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gh-119786: move frames documentation to InternalDocs and add details (#121009)

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Irit Katriel 2024-06-26 13:18:20 +01:00 committed by GitHub
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InternalDocs/README.md
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@ -14,6 +14,8 @@ it is not, please report that through the
[Compiler Design](compiler.md)
[Frames](frames.md)
[Adaptive Instruction Families](adaptive.md)
[The Source Code Locations Table](locations.md)

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Objects/frame_layout.md → InternalDocs/frames.md
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@ -1,51 +1,47 @@
# The Frame Stack
# Frames
Each call to a Python function has an activation record,
commonly known as a "frame".
Python semantics allows frames to outlive the activation,
so they have (before 3.11) been allocated on the heap.
This is expensive as it requires many allocations and
results in poor locality of reference.
In 3.11, rather than have these frames scattered about memory,
as happens for heap-allocated objects, frames are allocated
contiguously in a per-thread stack.
This improves performance significantly for two reasons:
* It reduces allocation overhead to a pointer comparison and increment.
* Stack allocated data has the best possible locality and will always be in
CPU cache.
Generator and coroutines still need heap allocated activation records, but
can be linked into the per-thread stack so as to not impact performance too much.
## Layout
Each activation record consists of four conceptual sections:
Each call to a Python function has an activation record, commonly known as a
"frame". It contains information about the function being executed, consisting
of three conceptual sections:
* Local variables (including arguments, cells and free variables)
* Evaluation stack
* Specials: The per-frame object references needed by the VM: globals dict,
code object, etc.
* Linkage: Pointer to the previous activation record, stack depth, etc.
* Specials: The per-frame object references needed by the VM, including
globals dict, code object, instruction pointer, stack depth, the
previous frame, etc.
### Layout
The definition of the ``_PyInterpreterFrame`` struct is in
[Include/internal/pycore_frame.h](https://github.com/python/cpython/blob/main/Include/internal/pycore_frame.h).
The specials and linkage sections are a fixed size, so are grouped together.
# Allocation
Python semantics allows frames to outlive the activation, so they need to
be allocated outside the C call stack. To reduce overhead and improve locality
of reference, most frames are allocated contiguously in a per-thread stack
(see ``_PyThreadState_PushFrame`` in
[Python/pystate.c](https://github.com/python/cpython/blob/main/Python/pystate.c)).
Frames of generators and coroutines are embedded in the generator and coroutine
objects, so are not allocated in the per-thread stack. See ``PyGenObject`` in
[Include/internal/pycore_genobject.h](https://github.com/python/cpython/blob/main/Include/internal/pycore_genobject.h).
## Layout
Each activation record is laid out as:
* Specials and linkage
* Specials
* Locals
* Stack
This seems to provide the best performance without excessive complexity.
It needs the interpreter to hold two pointers, a frame pointer and a stack pointer.
The specials have a fixed size, so the offset of the locals is know. The
interpreter needs to hold two pointers, a frame pointer and a stack pointer.
#### Alternative layout
An alternative layout that was used for part of 3.11 alpha was:
* Locals
* Specials and linkage
* Specials
* Stack
This has the advantage that no copying is required when making a call,
@ -53,19 +49,6 @@ as the arguments on the stack are (usually) already in the correct
location for the parameters. However, it requires the VM to maintain
an extra pointer for the locals, which can hurt performance.
A variant that only needs the need two pointers is to reverse the numbering
of the locals, so that the last one is numbered `0`, and the first in memory
is numbered `N-1`.
This allows the locals, specials and linkage to accessed from the frame pointer.
We may implement this in the future.
#### Note:
> In a contiguous stack, we would need to save one fewer registers, as the
> top of the caller's activation record would be the same at the base of the
> callee's. However, since some activation records are kept on the heap we
> cannot do this.
### Generators and Coroutines
Generators and coroutines contain a `_PyInterpreterFrame`
@ -92,25 +75,23 @@ section. The `frame_obj` field is initially `NULL`.
The `PyFrameObject` may outlive a stack-allocated `_PyInterpreterFrame`.
If it does then `_PyInterpreterFrame` is copied into the `PyFrameObject`,
except the evaluation stack which must be empty at this point.
The linkage section is updated to reflect the new location of the frame.
The previous frame link is updated to reflect the new location of the frame.
This mechanism provides the appearance of persistent, heap-allocated
frames for each activation, but with low runtime overhead.
### Generators and Coroutines
Generator objects have a `_PyInterpreterFrame` embedded in them.
This means that creating a generator requires only a single allocation,
reducing allocation overhead and improving locality of reference.
The embedded frame is linked into the per-thread frame when iterated or
awaited.
Generators (objects of type ``PyGen_Type``, ``PyCoro_Type`` or
``PyAsyncGen_Type``) have a `_PyInterpreterFrame` embedded in them, so
that they can be created with a single memory allocation.
When such an embedded frame is iterated or awaited, it can be linked with
frames on the per-thread stack via the linkage fields.
If a frame object associated with a generator outlives the generator, then
the embedded `_PyInterpreterFrame` is copied into the frame object.
All the above applies to coroutines and async generators as well.
the embedded `_PyInterpreterFrame` is copied into the frame object (see
``take_ownership()`` in
[Python/frame.c](https://github.com/python/cpython/blob/main/Python/frame.c)).
### Field names
@ -119,7 +100,7 @@ Thus, some of the field names may be a bit misleading.
For example the `f_globals` field has a `f_` prefix implying it belongs to the
`PyFrameObject` struct, although it belongs to the `_PyInterpreterFrame` struct.
We may rationalize this naming scheme for 3.12.
We may rationalize this naming scheme for a later version.
### Shim frames