3 ms·

You mean add a level of indirection to every class access? Also there is already compression for that pointer, making it 32 instead of 64 bits.

• saagarjha 4y ago
  Turning a pointer loading into an addition and then a pointer load is not that much overhead.
  • remram 4y ago
    That's how the current compression works, I guess I don't understand what you propose.
    • saagarjha 4y ago
      I guess I am confused as to what the extra indirection is.
      • chrisseaton 4y ago
        The current compression scheme doesn’t involve reading any memory. Indirection through a lookup table would. But who cares? 99% of the time you just need class identity, not values from it.
        • saagarjha 4y ago
          If you need identity you can just compare indices instead of pointers, no?
          • chrisseaton 4y ago
            What's what I just said? > 99% of the time you just need class identity, not values from it. So no need to deference the class pointer, or the table index.
            • saagarjha 4y ago
              Ok taking a step back I just realized you aren't 'remram :P But anyways my point was that in the old scheme if you wanted to know where the class was you would do some arithmetic on the compressed pointer to get a full pointer and now you can do a comparison, or if you want to read something do a dereference. Assuming the new layout is an array of classes you have a similar scheme where you have an index and you can compare those directly, and if you want data in the class you deref.
              • chrisseaton 4y ago
                > in the old scheme if you wanted to know where the class was you would do some arithmetic on the compressed pointer to get a full pointer and now you can do a comparison Well also you didn't actually need to decompress the pointer first - compressed pointers are unique as well of course.
                • saagarjha 4y ago
                  Ah, yes, true.
              • twic 4y ago
                Also, i believe that 'classes' aren't fixed size, because they contain the class's vtable [1]. As such, you couldn't keep them in an indexable array anyway. [1] in HotSpot they are instances of InstanceKlass: https://github.com/openjdk/jdk/blob/master/src/hotspot/share/oops/instanceKlass.hpp#L47 https://github.com/openjdk/jdk/blob/master/src/hotspot/share...
                • chrisseaton 4y ago
                  I think they mean keep references to them in an indexable array.
                  • remram 4y ago
                    That is the extra indirection I was talking about.