[Python-ideas] Add specialized bytecode with guards to functions

[Victor Stinner]

2015-10-21 23:03 GMT+02:00 Antoine Pitrou <antoine at python.org>:
> I think adding a version number to dicts would be a very cheap change
> and would potentially help several CPython-based projects (including
> perhaps Numba some day, though nowadays it wouldn't benefit).

I tried to leave the dict type unchanged (it's not true in the PoC
right now, but I'm working on fixing this). Currently, the versionned
dictionary type (fat.verdict) is only used when Python starts in FAT
mode and only in a few places: module, class and instance
dictionaries. I'm not sure that it's worth to add the versionning
feature to the base dict type. It uses a little bit more memory, and
IMHO the dict type is already heavy in term of memory usage (compared
to a compact C array).

I'm trying to keep all FAT features optional to be able to easily
compare performances, but also ensure that the FAT mode can be added
to CPython without hurting performances (CPU and memory) when it's
disabled.

> However, I would also like to challenge the idea that an accelerator
> has to be 100% compatible. If your API is opt-in (which Numba's is),
> then you can happily relax some constraints (provided it is documented)
> in exchange for much faster code. Our experience with Numba is that
> nobody cares if you can't monkeypatch the len() builtin, for example.

Sure, it will be possible to skip some guards if you know that
builtins are never monkey-patched, that a module variable is constant,
that a class will never be modified, etc. This will reduce even more
the cost of guards (especially when all guards are skipped :-)).

But I prefer to start with something that doesn't change the Python
semantic at all, and then provide opt-in options to give more freedom
to the optimizer.

Victor