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bpo-38373: Change list overallocating strategy. (GH-18952)

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* Add padding to make the allocated size multiple of 4.
* Do not overallocate if the new size is closer to overalocated size
  than to the old size.
This commit is contained in:
Serhiy Storchaka
2020-03-17 23:46:00 +02:00
committed by GitHub
parent d469d666b8
commit 2fe815edd6
2 changed files with 10 additions and 6 deletions
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2
Misc/NEWS.d/next/Core and Builtins/2020-03-11-12-28-16.bpo-38373.FE9S21.rst Normal file
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@@ -0,0 +1,2 @@
Chaged list overallocation strategy. It no longer overallocates if the new
size is closer to overalocated size than to the old size and adds padding.

14
Objects/listobject.c
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@@ -54,15 +54,17 @@ list_resize(PyListObject *self, Py_ssize_t newsize)
* enough to give linear-time amortized behavior over a long
* sequence of appends() in the presence of a poorly-performing
* system realloc().
* The growth pattern is: 0, 4, 8, 16, 25, 35, 46, 58, 72, 88, ...
* Add padding to make the allocated size multiple of 4.
* The growth pattern is: 0, 4, 8, 16, 24, 32, 40, 52, 64, 76, ...
* Note: new_allocated won't overflow because the largest possible value
* is PY_SSIZE_T_MAX * (9 / 8) + 6 which always fits in a size_t.
*/
new_allocated = (size_t)newsize + (newsize >> 3) + (newsize < 9 ? 3 : 6);
if (new_allocated > (size_t)PY_SSIZE_T_MAX / sizeof(PyObject *)) {
PyErr_NoMemory();
return -1;
}
new_allocated = ((size_t)newsize + (newsize >> 3) + 6) & ~(size_t)3;
/* Do not overallocate if the new size is closer to overalocated size
* than to the old size.
*/
if (newsize - Py_SIZE(self) > (Py_ssize_t)(new_allocated - newsize))
new_allocated = ((size_t)newsize + 3) & ~(size_t)3;
if (newsize == 0)
new_allocated = 0;