RFR: JDK-8216437 : PPC64: Add intrinsic for GHASH algorithm [v28]

[Suchismith Roy]

On Sat, 8 Mar 2025 18:14:48 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

>> @TheRealMDoerr  Yes. The tests do not pass with this. 
>> Trying to find a scope to reduce instructions. 
>> masm->vsldoi(vLowProduct, vLowProduct, vLowProduct, 8);           // Swap
>>     masm->vxor(vLowProduct, vLowProduct, vReducedLow);                // Reduction using constant
>>     masm->vsldoi(vCombinedResult, vLowProduct, vLowProduct, 8);       // Swap 
>>    
>>    
>>    can be brought down to 2 instructions. 
>>    Still looking for scope to reduce. Let me know your inputs
>
> I still find it hard to read. Can you describe the algorithm in pseudo code or mathematical equations? We can try to map it to a shorter instruction sequence.
> Btw. the comment looks wrong here: vxor(vLowProduct, vLowProduct, vReducedLow); // Reduction using constant

@TheRealMDoerr  
https://www.researchgate.net/publication/285612706_Implementing_GCM_on_ARMv8

I think the same algorithm used for polynomial reduction -Section 4.3

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PR Review Comment: https://git.openjdk.org/jdk/pull/20235#discussion_r2000541492