Re: [Haskell-cafe] Fast JSON validation - reducing allocations

11 May 2017


      David Turner  writes:
...
Ben, Eric, thanks for your help. A whole new world of low-level statistics
opens up before me...
No worries!
...
You're not wrong.
I've trawled through the STG as best as I can for a newbie. The function
with a large number in the 'Alloc' column looks pretty much to be the heart
of the `step` function. There's a lot of nesting but it looks largely to be
just checking the bounds on array indices, which I don't think allocates
anything.
Something that I should have mentioned earlier is that STG has the nice
property that all allocation is syntactically obvious: allocated
closures manifest as `let`s. This makes it fairly easy to pick out
possible allocation sites, even in large dumps.
...
However I see this this little snippet at the very bottom of the nesting:
case
    indexArray# [ds1_sl1b
                 y2_sl5H]
of
_ [Occ=Dead]
{ (##) ipv8_sl60 [Occ=Once!] ->
      case
          ipv8_sl60
      of
      _ [Occ=Dead]
      { GHC.Word.W8# dt6_sl62 [Occ=Once] ->
            case
                ss_sl5v
            of
            dt7_sl63
            { __DEFAULT ->
                  (#,,#) [__word 1
                          dt6_sl62
                          dt7_sl63];
            };
      };
};
I'm guessing that the `indexArray` call is the line that reads `nextState =
aTransitionsTable makeAutomaton AU.! (currentState, nextByte)`, and to me
it looks like it might be unboxing the thing it's getting out of the array.
Not sure that counts as an allocation, but I'm surprised anyway.
The unboxing should just involve looking at the value field of the W8#
closure (e.g. a memory dereference) and putting the value in a machine
register. There is no allocation here as far as I can see.
...
I'll keep digging anyway, but it'd be good to hear of any progress at your
end too. Hope it didn't spoil your lunch!
Far from it! It was nice to have a puzzle to ponder.

Cheers,

- Ben

Re: [Haskell-cafe] Fast JSON validation - reducing allocations

Ben Gamari