(original) (raw)

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.