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fix duplicated sections in frames.md file In the frames.md file at InternalDocs, change the first duplicated title to Specials, and remove unnecessary content under the Specials section.
136 lines
5.7 KiB
Markdown
136 lines
5.7 KiB
Markdown
# Frames
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Each call to a Python function has an activation record, commonly known as a
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"frame". It contains information about the function being executed, consisting
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of three conceptual sections:
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* Local variables (including arguments, cells and free variables)
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* Evaluation stack
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* Specials: The per-frame object references needed by the VM, including
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globals dict, code object, instruction pointer, stack depth, the
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previous frame, etc.
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The definition of the `_PyInterpreterFrame` struct is in
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[Include/internal/pycore_frame.h](../Include/internal/pycore_frame.h).
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# Allocation
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Python semantics allows frames to outlive the activation, so they need to
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be allocated outside the C call stack. To reduce overhead and improve locality
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of reference, most frames are allocated contiguously in a per-thread stack
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(see `_PyThreadState_PushFrame` in [Python/pystate.c](../Python/pystate.c)).
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Frames of generators and coroutines are embedded in the generator and coroutine
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objects, so are not allocated in the per-thread stack. See `PyGenObject` in
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[Include/internal/pycore_genobject.h](../Include/internal/pycore_genobject.h).
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## Layout
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Each activation record is laid out as:
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* Specials
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* Locals
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* Stack
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This seems to provide the best performance without excessive complexity.
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The specials have a fixed size, so the offset of the locals is known. The
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interpreter needs to hold two pointers, a frame pointer and a stack pointer.
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#### Alternative layout
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An alternative layout that was used for part of 3.11 alpha was:
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* Locals
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* Specials
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* Stack
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This has the advantage that no copying is required when making a call,
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as the arguments on the stack are (usually) already in the correct
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location for the parameters. However, it requires the VM to maintain
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an extra pointer for the locals, which can hurt performance.
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### Specials
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The specials section contains the following pointers:
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* Globals dict
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* Builtins dict
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* Locals dict (not the "fast" locals, but the locals for eval and class creation)
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* Code object
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* Heap allocated `PyFrameObject` for this activation record, if any.
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* The function.
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The pointer to the function is not strictly required, but it is cheaper to
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store a strong reference to the function and borrowed references to the globals
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and builtins, than strong references to both globals and builtins.
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### Frame objects
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When creating a backtrace or when calling `sys._getframe()` the frame becomes
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visible to Python code. When this happens a new `PyFrameObject` is created
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and a strong reference to it is placed in the `frame_obj` field of the specials
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section. The `frame_obj` field is initially `NULL`.
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The `PyFrameObject` may outlive a stack-allocated `_PyInterpreterFrame`.
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If it does then `_PyInterpreterFrame` is copied into the `PyFrameObject`,
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except the evaluation stack which must be empty at this point.
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The previous frame link is updated to reflect the new location of the frame.
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This mechanism provides the appearance of persistent, heap-allocated
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frames for each activation, but with low runtime overhead.
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### Generators and Coroutines
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Generators (objects of type `PyGen_Type`, `PyCoro_Type` or
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`PyAsyncGen_Type`) have a `_PyInterpreterFrame` embedded in them, so
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that they can be created with a single memory allocation.
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When such an embedded frame is iterated or awaited, it can be linked with
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frames on the per-thread stack via the linkage fields.
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If a frame object associated with a generator outlives the generator, then
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the embedded `_PyInterpreterFrame` is copied into the frame object (see
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`take_ownership()` in [Python/frame.c](../Python/frame.c)).
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### Field names
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Many of the fields in `_PyInterpreterFrame` were copied from the 3.10 `PyFrameObject`.
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Thus, some of the field names may be a bit misleading.
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For example the `f_globals` field has a `f_` prefix implying it belongs to the
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`PyFrameObject` struct, although it belongs to the `_PyInterpreterFrame` struct.
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We may rationalize this naming scheme for a later version.
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### Shim frames
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On entry to `_PyEval_EvalFrameDefault()` a shim `_PyInterpreterFrame` is pushed.
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This frame is stored on the C stack, and popped when `_PyEval_EvalFrameDefault()`
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returns. This extra frame is inserted so that `RETURN_VALUE`, `YIELD_VALUE`, and
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`RETURN_GENERATOR` do not need to check whether the current frame is the entry frame.
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The shim frame points to a special code object containing the `INTERPRETER_EXIT`
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instruction which cleans up the shim frame and returns.
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### The Instruction Pointer
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`_PyInterpreterFrame` has two fields which are used to maintain the instruction
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pointer: `instr_ptr` and `return_offset`.
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When a frame is executing, `instr_ptr` points to the instruction currently being
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executed. In a suspended frame, it points to the instruction that would execute
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if the frame were to resume. After `frame.f_lineno` is set, `instr_ptr` points to
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the next instruction to be executed. During a call to a python function,
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`instr_ptr` points to the call instruction, because this is what we would expect
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to see in an exception traceback.
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The `return_offset` field determines where a `RETURN` should go in the caller,
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relative to `instr_ptr`. It is only meaningful to the callee, so it needs to
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be set in any instruction that implements a call (to a Python function),
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including CALL, SEND and BINARY_SUBSCR_GETITEM, among others. If there is no
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callee, then return_offset is meaningless. It is necessary to have a separate
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field for the return offset because (1) if we apply this offset to `instr_ptr`
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while executing the `RETURN`, this is too early and would lose us information
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about the previous instruction which we could need for introspecting and
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debugging. (2) `SEND` needs to pass two offsets to the generator: one for
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`RETURN` and one for `YIELD`. It uses the `oparg` for one, and the
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`return_offset` for the other.
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