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> fucking cpython is allocating its integers on the heap and motherfucking reference-counting them And here I thought that it was shocking to learn that v8 all

> fucking cpython is allocating its integers on the heap and motherfucking reference-counting them And here I thought that it was shocking to learn that v8 allocates doubles on the heap recently. (I mean, I'm not a compiler writer, I have no idea how hard it would be to avoid this, but it feels like mandatory boxed floats would hurt performance a lot)

kragen 3y ago
nanboxing as used in spidermonkey (https://piotrduperas.com/posts/nan-boxing https://piotrduperas.com/posts/nan-boxing) is a possible alternative, but i think v8 works pretty hard to not use floats, and i don't think local-variable or temporary floats end up on the heap in v8 the way they do in cpython. i'm not that familiar with v8 tho (but i'm pretty sure it doesn't refcount things)
- vanderZwan 3y ago
  > i think v8 works pretty hard to not use floats Correct, to the point where at work a colleague and I actually have looked into how to force using floats even if we initiate objects with a small-integer number (the idea being that ensuring our objects having the correct hidden class the first time might help the JIT, and avoids wasting time on integer-to-float promotion in tight loops). Via trial and error in Node we figured that using -0 as a number literal works, but (say) 1.0 does not. > i don't think local-variable or temporary floats end up on the heap in v8 the way they do in cpython This would also make sense - v8 already uses pools to re-use common temporary object shapes in general IIRC, I see no reason why it wouldn't do at least that with heap-allocated doubles too.
  - kragen 3y ago
    so then the remaining performance-critical case is where you have a big array of floats you're looping over. in firefox that works fine (one allocation per lowest-level array, not one allocation and unprefetchable pointer dereference per float), but maybe in chrome you'd want to use a typedarray?
    - vanderZwan 3y ago
      Maybe, at that point it is basically similar to the struct-of-arrays vs array-of-structs trade-off, except with significantly worse ergonomics and less pay-off.
    - IainIreland 3y ago
      As I understand it, V8 keeps track of an ElementsKind for each array (or, more precisely, for the elements of every object; arrays are not special in this sense). If an array only contains floats, then they will all be stored unboxed and inline. See here: https://source.chromium.org/chromium/chromium/src/+/main:v8/src/objects/elements-kind.h;l=78;bpv=1 https://source.chromium.org/chromium/chromium/src/+/main:v8/... I assume that integers are coerced to floats in this mode, and that there's a performance cliff if you store a non-number in such an array, but in both cases I'm just guessing. In SpiderMonkey, as you say, we store all our values as doubles, and disguise the non-float values as NaNs.
      - kragen 3y ago
        thank you for the correction!

kragen 3y ago

nanboxing as used in spidermonkey (https://piotrduperas.com/posts/nan-boxing https://piotrduperas.com/posts/nan-boxing) is a possible alternative, but i think v8 works pretty hard to not use floats, and i don't think local-variable or temporary floats end up on the heap in v8 the way they do in cpython. i'm not that familiar with v8 tho (but i'm pretty sure it doesn't refcount things)

vanderZwan 3y ago
> i think v8 works pretty hard to not use floats Correct, to the point where at work a colleague and I actually have looked into how to force using floats even if we initiate objects with a small-integer number (the idea being that ensuring our objects having the correct hidden class the first time might help the JIT, and avoids wasting time on integer-to-float promotion in tight loops). Via trial and error in Node we figured that using -0 as a number literal works, but (say) 1.0 does not. > i don't think local-variable or temporary floats end up on the heap in v8 the way they do in cpython This would also make sense - v8 already uses pools to re-use common temporary object shapes in general IIRC, I see no reason why it wouldn't do at least that with heap-allocated doubles too.
- kragen 3y ago
  so then the remaining performance-critical case is where you have a big array of floats you're looping over. in firefox that works fine (one allocation per lowest-level array, not one allocation and unprefetchable pointer dereference per float), but maybe in chrome you'd want to use a typedarray?
  - vanderZwan 3y ago
    Maybe, at that point it is basically similar to the struct-of-arrays vs array-of-structs trade-off, except with significantly worse ergonomics and less pay-off.
  - IainIreland 3y ago
    As I understand it, V8 keeps track of an ElementsKind for each array (or, more precisely, for the elements of every object; arrays are not special in this sense). If an array only contains floats, then they will all be stored unboxed and inline. See here: https://source.chromium.org/chromium/chromium/src/+/main:v8/src/objects/elements-kind.h;l=78;bpv=1 https://source.chromium.org/chromium/chromium/src/+/main:v8/... I assume that integers are coerced to floats in this mode, and that there's a performance cliff if you store a non-number in such an array, but in both cases I'm just guessing. In SpiderMonkey, as you say, we store all our values as doubles, and disguise the non-float values as NaNs.
    - kragen 3y ago
      thank you for the correction!