SF patch #687598, array.append is sloooow

nnorwitz · nnorwitz · commit 937ca98e34ba · 2003-02-24T02:08:42.000Z
This improves speed by about 5.6% for me.
diff --git a/Modules/arraymodule.c b/Modules/arraymodule.c
@@ -13,6 +13,52 @@
 #endif /* DONT_HAVE_SYS_TYPES_H */
 #endif /* !STDC_HEADERS */
 
+/* Shamelessy stolen from listobject.c */
+static int
+roundupsize(int n)
+{
+	unsigned int nbits = 0;
+	unsigned int n2 = (unsigned int)n >> 5;
+
+	/* Round up:
+	 * If n <       256, to a multiple of        8.
+	 * If n <      2048, to a multiple of       64.
+	 * If n <     16384, to a multiple of      512.
+	 * If n <    131072, to a multiple of     4096.
+	 * If n <   1048576, to a multiple of    32768.
+	 * If n <   8388608, to a multiple of   262144.
+	 * If n <  67108864, to a multiple of  2097152.
+	 * If n < 536870912, to a multiple of 16777216.
+	 * ...
+	 * If n < 2**(5+3*i), to a multiple of 2**(3*i).
+	 *
+	 * This over-allocates proportional to the list size, making room
+	 * for additional growth.  The over-allocation is mild, but is
+	 * enough to give linear-time amortized behavior over a long
+	 * sequence of appends() in the presence of a poorly-performing
+	 * system realloc() (which is a reality, e.g., across all flavors
+	 * of Windows, with Win9x behavior being particularly bad -- and
+	 * we've still got address space fragmentation problems on Win9x
+	 * even with this scheme, although it requires much longer lists to
+	 * provoke them than it used to).
+	 */
+	do {
+		n2 >>= 3;
+		nbits += 3;
+	} while (n2);
+	return ((n >> nbits) + 1) << nbits;
+ }
+
+#define NRESIZE(var, type, nitems)				\
+do {								\
+	size_t _new_size = roundupsize(nitems);			\
+	if (_new_size <= ((~(size_t)0) / sizeof(type)))		\
+		PyMem_RESIZE(var, type, _new_size);		\
+	else							\
+		var = NULL;					\
+} while (0)
+/* END SHAMELESSLY STOLEN CODE */
+
 struct arrayobject; /* Forward */
 
 /* All possible arraydescr values are defined in the vector "descriptors"
@@ -419,8 +465,7 @@ ins1(arrayobject *self, int where, PyObject *v)
 	if ((*self->ob_descr->setitem)(self, -1, v) < 0)
 		return -1;
 	items = self->ob_item;
-	PyMem_RESIZE(items, char,
-		     (self->ob_size+1) * self->ob_descr->itemsize);
+	NRESIZE(items, char, (self->ob_size+1) * self->ob_descr->itemsize);
 	if (items == NULL) {
 		PyErr_NoMemory();
 		return -1;
