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  1. JDK
  2. JDK-8074124

Most Unsafe.get*() access shapes are losing vs. the plain Java accesses

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      If you run a simple benchmark like this:

          @Benchmark
          public int stream_unsafe_char() {
              int s = 0;
              for (int c = 0; c < size; c++) {
                  s += U.getChar(charArr, CHAR_ARR_OFFSET + CHAR_ARR_SCALE * c);
              }
              return s;
          }

      ...then you will notice that generated code has a stray "movslq":

       0x00007ffad8741180: movslq %r8d,%r9 <--- unnecessary sign extension
       0x00007ffad8741183: movzwl 0x10(%r10,%r9,2)
       0x00007ffad8741189: add %ecx,%edx
       0x00007ffad874118b: inc %r8d
       0x00007ffad874118e: cmp %r11d,%r8d
       0x00007ffad8741191: jl 0x00007ffad8741180

      Indeed, the baseline test that does the plain Java access:

          @Benchmark
          public int plain() {
              int s = 0;
              for (int c = 0; c < size; c++) {
                  s += charArr[c];
              }
              return s;
          }

      ...does without sign extension:

       0x00007fc3d0f84ea0: movzwl 0x10(%r11,%r8,2),%ecx
       0x00007fc3d0f84ea6: add %ecx,%edx
       0x00007fc3d0f84ea8: inc %r8d
       0x00007fc3d0f84eab: cmp %r10d,%r8d
       0x00007fc3d0f84eae: jl 0x00007fc3d0f84ea0

      This conversion costs some cycles on my 1x4x2 4 GHz Haswell, running with Linux x86_64, 8u40 EA:

      Benchmark (size) Mode Cnt Score Error Units
      UnsafeMovslq.plain 1000 avgt 5 343.719 ± 0.694 ns/op
      UnsafeMovslq.unsafe 1000 avgt 5 362.350 ± 5.302 ns/op

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              • Assignee:
                Unassigned
                Reporter:
                shade Aleksey Shipilev
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                • Created:
                  Updated: