* Optimized list appends and pops by making fewer calls the underlying system
realloc(). This is achieved by tracking the overallocation size in a new
field and using that information to skip calls to realloc() whenever
possible.
* Simplified and tightened the amount of overallocation. For larger lists,
this overallocates by 1/8th (compared to the previous scheme which ranged
between 1/4th to 1/32nd over-allocation). For smaller lists (n<6), the
maximum overallocation is one byte (formerly it could be upto eight bytes).
This saves memory in applications with large numbers of small lists.
* Eliminated the NRESIZE macro in favor of a new, static list_resize function
that encapsulates the resizing logic. Coverting this back to macro would
give a small (under 1%) speed-up. This was too small to warrant the loss
of readability, maintainability, and de-coupling.
* Some functions using NRESIZE had grown unnecessarily complex in their
efforts to bend to the macro's calling pattern. With the new list_resize
function in place, those other functions could be simplified. That is
being saved for a separate patch.
* The ob_item==NULL check could be eliminated from the new list_resize
function. This would entail finding each piece of code that sets ob_item
to NULL and adding a new line to invalidate the overallocation tracking
field. Rather than impose a new requirement on other pieces of list code,
it was preferred to leave the NULL check in place and retain the benefits
of decoupling, maintainability and information hiding (only PyList_New()
and list_sort() need to know about the new field). This approach also
reduces the odds of breaking an extension module.
(Collaborative effort by Raymond Hettinger, Hye-Shik Chang, Tim Peters,
and Armin Rigo.)
diff --git a/Misc/NEWS b/Misc/NEWS
index d9778a3..275593b 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,17 @@
Core and builtins
-----------------
+- Optimized list resize operations to make fewer calls to the system
+ realloc(). Significantly speeds up list appends, list pops,
+ list comprehensions, and the list contructor (when the input iterable
+ length is not known).
+
+- Changed the internal list over-allocation scheme. For larger lists,
+ overallocation ranged between 3% and 25%. Now, it is a constant 12%.
+ For smaller lists (n<=5), overallocation was upto eight bytes. Now,
+ the overallocation is no more than one byte -- this improves space
+ utilization for applications that have large numbers of small lists.
+
- Support for arbitrary objects supporting the read-only buffer
interface as the co_code field of code objects (something that was
only possible to create from C code) has been removed.