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optimize nextPowOfTwo#5347

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headius merged 2 commits intojruby:masterfrom
ahorek:pow2
Oct 11, 2018
Merged

optimize nextPowOfTwo#5347
headius merged 2 commits intojruby:masterfrom
ahorek:pow2

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@ahorek
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@ahorek ahorek commented Oct 4, 2018 •
edited
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numberOfLeadingZeros is vectorized and more effective than the original version

It's also possible to use highestOneBit, but I didn't because it was optimized the same way only since java 11 see https://bugs.openjdk.java.net/browse/JDK-8199843

require 'benchmark/ips'
require 'set'
Benchmark.ips do |x|
  x.report('set') { 100.times { Set[:x, :a, :h, :h, :h, :g, :h, :h, :h, :i] } }
end

patch

                      8.632k (± 1.9%) i/s -     43.316k in   5.020055s

master

                      8.384k (±10.6%) i/s -     41.477k in   5.035840s

patch

         0: ldc           #3                  // int -2147483648
         2: iload_0
         3: invokestatic  #4                  // Method java/lang/Integer.numberOfLeadingZeros:(I)I
         6: iushr
         7: istore_1
         8: iload_0
         9: iload_1
        10: if_icmpne     17
        13: iload_0
        14: goto          20
        17: iload_1
        18: iconst_1
        19: ishl
        20: ireturn

master

         0: iload_0
         1: istore_1
         2: iinc          1, -1
         5: iload_1
         6: iload_1
         7: iconst_1
         8: ishr
         9: ior
        10: istore_1
        11: iload_1
        12: iload_1
        13: iconst_2
        14: ishr
        15: ior
        16: istore_1
        17: iload_1
        18: iload_1
        19: iconst_4
        20: ishr
        21: ior
        22: istore_1
        23: iload_1
        24: iload_1
        25: bipush        8
        27: ishr
        28: ior
        29: istore_1
        30: iload_1
        31: iload_1
        32: bipush        16
        34: ishr
        35: ior
        36: istore_1
        37: iinc          1, 1
        40: iload_1
        41: ireturn

@ahorek ahorek changed the title optimalize nextPowOfTwo optimize nextPowOfTwo Oct 5, 2018
@kares
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kares commented Oct 11, 2018 •
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heh, interesting ... maybe the branch (last line) could be eliminated as well?

@ahorek
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ahorek commented Oct 11, 2018

I took a safe path, the branch could be eliminated only if we can ensure that the input will always be positive and not 0 or 1. Let me confirm that.

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ahorek commented Oct 11, 2018 •
edited
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         0: ldc           #3                  // int -2147483648
         2: iload_0
         3: iconst_1
         4: isub
         5: invokestatic  #4                  // Method java/lang/Integer.numberOfLeadingZeros:(I)I
         8: iushr
         9: iconst_1
        10: ishl
        11: ireturn

        8.787k (±14.2%) i/s -     43.036k in   5.046647s

inspiration jemalloc/jemalloc#1303

@headius headius merged commit 825026b into jruby:master Oct 11, 2018
@headius headius added this to the JRuby 9.2.1.0 milestone Oct 11, 2018
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@ahorek @kares @headius