RFR: 8319716: RISC-V: Add SHA-2

[Robbin Ehn]

On Tue, 21 Nov 2023 08:47:37 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>>> Depending on hardware pipeline depth this load can actually be executed after "__ vadd_vv(v14, v15, v10);" thus that instruction maybe already be retired when reaching round 1.
>>> 
>>> Preloading these, depending on the number of V-load ports, the preloading it self can be very costly as they can't be executed out-of-order in parallel.
>> 
>> Make sense. I was expecting those to retire when reaching the first round (round0).
>> 
>>> So hiding the load in previous round can be faster, therefore my fast conclusion without numbers was at least for single pass no preloading _should_ be better on bigger hardware.
>> 
>> But I see that there is a true data dependence on the vector load for each round. Any thing I missed?
>> Say, for round2:
>> 
>>     // Quad-round 2 (+2, v12->v13->v10->v11)
>>     __ vl1re32_v(v15, consts);     ----> Define v15
>>     __ addi(consts, consts, 16);
>>     __ vadd_vv(v14, v15, v12);    ----> Use v15
>
> __ vadd_vv(v14, v15, v11);
>     <load can start>             <<----------------------------------------------------------|
>     __ vsha2cl_vv(v17, v16, v14);                                                            |
>     __ vsha2ch_vv(v16, v17, v14);                                                            |
>     __ vmerge_vvm(v14, v13, v12);                                                            |
>     __ vsha2ms_vv(v11, v14, v10); // Generate W[23:20]                                       |
>     //--------------------------------------------------------------------------------       |
>     // Quad-round 2 (+2, v12->v13->v10->v11)                                                 |
>     __ vl1re32_v(v15, consts); ---------------------------------------------------------------
> 
> 
> No ?

If you consider register renaming load should be enable to start even earlier.
E.g. load into vX, then rename vX to v15 after the add that uses v15.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16562#discussion_r1400231771