mirrors/cpython
1
0
Fork 0
mirror of https://github.com/python/cpython.git synced 2025-09-26 10:19:53 +00:00
Code Issues Projects Releases Packages Wiki Activity Actions 8

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

Browse source 
* 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
No known key found for this signature in database
GPG key ID: 4AEE18F83AFDEB23
2 changed files with 10 additions and 6 deletions
Download patch file Download diff file Expand all files Collapse all files

2
Misc/NEWS.d/next/Core and Builtins/2020-03-11-12-28-16.bpo-38373.FE9S21.rst Normal file
View file

@ -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
View file

@ -54,15 +54,17 @@ list_resize(PyListObject *self, Py_ssize_t newsize)
* enough to give linear-time amortized behavior over a long * enough to give linear-time amortized behavior over a long
* sequence of appends() in the presence of a poorly-performing * sequence of appends() in the presence of a poorly-performing
* system realloc(). * 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 * 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. * 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); new_allocated = ((size_t)newsize + (newsize >> 3) + 6) & ~(size_t)3;
if (new_allocated > (size_t)PY_SSIZE_T_MAX / sizeof(PyObject *)) { /* Do not overallocate if the new size is closer to overalocated size
PyErr_NoMemory(); * than to the old size.
return -1; */
} if (newsize - Py_SIZE(self) > (Py_ssize_t)(new_allocated - newsize))
new_allocated = ((size_t)newsize + 3) & ~(size_t)3;
if (newsize == 0) if (newsize == 0)
new_allocated = 0; new_allocated = 0;