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

[Brett Cannon]

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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