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

[Maciej Fijalkowski]

On Wed, Jan 20, 2016 at 7:22 PM, Brett Cannon <brett at python.org> wrote:
>
>
> On Wed, 20 Jan 2016 at 10:11 Yury Selivanov <yselivanov.ml at gmail.com> wrote:
>>
>> 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.
>
>
> Ooh, now my brain is trying to figure out the design of the cache. :)
>
>>
>>
>> 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.
>
>
> Why can't you simply use the id of the dict object as the globally unique
> dict ID? It's already globally unique amongst all Python objects which makes
> it inherently unique amongst dicts.
>
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Brett, you need two things - the ID of the dict and the version tag.
What we do in pypy is we have a small object (called, surprisingly,
VersionTag()) and we use the ID of that. That way you can change the
version id of an existing dict and have only one field.