RFR: 8358032: Use crypto pmull for CRC32/CRC32C intrinsics on Ampere CPU [v2]

[Andrew Haley]

On Wed, 4 Jun 2025 03:30:11 GMT, Liming Liu <lliu at openjdk.org> wrote:

>> This PR is to enable the use of crypto pmull for CRC32/CRC32C intrinsics on Ampere CPU. There is an option UseCryptoPmullForCRC32 that can enable crypto pmull, but directly enabling it on Ampere CPU will cause the following problems.
>> 
>> 1. There will be regressions (-14% ~ -8%) on Ampere1 when the length is 64. When <= 128, both kernel_crc32_using_crc32 and kernel_crc32_using_crypto_pmull use the loop labeled as CRC_by32_loop, but their implements are a little different, and the loop in kernel_crc32_using_crc32 is better at hiding latency on Ampere1. So this PR takes the loop in kernel_crc32_using_crc32 to kernel_crc32_using_crypto_pmull, and does the same for CRC32C intrinsic.
>> 
>> 2. The intrinsics only use crypto pmull when the length is higher than 383, while the loop in kernel_crc32_common_fold_using_crypto_pmull looks able to handle 256, and if it handles 256 on Ampere1, the improvements can be as high as 110% compared with kernel_crc32_using_crc32/kernel_crc32c_using_crc32c. However, there are regressions (~-6%) on Neoverse V1 when the length is 256. So this PR introduces a new option named CryptoPmullForCRC32LowLimit. It defaults to 256 since the code could handle 256, while it is set to 384 for V1/V2 to keep the old behavior on these platforms.
>> 
>> The performance regressions and improvements were measured with the following microbenchmarks:
>> org.openjdk.bench.java.util.TestCRC32.testCRC32Update
>> org.openjdk.bench.java.util.TestCRC32C.testCRC32CUpdate
>> 
>> Ran the following JTReg tests on Ampere1 and did not find problems:
>> test/hotspot/jtreg/compiler/intrinsics/zip/TestCRC32.java
>> test/hotspot/jtreg/compiler/intrinsics/zip/TestCRC32C.java
>
> Liming Liu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Make it be a diagnostic flag

src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 4355:

> 4353:     add(buf, buf, 32);
> 4354:     crc32x(crc, crc, tmp2);
> 4355:     subs(len, len, 32);

What is the point of these changes?

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/25609#discussion_r2126035063