gh-142183: Change data stack to use a resizable array

Include/cpython/pystate.h

@@ -55,14 +55,14 @@ typedef struct _err_stackitem {
 } _PyErr_StackItem;
 
 typedef struct _stack_chunk {
-    struct _stack_chunk *previous;
     size_t size;
-    size_t top;
-    PyObject * data[1]; /* Variable sized */
+    struct _stack_chunk *previous;
+    PyObject *data[1]; /* Variable sized */
 } _PyStackChunk;
 
-/* Minimum size of data stack chunk */
-#define _PY_DATA_STACK_CHUNK_SIZE (16*1024)
+#define _PY_STACK_CHUNK_MIN_SIZE 4096
+#define _PY_STACK_CHUNK_OVERHEADS (offsetof(_PyStackChunk, data))
+
 struct _ts {
     /* See Python/ceval.c for comments explaining most fields */
 
@@ -195,10 +195,9 @@ struct _ts {
     /* Unique thread state id. */
     uint64_t id;
 
-    _PyStackChunk *datastack_chunk;
-    PyObject **datastack_top;
-    PyObject **datastack_limit;
-    _PyStackChunk *datastack_cached_chunk;
+    _PyStackChunk *stack_chunk_list;
+    PyObject **stack_top;
+    PyObject **stack_limit;
     /* XXX signal handlers should also be here */
 
     /* The following fields are here to avoid allocation during init.

Include/internal/pycore_debug_offsets.h

@@ -106,7 +106,7 @@ typedef struct _Py_DebugOffsets {
         uint64_t last_profiled_frame;
         uint64_t thread_id;
         uint64_t native_thread_id;
-        uint64_t datastack_chunk;
+        uint64_t stack_chunk_list;
         uint64_t status;
         uint64_t holds_gil;
         uint64_t gil_requested;
@@ -287,7 +287,7 @@ typedef struct _Py_DebugOffsets {
         .last_profiled_frame = offsetof(PyThreadState, last_profiled_frame), \
         .thread_id = offsetof(PyThreadState, thread_id), \
         .native_thread_id = offsetof(PyThreadState, native_thread_id), \
-        .datastack_chunk = offsetof(PyThreadState, datastack_chunk), \
+        .stack_chunk_list = offsetof(PyThreadState, stack_chunk_list), \
         .status = offsetof(PyThreadState, _status), \
         .holds_gil = offsetof(PyThreadState, holds_gil), \
         .gil_requested = offsetof(PyThreadState, gil_requested), \

Include/internal/pycore_interpframe.h

@@ -330,12 +330,12 @@ static inline bool
 _PyThreadState_HasStackSpace(PyThreadState *tstate, int size)
 {
     assert(
-        (tstate->datastack_top == NULL && tstate->datastack_limit == NULL)
+        (tstate->stack_top == NULL && tstate->stack_limit == NULL)
         ||
-        (tstate->datastack_top != NULL && tstate->datastack_limit != NULL)
+        (tstate->stack_top != NULL && tstate->stack_limit != NULL)
     );
-    return tstate->datastack_top != NULL &&
-        size < tstate->datastack_limit - tstate->datastack_top;
+    return tstate->stack_top != NULL &&
+        size < tstate->stack_limit - tstate->stack_top;
 }
 
 extern _PyInterpreterFrame *
@@ -352,9 +352,9 @@ _PyFrame_PushUnchecked(PyThreadState *tstate, _PyStackRef func, int null_locals_
     CALL_STAT_INC(frames_pushed);
     PyFunctionObject *func_obj = (PyFunctionObject *)PyStackRef_AsPyObjectBorrow(func);
     PyCodeObject *code = (PyCodeObject *)func_obj->func_code;
-    _PyInterpreterFrame *new_frame = (_PyInterpreterFrame *)tstate->datastack_top;
-    tstate->datastack_top += code->co_framesize;
-    assert(tstate->datastack_top < tstate->datastack_limit);
+    _PyInterpreterFrame *new_frame = (_PyInterpreterFrame *)tstate->stack_top;
+    tstate->stack_top += code->co_framesize;
+    assert(tstate->stack_top < tstate->stack_limit);
     _PyFrame_Initialize(tstate, new_frame, func, NULL, code, null_locals_from,
                         previous);
     return new_frame;
@@ -366,9 +366,9 @@ static inline _PyInterpreterFrame *
 _PyFrame_PushTrampolineUnchecked(PyThreadState *tstate, PyCodeObject *code, int stackdepth, _PyInterpreterFrame * previous)
 {
     CALL_STAT_INC(frames_pushed);
-    _PyInterpreterFrame *frame = (_PyInterpreterFrame *)tstate->datastack_top;
-    tstate->datastack_top += code->co_framesize;
-    assert(tstate->datastack_top < tstate->datastack_limit);
+    _PyInterpreterFrame *frame = (_PyInterpreterFrame *)tstate->stack_top;
+    tstate->stack_top += code->co_framesize;
+    assert(tstate->stack_top < tstate->stack_limit);
     frame->previous = previous;
     frame->f_funcobj = PyStackRef_None;
     frame->f_executable = PyStackRef_FromPyObjectNew(code);
@@ -404,6 +404,54 @@ PyAPI_FUNC(_PyInterpreterFrame *)
 _PyEvalFramePushAndInit_Ex(PyThreadState *tstate, _PyStackRef func,
     PyObject *locals, Py_ssize_t nargs, PyObject *callargs, PyObject *kwargs, _PyInterpreterFrame *previous);
 
