RFR: JDK-8300584: Accelerate AVX-512 CRC32C for small buffers

[Vladimir Kozlov]

On Wed, 18 Jan 2023 19:21:58 GMT, Scott Gibbons <duke at openjdk.org> wrote:

> Use AVX2 code for CRC32C for small buffers in the AVX-512 path.  Breakeven buffer size between the two algorithms is on the order of 384 bytes.
> 
> **Performance numbers for previous:**
> 
> Benchmark                    (count)   Mode  Cnt      Score     Error   Units
> TestCRC32C.testCRC32CUpdate       64  thrpt    4  66974.957 ±   8.872  ops/ms
> TestCRC32C.testCRC32CUpdate      128  thrpt    4  44224.810 ±  11.801  ops/ms
> TestCRC32C.testCRC32CUpdate      256  thrpt    4  63997.611 ± 173.577  ops/ms
> TestCRC32C.testCRC32CUpdate      512  thrpt    4  56068.683 ± 269.582  ops/ms
> TestCRC32C.testCRC32CUpdate     2048  thrpt    4  27048.098 ±  87.350  ops/ms
> TestCRC32C.testCRC32CUpdate    16384  thrpt    4   4066.736 ±  10.318  ops/ms
> TestCRC32C.testCRC32CUpdate    65536  thrpt    4   1040.754 ±   6.419  ops/ms
> 
> 
> **Performance numbers for this version:**
> 
> Benchmark                    (count)   Mode  Cnt       Score     Error   Units
> TestCRC32C.testCRC32CUpdate       64  thrpt    3  161659.326 ±  74.974  ops/ms
> TestCRC32C.testCRC32CUpdate      128  thrpt    3   88456.935 ±  11.940  ops/ms
> TestCRC32C.testCRC32CUpdate      256  thrpt    3   73254.993 ±   5.004  ops/ms
> TestCRC32C.testCRC32CUpdate      512  thrpt    3   56508.541 ± 298.229  ops/ms
> TestCRC32C.testCRC32CUpdate     2048  thrpt    3   26701.995 ±  31.369  ops/ms
> TestCRC32C.testCRC32CUpdate    16384  thrpt    3    4110.819 ±   4.618  ops/ms
> TestCRC32C.testCRC32CUpdate    65536  thrpt    3    1045.821 ±   2.037  ops/ms

We should avoid duplicating code. Since `crc32c_ipl_alg2_alt2` is used in both cases we can reshape code like next (pseudo code):


  if (supports_avx512()) {
    if (len > 384) {
       kernel_crc32_avx512();
       jmp Exit;
     }
  }
  crc32c_ipl_alg2_alt2();
Exit:
...

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

Changes requested by kvn (Reviewer).

PR: https://git.openjdk.org/jdk/pull/12079