address(haystack, index, Address::times_8));
>>       __ movq(Address(rsp, index, Address::times_8), XMM_TMP1);
>>       __ subq(index, 1);
>>       __ jcc(Assembler::positive, L_loop);
>
> Yes, and that is intentional.
> 
> Say, haystack_len is 7, then the first block computes the adjustment of the haystack, which is 8 - (7 % 8) = 1. We adjust the haystack pointer one byte down, so that when we copy (multiple of) 8 bytes, we land on the last byte. We do copy a few bytes that are preceding the array, which is part of the object header and guaranteed to be >= 8 bytes.
> 
> Then we compute the number of words to copy, but make it 0-based. That is '0' is 1 word, '1' is 2 words, etc. It makes the loop nicer. In this example we get 0, which means we copy one word from the adjusted haystack, which is correct.
> 
> Then comes the actual loop.
> 
> Afterwards we adjust the haystack pointer so that it points to the first array element that we just copied onto the stack, ignoring the few garbage bytes that we also copied.

@rkennke Thanks for the explanation. I attach here a fix which is an extension of existing way of copying while taking care of the smaller object header. Also there are two instances of this in the intrinsic so I have factored the new code into a method and call it from both the places. 
[indexof_fix.patch](https://github.com/user-attachments/files/17285239/indexof_fix.patch)

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1790967275