Index: longobject.c
===================================================================
--- longobject.c	(revision 66563)
+++ longobject.c	(working copy)
@@ -2495,18 +2495,48 @@
 		}
 	}
 	else {	/* a is not the same as b -- gradeschool long mult */
-		for (i = 0; i < size_a; ++i) {
-			twodigits carry = 0;
-			twodigits f = a->ob_digit[i];
+		for (i = 0; i < size_a - 1; i += 2) {
+			twodigits f0 = a->ob_digit[i];
+			twodigits f1 = a->ob_digit[i+1];
 			digit *pz = z->ob_digit + i;
 			digit *pb = b->ob_digit;
-			digit *pbend = b->ob_digit + size_b;
+			twodigits carry = *pz + pb[0] * f0;
+			*pz++ = (digit)(carry & PyLong_MASK);
+			carry >>= PyLong_SHIFT;
 
 			SIGCHECK({
 				Py_DECREF(z);
 				return NULL;
 			})
-
+			int j;
+			/* carry + *pz + pb[j+1] * f0 + pb[j] * f1 <=
+			*  2*MASK + 2*MASK*MASK is contained in a twodigits,
+			* see longintrepr.h */
+			for(j=0; j < size_b-1; j++) {
+				carry += *pz + pb[j+1] * f0 + pb[j] * f1;
+				*pz++ = (digit)(carry & PyLong_MASK);
+				carry >>= PyLong_SHIFT;
+				assert(carry <= PyLong_MASK);
+			}
+			carry += *pz + pb[size_b-1] * f1;
+			*pz++ = (digit)(carry & PyLong_MASK);
+			carry >>= PyLong_SHIFT;
+			if (carry)
+				*pz += (digit)(carry & PyLong_MASK);
+			assert((carry >> PyLong_SHIFT) == 0);
+		}
+		if (size_a&1) {
+			twodigits carry = 0;
+			twodigits f = a->ob_digit[size_a - 1];
+			digit *pz = z->ob_digit + size_a - 1;
+			digit *pb = b->ob_digit;
+			digit *pbend = b->ob_digit + size_b;
+        
+			SIGCHECK({
+				Py_DECREF(z);
+				return NULL;
+			})
+        
 			while (pb < pbend) {
 				carry += *pz + *pb++ * f;
 				*pz++ = (digit)(carry & PyLong_MASK);