RFR: 8320448: Accelerate IndexOf using AVX2 [v7]

[Scott Gibbons]

On Mon, 22 Jan 2024 07:08:31 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> src/hotspot/cpu/x86/stubGenerator_x86_64_string.cpp line 505:
>> 
>>> 503:     __ cmpb(Address(rbx, r15, Address::times_1, -0xa), rax);
>>> 504:     __ jne(L_top_loop_1);
>>> 505:     __ jmp(L_0x406019);
>> 
>> Instead of having special handling for each tail size (3 - 31 bytes), can we directly use 32 bytes VMASKMOVPS with appropriate mask for different tail sizes and only residual part (0 - 3 bytes) can fall over to scalar tail.
>
> Basically tail size can be rounded to nearest multiple of doubleword.

I have since changed the algorithm due to request from @sviswa7

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PR Review Comment: https://git.openjdk.org/jdk/pull/16753#discussion_r1610120366