+static inline bool
+ptr_in_chunk(const char *ptr, const _PyStackChunk *chunk)
+{
+    assert(chunk != NULL);
+    const char *start = (char *)&chunk->data[0];
+    const intptr_t offset = ptr - start;
+    const intptr_t usable_size = (intptr_t)(chunk->size - _PY_STACK_CHUNK_OVERHEADS);
+    return offset >= 0 && offset < usable_size && start + offset == ptr;
+}
+
+static inline uintptr_t
+get_offset_in_chunk(const char *ptr, const _PyStackChunk *chunk)
+{
+    assert(chunk != NULL);
+    assert(chunk->data != NULL);
+    assert(ptr_in_chunk(ptr, chunk));
+
+    return ptr - (char *)chunk;
+}
+
+static inline uintptr_t
+get_offset_in_chunk_list(char *base, _PyStackChunk *stack_chunk_list)
+{
+    assert(stack_chunk_list != NULL);
+    assert(base != NULL);
+    _PyStackChunk *chunk = stack_chunk_list;
+    do {
+        if (ptr_in_chunk(base, chunk)) {
+            return get_offset_in_chunk(base, chunk);
+        }
+        chunk = chunk->previous;
+    } while (chunk);
+    assert(false);  // did not find correct chunk
+    Py_UNREACHABLE();
+}
+
+static inline void *
+_Py_ensure_frame_in_current_stack_chunk(PyThreadState *tstate, char *frame)
+{
+    assert(tstate != NULL);
+    assert(frame != NULL);
+    if (ptr_in_chunk(frame, tstate->stack_chunk_list)) {
+        return frame;
+    }
+    uintptr_t offset = get_offset_in_chunk_list(frame, tstate->stack_chunk_list->previous);
+    return ((char *)tstate->stack_chunk_list) + offset;
+}
+
 #ifdef __cplusplus
 }
 #endif

Include/internal/pycore_runtime_init.h

@@ -179,6 +179,9 @@ extern PyTypeObject _PyExc_MemoryError;
         ._whence = _PyThreadState_WHENCE_NOTSET, \
         .py_recursion_limit = Py_DEFAULT_RECURSION_LIMIT, \
         .context_ver = 1, \
+        .stack_chunk_list = NULL, \
+        .stack_limit = NULL, \
+        .stack_top = NULL, \
     }
 
 

Misc/NEWS.d/next/Core_and_Builtins/2026-04-17-12-27-33.gh-issue-142183.jKFUky.rst

@@ -0,0 +1 @@
+Change the resizing implementation of the Python stack to use a resizable array instead of a chain of arrays, to avoid degenerate performance cases and allow further JIT optimizations.

