diff options
Diffstat (limited to 'Modules/_collectionsmodule.c')
| -rw-r--r-- | Modules/_collectionsmodule.c | 18 |
1 files changed, 11 insertions, 7 deletions
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 *freeblocks[MAXFREEBLOCKS]; 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"); |