8000 gh-92914: Round the allocated size for lists up to the even number by serhiy-storchaka · Pull Request #92915 · python/cpython · GitHub
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gh-92914: Round the allocated size for lists up to the even number #92915

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Merged
merged 5 commits into from
May 19, 2022
Merged

gh-92914: Round the allocated size for lists up to the even number #92915

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9a2ed09
gh-92914: Always round up the size allocated for lists up to 16 bytes
serhiy-storchaka May 18, 2022
a6b1a79
Round up to the even number.
serhiy-storchaka May 18, 2022
8599f81
Fix tests.
serhiy-storchaka May 18, 2022
67779ed
Update 2022-05-18-08-32-33.gh-issue-92914.tJUeTD.rst
serhiy-storchaka May 18, 2022
7025f22
Update 2022-05-18-08-32-33.gh-issue-92914.tJUeTD.rst
serhiy-storchaka May 18, 2022
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2022-05-18-08-32-33.gh-issue-92914.tJUeTD.rst
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@@ -0,0 +1 @@
Always round up the size allocated for lists up to 16 bytes.
10 changes: 10 additions & 0 deletions Objects/listobject.c
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Expand Up @@ -94,6 +94,16 @@ list_preallocate_exact(PyListObject *self, Py_ssize_t size)
assert(self->ob_item == NULL);
assert(size > 0);

/* Since the Python memory allocator has granularity of 16 bytes,
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alignment is 8bytes on 32bit platform.

cpython/Objects/obmalloc.c

Lines 878 to 884 in b86d783

#if SIZEOF_VOID_P > 4
#define ALIGNMENT 16 /* must be 2^N */
#define ALIGNMENT_SHIFT 4
#else
#define ALIGNMENT 8 /* must be 2^N */
#define ALIGNMENT_SHIFT 3
#endif

So I think we don't need the else { block.

* there is no benefit of allocating space for the odd number of items,
* and there is no drawback of rounding it up.
*/
if (sizeof(PyObject*) > 4) {
size = (size + 1) & ~(size_t)1;
}
else {
size = (size + 3) & ~(size_t)3;
}
PyObject **items = PyMem_New(PyObject*, size);
if (items == NULL) {
PyErr_NoMemory();
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