RFR: 8261542: X86 slice and unslice intrinsics for 256-bit byte/short vectors

[Vladimir Kozlov]

On Thu, 11 Feb 2021 02:37:35 GMT, Sandhya Viswanathan <sviswanathan at openjdk.org> wrote:

> The slice and unslice intrinsics for 256-bit byte/short vectors can be implemented for x86 platforms supporting AVX2 using a sequence of instructions.
> 
> JBS: https://bugs.openjdk.java.net/browse/JDK-8261542
> 
> The PerfSliceOrigin.java jmh test attached to the JBS shows the following performance on AVX2 platform.
> 
> Before:
> Benchmark                                 (size)   Mode  Cnt   Score   Error   Units
> PerfSliceOrigin.vectorSliceOrigin           1024  thrpt    5  18.887 ± 1.128  ops/ms
> PerfSliceOrigin.vectorSliceUnsliceOrigin    1024  thrpt    5   9.374 ± 0.370  ops/ms
> 
> After:
> Benchmark                                 (size)   Mode  Cnt      Score     Error   Units
> PerfSliceOrigin.vectorSliceOrigin           1024  thrpt    5  13861.420 ±  19.071  ops/ms
> PerfSliceOrigin.vectorSliceUnsliceOrigin    1024  thrpt    5   7895.199 ± 142.580  ops/ms

Please, add a test which verifies correctness of results when this code is used. If we don't have it already.

src/hotspot/cpu/x86/x86.ad line 7506:

> 7504: instruct rearrangeB_avx(legVec dst, legVec src, vec shuffle, legVec vtmp1, legVec vtmp2, rRegP scratch) %{
> 7505:   predicate(vector_element_basic_type(n) == T_BYTE &&
> 7506:             vector_length(n) == 32 && !VM_Version::supports_avx512_vbmi());

Predicate matches bail-out condition in match_rule_supported_vector(). Does it mean this code never used before?
So you are implementing it now. Right?

src/hotspot/cpu/x86/x86.ad line 7550:

> 7548:     // only byte shuffle instruction available on these platforms
> 7549:     int vlen_in_bytes = vector_length_in_bytes(this);
> 7550:     if (UseAVX == 0) {

This code will not be executed with vector length 16 because match_rule_supported_vector() bailout with (size_in_bits == 256 && UseAVX < 2).

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

Changes requested by kvn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2520