Modules/_remote_debugging/debug_offsets_validation.h

@@ -252,7 +252,7 @@ validate_fixed_field(
 #define PY_REMOTE_DEBUG_THREAD_STATE_FIELDS(APPLY, buffer_size) \
     APPLY(thread_state, native_thread_id, sizeof(unsigned long), _Alignof(long), buffer_size); \
     APPLY(thread_state, interp, sizeof(uintptr_t), _Alignof(uintptr_t), buffer_size); \
-    APPLY(thread_state, datastack_chunk, sizeof(uintptr_t), _Alignof(uintptr_t), buffer_size); \
+    APPLY(thread_state, stack_chunk_list, sizeof(uintptr_t), _Alignof(uintptr_t), buffer_size); \
     APPLY(thread_state, status, FIELD_SIZE(PyThreadState, _status), _Alignof(unsigned int), buffer_size); \
     APPLY(thread_state, holds_gil, sizeof(int), _Alignof(int), buffer_size); \
     APPLY(thread_state, gil_requested, sizeof(int), _Alignof(int), buffer_size); \

Modules/_remote_debugging/frames.c

@@ -27,7 +27,7 @@ process_single_stack_chunk(
     StackChunkInfo *chunk_info
 ) {
     // Start with default size assumption
-    size_t current_size = _PY_DATA_STACK_CHUNK_SIZE;
+    size_t current_size = _PY_STACK_CHUNK_MIN_SIZE;
 
     char *this_chunk = PyMem_RawMalloc(current_size);
     if (!this_chunk) {
@@ -87,45 +87,32 @@ copy_stack_chunks(RemoteUnwinderObject *unwinder,
     size_t count = 0;
     size_t max_chunks = 16;
 
-    if (read_ptr(unwinder, tstate_addr + (uintptr_t)unwinder->debug_offsets.thread_state.datastack_chunk, &chunk_addr)) {
+    if (read_ptr(unwinder, tstate_addr + (uintptr_t)unwinder->debug_offsets.thread_state.stack_chunk_list, &chunk_addr)) {
         set_exception_cause(unwinder, PyExc_RuntimeError, "Failed to read initial stack chunk address");
         return -1;
     }
 
+    if (!chunk_addr) {
+        out_chunks->chunks = NULL;
+        out_chunks->count = 0;
+        return 0;
+    }
+
     chunks = PyMem_RawMalloc(max_chunks * sizeof(StackChunkInfo));
     if (!chunks) {
         PyErr_NoMemory();
         set_exception_cause(unwinder, PyExc_MemoryError, "Failed to allocate stack chunks array");
         return -1;
     }
 
-    const size_t MAX_STACK_CHUNKS = 4096;
-    while (chunk_addr != 0 && count < MAX_STACK_CHUNKS) {
-        // Grow array if needed
-        if (count >= max_chunks) {
-            max_chunks *= 2;
-            StackChunkInfo *new_chunks = PyMem_RawRealloc(chunks, max_chunks * sizeof(StackChunkInfo));
-            if (!new_chunks) {
-                PyErr_NoMemory();
-                set_exception_cause(unwinder, PyExc_MemoryError, "Failed to grow stack chunks array");
-                goto error;
-            }
-            chunks = new_chunks;
-        }
-
-        // Process this chunk
-        if (process_single_stack_chunk(unwinder, chunk_addr, &chunks[count]) < 0) {
-            set_exception_cause(unwinder, PyExc_RuntimeError, "Failed to process stack chunk");
-            goto error;
-        }
-
-        // Get next chunk address and increment count
-        chunk_addr = GET_MEMBER(uintptr_t, chunks[count].local_copy, offsetof(_PyStackChunk, previous));
-        count++;
+    // Process this chunk
+    if (process_single_stack_chunk(unwinder, chunk_addr, &chunks[count]) < 0) {
+        set_exception_cause(unwinder, PyExc_RuntimeError, "Failed to process stack chunk");
+        goto error;
     }
 
     out_chunks->chunks = chunks;
-    out_chunks->count = count;
+    out_chunks->count = 1;
     return 0;
 
 error:

Python/ceval.c

@@ -1925,8 +1925,13 @@ clear_thread_frame(PyThreadState *tstate, _PyInterpreterFrame * frame)
     assert(frame->owner == FRAME_OWNED_BY_THREAD);
     // Make sure that this is, indeed, the top frame. We can't check this in
     // _PyThreadState_PopFrame, since f_code is already cleared at that point:
-    assert((PyObject **)frame + _PyFrame_GetCode(frame)->co_framesize ==
-        tstate->datastack_top);
+    assert(
+        _Py_ensure_frame_in_current_stack_chunk(  // the frame might be in a previous stack chunk
+            tstate,
+            (char *)((PyObject **)frame + _PyFrame_GetCode(frame)->co_framesize)
+        )
+        == tstate->stack_top
+    );
     assert(frame->frame_obj == NULL || frame->frame_obj->f_frame == frame);
     _PyFrame_ClearExceptCode(frame);
     PyStackRef_CLEAR(frame->f_executable);