RFR: 8355216: Accelerate P-256 arithmetic on aarch64

[Andrew Haley]

On Tue, 18 Nov 2025 19:00:21 GMT, Ben Perez <bperez at openjdk.org> wrote:

> Is there a reason hotspot doesn't leave `r9` open for use as a caller saved local variable like in the ARM docs https://developer.arm.com/documentation/102374/0103/Procedure-Call-Standard. Either way will fix.

Yes, because a ton of convenience macros in `MacroAssembler` use it. HotSpot internally doesn't use the APCS.

>> src/hotspot/cpu/aarch64/stubGenerator_aarch64.cpp line 7169:
>> 
>>> 7167:     Register mul_tmp = r14;
>>> 7168:     Register n = r15;
>>> 7169: 
>> 
>> Here, you could do something like
>> 
>> 
>>     RegSet scratch = RegSet::range(r3, r28) - rscratch1 - rscratch2;
>> 
>>     {
>>       auto r_it = scratch.begin();
>>       Register
>>         c_ptr = *r_it++,
>>         a_i = *r_it++,
>>         c_idx = *r_it++, //c_idx is not used at the same time as a_i
>>         limb_mask_scalar = *r_it++,
>>         b_j = *r_it++,
>>         mod_j = *r_it++,
>>         mod_ptr = *r_it++,
>>         mul_tmp = *r_it++,
>>         n = *r_it++;
>>        ...
>>     }
>> 
>> 
>> 
>> Note that a RegSet iterator doesn't affect the RegSet it was created from, so once this block has ended you can allocate again from the set of scratch registers.
>
> Is there by any chance documentation for `RegSet` that I can reference while making these changes?

I could talk you through it, or give you a few more examples. It's easy to use once you get used to it.

> thanks for the suggestion! Does using `T16B` improve performance? Similarly, should this be applied to `EOR` as well?

Yes. According to the ARM, only 8B and 16B forms are the official names.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27946#discussion_r2541919496
PR Review Comment: https://git.openjdk.org/jdk/pull/27946#discussion_r2477261216
PR Review Comment: https://git.openjdk.org/jdk/pull/27946#discussion_r2476966812