Introduced helper functions v_iadd and v_isub, for in-place digit-vector

addition and subtraction.  Reworked the tail end of k_mul() to use them.
This saves oodles of one-shot longobject allocations (this is a triply-
recursive routine, so saving one allocation in the body saves 3**n
allocations at depth n; we actually save 2 allocations in the body).

Objects/longobject.c

@@ -775,6 +775,57 @@ convert_binop(PyObject *v, PyObject *w, PyLongObject **a, PyLongObject **b) {
 		return Py_NotImplemented; \
 	}
 
+/* x[0:m] and y[0:n] are digit vectors, LSD first, m >= n required.  x[0:n]
+ * is modified in place, by adding y to it.  Carries are propagated as far as
+ * x[m-1], and the remaining carry (0 or 1) is returned.
+ */
+static digit
+v_iadd(digit *x, int m, digit *y, int n)
+{
+	int i;
+	digit carry = 0;
+
+	assert(m >= n);
+	for (i = 0; i < n; ++i) {
+		carry += x[i] + y[i];
+		x[i] = carry & MASK;
+		carry >>= SHIFT;
+		assert((carry & 1) == carry);
+	}
+	for (; carry && i < m; ++i) {
+		carry += x[i];
+		x[i] = carry & MASK;
+		carry >>= SHIFT;
+		assert((carry & 1) == carry);
+	}
+	return carry;
+}
+
+/* x[0:m] and y[0:n] are digit vectors, LSD first, m >= n required.  x[0:n]
+ * is modified in place, by subtracting y from it.  Borrows are propagated as
+ * far as x[m-1], and the remaining borrow (0 or 1) is returned.
+ */
+static digit
+v_isub(digit *x, int m, digit *y, int n)
+{
+	int i;
+	digit borrow = 0;
+
+	assert(m >= n);
+	for (i = 0; i < n; ++i) {
+		borrow = x[i] - y[i] - borrow;
+		x[i] = borrow & MASK;
+		borrow >>= SHIFT;
+		borrow &= 1;	/* keep only 1 sign bit */
+	}
+	for (; borrow && i < m; ++i) {
+		borrow = x[i] - borrow;
+		x[i] = borrow & MASK;
+		borrow >>= SHIFT;
+		borrow &= 1;
+	}
+	return borrow;
+}
 
 /* Multiply by a single digit, ignoring the sign. */
 
@@ -1558,7 +1609,9 @@ k_mul(PyLongObject *a, PyLongObject *b)
 	PyLongObject *t1, *t2;
 	int shift;	/* the number of digits we split off */
 	int i;
-
+#ifdef Py_DEBUG
+	digit d;
+#endif
 	/* (ah*X+al)(bh*X+bl) = ah*bh*X*X + (ah*bl + al*bh)*X + al*bl
 	 * Let k = (ah+al)*(bh+bl) = ah*bl + al*bh  + ah*bh + al*bl
 	 * Then the original product is
@@ -1629,39 +1682,32 @@ k_mul(PyLongObject *a, PyLongObject *b)
 	Py_DECREF(t2);
 	if (k == NULL) goto fail;
 
-	/* Subtract ahbh and albl from k.  Note that this can't become
-	 * negative, since k = ahbh + albl + other stuff.
+	/* Add k into the result, starting at the shift'th LSD. */
+	i = ret->ob_size - shift;  /* # digits after shift */
+#ifdef Py_DEBUG
+	d =
+#endif
+	v_iadd(ret->ob_digit + shift, i, k->ob_digit, k->ob_size);
+	assert(d == 0);
+	Py_DECREF(k);
+
+	/* Subtract ahbh and albl from the result.  Note that this can't
+	 * become negative, since k = ahbh + albl + other stuff.
 	 */
-	if ((t1 = x_sub(k, ahbh)) == NULL) goto fail;
-	Py_DECREF(k);
-	k = t1;
+#ifdef Py_DEBUG
+	d =
+#endif
+	v_isub(ret->ob_digit + shift, i, ahbh->ob_digit, ahbh->ob_size);
+	assert(d == 0);
 	Py_DECREF(ahbh);
-	ahbh = NULL;
 
-	if ((t1 = x_sub(k, albl)) == NULL) goto fail;
-	Py_DECREF(k);
-	k = t1;
+#ifdef Py_DEBUG
+	d =
+#endif
+	v_isub(ret->ob_digit + shift, i, albl->ob_digit, albl->ob_size);
+	assert(d == 0);
 	Py_DECREF(albl);
-	albl = NULL;
 
-	/* Add k into the result, at the shift-th least-significant digit. */
-	{
-		int j;		/* index into k */
-		digit carry = 0;
-
-		for (i = shift, j = 0; j < k->ob_size; ++i, ++j) {
-			carry += ret->ob_digit[i] + k->ob_digit[j];
-			ret->ob_digit[i] = carry & MASK;
-			carry >>= SHIFT;
-		}
-		for (; carry && i < ret->ob_size; ++i) {
-			carry += ret->ob_digit[i];
-			ret->ob_digit[i] = carry & MASK;
-			carry >>= SHIFT;
-		}
-		assert(carry == 0);
-	}
-	Py_DECREF(k);
 	return long_normalize(ret);
 
  fail: