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bpo-33234 Improve list() pre-sizing for inputs with known lengths (no __length_hint__) #9846
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Original file line number | Diff line number | Diff line change |
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@@ -0,0 +1,2 @@ | ||
The list constructor will pre-size and not over-allocate when | ||
the input lenght is known. |
Original file line number | Diff line number | Diff line change |
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@@ -76,6 +76,33 @@ list_resize(PyListObject *self, Py_ssize_t newsize) | |
return 0; | ||
} | ||
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static int | ||
list_preallocate_exact(PyListObject *self, Py_ssize_t size) | ||
{ | ||
assert(self->ob_item == NULL); | ||
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PyObject **items; | ||
size_t allocated; | ||
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allocated = (size_t)size; | ||
if (allocated > (size_t)PY_SSIZE_T_MAX / sizeof(PyObject *)) { | ||
PyErr_NoMemory(); | ||
return -1; | ||
} | ||
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if (size == 0) { | ||
allocated = 0; | ||
} | ||
items = (PyObject **)PyMem_New(PyObject*, allocated); | ||
if (items == NULL) { | ||
PyErr_NoMemory(); | ||
return -1; | ||
} | ||
self->ob_item = items; | ||
self->allocated = allocated; | ||
return 0; | ||
} | ||
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/* Debug statistic to compare allocations with reuse through the free list */ | ||
#undef SHOW_ALLOC_COUNT | ||
#ifdef SHOW_ALLOC_COUNT | ||
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@@ -2649,6 +2676,19 @@ list___init___impl(PyListObject *self, PyObject *iterable) | |
(void)_list_clear(self); | ||
} | ||
if (iterable != NULL) { | ||
if (_PyObject_HasLen(iterable)) { | ||
Py_ssize_t iter_len = PyObject_Size(iterable); | ||
if (iter_len == -1) { | ||
if (!PyErr_ExceptionMatches(PyExc_TypeError)) { | ||
return -1; | ||
} | ||
PyErr_Clear(); | ||
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There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. @pablogsal, @serhiy-storchaka: In a previous comment, I proposed to add an helper function to "probe" an object size: so move this code into a private helper function. Since the same code is used by PyObject_LengthHint(), it would now make sense, no? See also iter_len() ... which is different. There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. This code will be used just in two places and is not too complex. Adding yet one intermediate function will add a performance penalty for calling a function and checking its result, and will complicate the code. If this code will be used in more places, it can be refactored. |
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} | ||
if (iter_len > 0 && self->ob_item == NULL | ||
&& list_preallocate_exact(self, iter_len)) { | ||
return -1; | ||
} | ||
} | ||
PyObject *rv = list_extend(self, iterable); | ||
if (rv == NULL) | ||
return -1; | ||
|
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