backout 5accb0ac8bfb; needs more discussion on python-dev
diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index f175fca..abd80e0 100644
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -8,13 +8,12 @@
*/
/* The block length may be set to any number over 1. Larger numbers
- * reduce the number of calls to the memory allocator, give faster
- * indexing and rotation, and reduce the link::data overhead ratio.
- * Ideally, the block length should be a power-of-two for faster
- * division/modulo computations during indexing.
+ * reduce the number of calls to the memory allocator but take more
+ * memory. Ideally, BLOCKLEN should be set with an eye to the
+ * length of a cache line.
*/
-#define BLOCKLEN 64
+#define BLOCKLEN 62
#define CENTER ((BLOCKLEN - 1) / 2)
/* A `dequeobject` is composed of a doubly-linked list of `block` nodes.
@@ -59,8 +58,13 @@
static block *
newblock(block *leftlink, block *rightlink, Py_ssize_t len) {
block *b;
- /* To prevent len from overflowing PY_SSIZE_T_MAX on 32-bit machines, we
- * refuse to allocate new blocks if the current len is nearing overflow. */
+ /* To prevent len from overflowing PY_SSIZE_T_MAX on 64-bit machines, we
+ * refuse to allocate new blocks if the current len is dangerously
+ * close. There is some extra margin to prevent spurious arithmetic
+ * overflows at various places. The following check ensures that
+ * the blocks allocated to the deque, in the worst case, can only
+ * have PY_SSIZE_T_MAX-2 entries in total.
+ */
if (len >= PY_SSIZE_T_MAX - 2*BLOCKLEN) {
PyErr_SetString(PyExc_OverflowError,
"cannot add more blocks to the deque");