[mypyc] Allow defining a single-item free "list" for a native class - #19785
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It's quite common to have a class where we almost always have at most
a single allocated instance (per thread). Now these instances can be
allocated more quickly, by reusing the memory that was used for the
most recently freed instance (a separate memory block is reused for
each thread on free-threaded builds).
It's used like this (only the value 1 is supported for now):
```
from mypy_extensions import mypyc_attr
@mypyc_attr(free_list=1)
class Foo:
...
```
This makes a microbenchmark that only allocates and immediately
frees simple objects repeatedly around 3.8x faster.
It's probably worth extending this to support larger free lists in the
future.
We can later look into enabling this automatically for certain native
classes based on profile information.
for more information, see https://pre-commit.ci
p-sawicki
approved these changes
Sep 2, 2025
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It's quite common to have a class where we almost always have at most a single allocated instance (per thread). Now these instances can be allocated more quickly, by reusing the memory that was used for the most recently freed instance (a separate memory block is reused for each thread on free-threaded builds).
It's used like this (only the value 1 is supported for now):
This makes a microbenchmark that only allocates and immediately frees simple objects repeatedly around 3.8x faster.
It's probably worth extending this to support larger free lists in the future.
We can later look into enabling this automatically for certain native classes based on profile information.