[vector] AVX2 ByteVector.shiftR performance and semantics

[Richard Startin]

I noticed that in the current implementation ByteVector.shiftR is much slower than the equivalent logic using IntVector for AVX2. I don't fully understand the compiled code in the ByteVector case but most of the time is spent inside VectorIntrinsics::broadcastInt (see attachment):

 @Benchmark
  public int shiftRByte() {
    return ((ByteVector)YMM_LONG.fromArray(data, 0)
            .rebracket(YMM_BYTE)).shiftR(4).and((byte)0x0F)
            .get(0);
  }

  @Benchmark
  public int shiftRInt() {
    return ((ByteVector)((IntVector)YMM_LONG.fromArray(data, 0)
            .rebracket(YMM_INT)).shiftR(4).and(0x0F0F0F0F)
            .rebracket(YMM_BYTE))
            .get(0);
  }

Benchmark                         (size)   Mode  Cnt    Score    Error   Units
PopCount.shiftRByte         1024  thrpt    5   29.310 ±  0.680  ops/us
PopCount.shiftRInt             1024  thrpt    5  257.261 ± 17.210  ops/us

It's not clear whether shiftR is a signed or unsigned shift. If it is an unsigned shift, I think it can be implemented efficiently in terms of ints, which already works very well.

Currently, ByteVector.shiftR seems to preserve sign, at least in the interpreter. Printing

println(((ByteVector)YMM_LONG.fromArray(data, 0).rebracket(YMM_BYTE)));
println(((ByteVector)YMM_LONG.fromArray(data, 0).rebracket(YMM_BYTE)).shiftR(4));
println(((ByteVector)((IntVector)YMM_LONG.fromArray(data, 0).rebracket(YMM_INT)).shiftR(4).and(0x0F0F0F0F)
.rebracket(YMM_BYTE)));

printed this:

[9, 15, -41, 21, 85, 116, -18, 41, 30, -8, -40, -96, 119, -75, 119, -85, -48, -28, -55, 84, 118, 7, -46, 15, -103, -37, 48, -52, 14, -32, -105, -84]
[0, 0, -3, 1, 5, 7, -2, 2, 1, -1, -3, -6, 7, -5, 7, -6, -3, -2, -4, 5, 7, 0, -3, 0, -7, -3, 3, -4, 0, -2, -7, -6]
[0, 0, 13, 1, 5, 7, 14, 2, 1, 15, 13, 10, 7, 11, 7, 10, 13, 14, 12, 5, 7, 0, 13, 0, 9, 13, 3, 12, 0, 14, 9, 10]

On the other hand, I saw that the shr instruction was used in the JIT compiled code.

Thanks,
Richard




<http://0x80.pl/articles/sse-popcount.html>




-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: perfasm.txt
URL: <https://mail.openjdk.java.net/pipermail/panama-dev/attachments/20190129/50764e70/perfasm-0001.txt>