RFR: 8296411: AArch64: Accelerated Poly1305 intrinsics [v2]

[Andrew Haley]

On Wed, 24 May 2023 10:07:47 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> Andrew Haley has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Whitespace
>
> src/hotspot/cpu/aarch64/stubGenerator_aarch64.cpp line 7097:
> 
>> 7095:       // together partial products without any risk of needing to
>> 7096:       // propagate a carry out.
>> 7097:       wide_mul(U_0, U_0HI, S_0, R_0);  wide_madd(U_0, U_0HI, S_1, RR_1); wide_madd(U_0, U_0HI, S_2, RR_0);
> 
> What is `r` corresponding to here? This asserts that 'the top four bits of each 32-bit subword of "r" are zero'. If `r` is `R_0...R_2` it would seem broken since we're packing 26-bit values into `R_0...R_2` above in a way that would break this invariant?

No, it doesn't break the invariants.

R is the randomly-chosen 128-bit key. It is generated from an initial 128-bit-log string or random bits, then
`r &= 0x0ffffffc0ffffffc0ffffffc0fffffff`

This 128-bit-long string is split into 26-bit limbs before the intrinsic is called. The zero bits remain zero.
When we repack R into two 64-bit registers those zero bits are still zero.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14085#discussion_r1203850178