Revert "bpo-40521: Remove freelist from collections.deque() (GH-21073)" (GH-24944)

This reverts commit 32f2eda85957365d208f499b730d30b7eb419741.
It can be re-applied if the subinterpreter PEP is approved.
Otherwise, the commit degraded performance with no offsetting
benefit.

1 files changed, 21 insertions, 2 deletions

diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index ca63f71..8b01a7f 100644
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -118,9 +118,23 @@ static PyTypeObject deque_type;
 #define CHECK_NOT_END(link)
 #endif
 
+/* A simple freelisting scheme is used to minimize calls to the memory
+   allocator.  It accommodates common use cases where new blocks are being
+   added at about the same rate as old blocks are being freed.
+ */
+
+#define MAXFREEBLOCKS 16
+static Py_ssize_t numfreeblocks = 0;
+static block *freeblocks[MAXFREEBLOCKS];
+
 static block *
 newblock(void) {
-    block *b = PyMem_Malloc(sizeof(block));
+    block *b;
+    if (numfreeblocks) {
+        numfreeblocks--;
+        return freeblocks[numfreeblocks];
+    }
+    b = PyMem_Malloc(sizeof(block));
     if (b != NULL) {
         return b;
     }
@@ -131,7 +145,12 @@ newblock(void) {
 static void
 freeblock(block *b)
 {
-    PyMem_Free(b);
+    if (numfreeblocks < MAXFREEBLOCKS) {
+        freeblocks[numfreeblocks] = b;
+        numfreeblocks++;
+    } else {
+        PyMem_Free(b);
+    }
 }
 
 static PyObject *