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

Yes I need to modify the comments. 

 masm->vsldoi(vTmp8, vMidProduct, vZero, 8);                       // mL : Extract the lower 64 bits of M
    masm->vsldoi(vTmp9, vZero, vMidProduct, 8);                       // mH : Extract the higher 64 bits of M
    masm->vxor(vLowProduct, vLowProduct, vTmp8);                      // LL + mL : Partial result for lower half
    masm->vxor(vHighProduct, vHighProduct, vTmp9);                    // HH + mH : Partial result for upper half

The above 4 are solving the parts where we multiply the lower and higher halves with the middle product.

http://web.archive.org/web/20130609111954/http://www.intel.com/content/dam/www/public/us/en/documents/white-papers/communications-ia-galois-counter-mode-paper.pdf 

Page 11 explains it. 



I am figuring out how to write the maths equation for the reduction part using vConstC2.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20235#discussion_r1987519084