# Frames

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, including
  globals dict, code object, instruction pointer, stack depth, the
  previous frame, etc.

The definition of the `_PyInterpreterFrame` struct is in
[Include/internal/pycore_interpframe_structs.h](../Include/internal/pycore_interpframe_structs.h).

# 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](../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_interpframe_structs.h](../Include/internal/pycore_interpframe_structs.h).

## Layout

Each activation record is laid out as:

* Specials
* Locals
* Stack

This seems to provide the best performance without excessive complexity.
The specials have a fixed size, so the offset of the locals is known. 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
* Stack

This has the advantage that no copying is required when making a call,
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.

### Specials

The specials section contains the following pointers:

* Globals dict
* Builtins dict
* Locals dict (not the "fast" locals, but the locals for eval and class creation)
* Code object
* Heap allocated `PyFrameObject` for this activation record, if any.
* The function.

The pointer to the function is not strictly required, but it is cheaper to
store a strong reference to the function and borrowed references to the globals
and builtins, than strong references to both globals and builtins.

### Frame objects

When creating a backtrace or when calling `sys._getframe()` the frame becomes
visible to Python code. When this happens a new `PyFrameObject` is created
and a strong reference to it is placed in the `frame_obj` field of the specials
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 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

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 (see
`take_ownership()` in [Python/frame.c](../Python/frame.c)).

### Field names

Many of the fields in `_PyInterpreterFrame` were copied from the 3.10 `PyFrameObject`.
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 a later version.


### Shim frames

On entry to `_PyEval_EvalFrameDefault()` a shim `_PyInterpreterFrame` is pushed.
This frame is stored on the C stack, and popped when `_PyEval_EvalFrameDefault()`
returns. This extra frame is inserted so that `RETURN_VALUE`, `YIELD_VALUE`, and
`RETURN_GENERATOR` do not need to check whether the current frame is the entry frame.
The shim frame points to a special code object containing the `INTERPRETER_EXIT`
instruction which cleans up the shim frame and returns.


### Base frame

Each thread state contains an embedded `_PyInterpreterFrame` called the "base frame"
that serves as a sentinel at the bottom of the frame stack. This frame is allocated
in `_PyThreadStateImpl` (the internal extension of `PyThreadState`) and initialized
when the thread state is created. The `owner` field is set to `FRAME_OWNED_BY_INTERPRETER`.

External profilers and sampling tools can validate that they have successfully unwound
the complete call stack by checking that the frame chain terminates at the base frame.
The `PyThreadState.base_frame` pointer provides the expected address to compare against.
If a stack walk doesn't reach this frame, the sample is incomplete (possibly due to a
race condition) and should be discarded.

The base frame is embedded in `_PyThreadStateImpl` rather than `PyThreadState` because
`_PyInterpreterFrame` is defined in internal headers that cannot be exposed in the
public API. A pointer (`PyThreadState.base_frame`) is provided for profilers to access
the address without needing internal headers.

See the initialization in `new_threadstate()` in [Python/pystate.c](../Python/pystate.c).

#### How profilers should use the base frame

External profilers should read `tstate->base_frame` before walking the stack, then
walk from `tstate->current_frame` following `frame->previous` pointers until reaching
a frame with `owner == FRAME_OWNED_BY_INTERPRETER`. After the walk, verify that the
last frame address matches `base_frame`. If not, discard the sample as incomplete
since the frame chain may have been in an inconsistent state due to concurrent updates.


### Remote Profiling Frame Cache

The `last_profiled_frame` field in `PyThreadState` supports an optimization for
remote profilers that sample call stacks from external processes. When a remote
profiler reads the call stack, it writes the current frame address to this field.
The eval loop then keeps this pointer valid by updating it to the parent frame
whenever a frame returns (in `_PyEval_FrameClearAndPop`).

This creates a "high-water mark" that always points to a frame still on the stack.
On subsequent samples, the profiler can walk from `current_frame` until it reaches
`last_profiled_frame`, knowing that frames from that point downward are unchanged
and can be retrieved from a cache. This significantly reduces the amount of remote
memory reads needed when call stacks are deep and stable at their base.

The update in `_PyEval_FrameClearAndPop` is guarded: it only writes when
`last_profiled_frame` is non-NULL AND matches the frame being popped. This
prevents transient frames (called and returned between profiler samples) from
corrupting the cache pointer, while avoiding any overhead when profiling is inactive.


### The Instruction Pointer

`_PyInterpreterFrame` has two fields which are used to maintain the instruction
pointer: `instr_ptr` and `return_offset`.

When a frame is executing, `instr_ptr` points to the instruction currently being
executed. In a suspended frame, it points to the instruction that would execute
if the frame were to resume. After `frame.f_lineno` is set, `instr_ptr` points to
the next instruction to be executed. During a call to a python function,
`instr_ptr` points to the call instruction, because this is what we would expect
to see in an exception traceback.

The `return_offset` field determines where a `RETURN` should go in the caller,
relative to `instr_ptr`.  It is only meaningful to the callee, so it needs to
be set in any instruction that implements a call (to a Python function),
including CALL, SEND and BINARY_OP_SUBSCR_GETITEM, among others. If there is no
callee, then return_offset is meaningless. It is necessary to have a separate
field for the return offset because (1) if we apply this offset to `instr_ptr`
while executing the `RETURN`, this is too early and would lose us information
about the previous instruction which we could need for introspecting and
debugging. (2) `SEND` needs to pass two offsets to the generator: one for
`RETURN` and one for `YIELD`. It uses the `oparg` for one, and the
`return_offset` for the other.