[Python-Dev] PEP 509: Add a private version to dict

[Yury Selivanov]

On 2016-01-18 5:43 PM, Victor Stinner wrote:
> Is someone opposed to this PEP 509?
>
> The main complain was the change on the public Python API, but the PEP
> doesn't change the Python API anymore.
>
> I'm not aware of any remaining issue on this PEP.

Victor,

I've been experimenting with the PEP to implement a per-opcode
cache in ceval loop (I'll share my progress on that in a few
days).  This allows to significantly speedup LOAD_GLOBAL and
LOAD_METHOD opcodes, to the point, where they don't require
any dict lookups at all.  Some macro-benchmarks (such as
chameleon_v2) demonstrate impressive ~10% performance boost.

I rely on your dict->ma_version to implement cache invalidation.

However, besides guarding against version change, I also want
to guard against the dict being swapped for another dict, to
avoid situations like this:


     def foo():
         print(bar)

     exec(foo.__code__, {'bar': 1}, {})
     exec(foo.__code__, {'bar': 2}, {})


What I propose is to add a pointer "ma_extra" (same 64bits),
which will be set to NULL for most dict instances (instead of
ma_version).  "ma_extra" can then point to a struct that has a
globally unique dict ID (uint64), and a version tag (unit64).
A macro like PyDict_GET_ID and PyDict_GET_VERSION could then
efficiently fetch the version/unique ID of the dict for guards.

"ma_extra" would also make it easier for us to extend dicts
in the future.

Yury