SF #1022910: Conserve memory with list.pop()

The list resizing scheme only downsized when more than 16 elements were
removed in a single step:  del a[100:120].   As a result, the list would
never shrink when popping elements off one at a time.

This patch makes it shrink whenever more than half of the space is unused.

Also, at Tim's suggestion, renamed _new_size to new_allocated.  This makes
the code easier to understand.

diff --git a/Objects/listobject.c b/Objects/listobject.c
index 996ddb95b1d3d91c043a7326ba2e65d82a912f77..1fb77b9760c3762ec23f075d61950a48cf3f76f5 100644 (file)
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -25,13 +25,14 @@ static int
 list_resize(PyListObject *self, int newsize)
 {
        PyObject **items;
-       size_t _new_size;
+       size_t new_allocated;
+       int allocated = self->allocated;
 
        /* Bypass realloc() when a previous overallocation is large enough
-          to accommodate the newsize.  If the newsize is 16 smaller than the
-          current size, then proceed with the realloc() to shrink the list.
+          to accommodate the newsize.  If the newsize falls lower than half
+          the allocated size, then proceed with the realloc() to shrink the list.
        */
-       if (self->allocated >= newsize && self->ob_size < newsize + 16) {
+       if (allocated >= newsize && newsize >= (allocated >> 1)) {
                assert(self->ob_item != NULL || newsize == 0);
                self->ob_size = newsize;
                return 0;
@@ -44,10 +45,12 @@ list_resize(PyListObject *self, int newsize)
         * system realloc().
         * The growth pattern is:  0, 4, 8, 16, 25, 35, 46, 58, 72, 88, ...
         */
-       _new_size = (newsize >> 3) + (self->ob_size < 8 ? 3 : 6) + newsize;
+       new_allocated = (newsize >> 3) + (newsize < 9 ? 3 : 6) + newsize;
+       if (newsize == 0)
+               new_allocated = 0;
        items = self->ob_item;
-       if (_new_size <= ((~(size_t)0) / sizeof(PyObject *)))
-               PyMem_RESIZE(items, PyObject *, _new_size);
+       if (new_allocated <= ((~(size_t)0) / sizeof(PyObject *)))
+               PyMem_RESIZE(items, PyObject *, new_allocated);
        else
                items = NULL;
        if (items == NULL) {
@@ -56,7 +59,7 @@ list_resize(PyListObject *self, int newsize)
        }
        self->ob_item = items;
        self->ob_size = newsize;
-       self->allocated = _new_size;
+       self->allocated = new_allocated;
        return 0;
 }