of differences between image elements and the corresponding
domain elements,
$$
-\Delta \, R \coloneqq
+\diff R \coloneqq
\vec s \mapsto
\{\, \vec \delta \in \Z^{d} \mid \exists \vec x \to \vec y \in R :
\vec \delta = \vec y - \vec x
$$
\end{example}
+The fact that we ignore some impure constraints clearly leads
+to a loss of accuracy. In some cases, some of this loss can be recovered
+by not considering the parameters in a special way.
+That is, instead of considering the set
+$$
+\Delta = \diff R =
+\vec s \mapsto
+\{\, \vec \delta \in \Z^{d} \mid \exists \vec x \to \vec y \in R :
+\vec \delta = \vec y - \vec x
+\,\}
+$$
+we consider the set
+$$
+\Delta' = \diff R' =
+\{\, \vec \delta \in \Z^{n+d} \mid \exists
+(\vec s, \vec x) \to (\vec s, \vec y) \in R' :
+\vec \delta = (\vec s - \vec s, \vec y - \vec x)
+\,\}
+.
+$$
+The first $n$ coordinates of every element in $\Delta'$ are zero.
+Projecting out these zero coordinates from $\Delta'$ is equivalent
+to projecting out the parameters in $\Delta$.
+The result is obviously a superset of $\Delta$, but all its constraints
+are of type \eqref{eq:transitive:non-parametric} and they can therefore
+all be used in the construction of $Q_i$.
+
+\begin{example}
+Consider the relation
+$$
+% [n] -> { [x, y] -> [1 + x, 1 - n + y] | n >= 2 }
+R = n \to \{\, (x, y) \to (1 + x, 1 - n + y) \mid n \ge 2 \,\}
+.
+$$
+We have
+$$
+\diff R = n \to \{\, (1, 1 - n) \mid n \ge 2 \,\}
+$$
+and so, by treating the parameters in a special way, we obtain
+the following approximation for $R^+$:
+$$
+n \to \{\, (x, y) \to (x', y') \mid n \ge 2 \wedge y' \le 1 - n + y \wedge x' \ge 1 + x \,\}
+.
+$$
+If we consider instead
+$$
+R' = \{\, (n, x, y) \to (n, 1 + x, 1 - n + y) \mid n \ge 2 \,\}
+$$
+then
+$$
+\diff R' = \{\, (0, 1, y) \mid y \le -1 \,\}
+$$
+and we obtain the approximation
+$$
+n \to \{\, (x, y) \to (x', y') \mid n \ge 2 \wedge x' \ge 1 + x \wedge y' \le x + y - x' \,\}
+.
+$$
+If we consider both $\diff R$ and $\diff R'$, then we obtain
+$$
+n \to \{\, (x, y) \to (x', y') \mid n \ge 2 \wedge y' \le 1 - n + y \wedge x' \ge 1 + x \wedge y' \le x + y - x' \,\}
+.
+$$
+Note, however, that this is not the most accurate affine approximation that
+can be obtained. That would be
+$$
+n \to \{\, (x, y) \to (x', y') \mid y' \le 2 - n + x + y - x' \wedge n \ge 2 \wedge x' \ge 1 + x \,\}
+.
+$$
+\end{example}
+
\subsection{Checking Exactness}
The approximation $T$ for the transitive closure $R^+$ can be obtained
* variables are non-zero and if moreover the parametric constant
* can never attain positive values.
* Return IMPURE otherwise.
+ *
+ * If div_purity is NULL then we are dealing with a non-parametric set
+ * and so the constraint is obviously PURE_VAR.
*/
static int purity(__isl_keep isl_basic_set *bset, isl_int *c, int *div_purity,
int eq)
int i;
int p = 0, v = 0;
+ if (!div_purity)
+ return PURE_VAR;
+
n_div = isl_basic_set_dim(bset, isl_dim_div);
d = isl_basic_set_dim(bset, isl_dim_set);
nparam = isl_basic_set_dim(bset, isl_dim_param);
return -1;
}
+/* If any of the constraints is found to be impure then this function
+ * sets *impurity to 1.
+ */
static __isl_give isl_basic_map *add_delta_constraints(
__isl_take isl_basic_map *path,
__isl_keep isl_basic_set *delta, unsigned off, unsigned nparam,
- unsigned d, int *div_purity, int eq)
+ unsigned d, int *div_purity, int eq, int *impurity)
{
int i, k;
int n = eq ? delta->n_eq : delta->n_ineq;
int p = purity(delta, delta_c[i], div_purity, eq);
if (p < 0)
goto error;
+ if (p != PURE_VAR && p != PURE_PARAM && !*impurity)
+ *impurity = 1;
if (p == IMPURE)
continue;
if (eq && p != MIXED) {
*
* In particular, let delta be defined as
*
- * \delta = [p] -> { [x] : A x + a >= and B p + b >= 0 and
+ * \delta = [p] -> { [x] : A x + a >= 0 and B p + b >= 0 and
* C x + C'p + c >= 0 and
* D x + D'p + d >= 0 }
*
* parameter dependent and others. Constraints containing
* any of the other existentially quantified variables are removed.
* This is safe, but leads to an additional overapproximation.
+ *
+ * If there are any impure constraints, then we also eliminate
+ * the parameters from \delta, resulting in a set
+ *
+ * \delta' = { [x] : E x + e >= 0 }
+ *
+ * and add the constraints
+ *
+ * E f + k e >= 0
+ *
+ * to the constructed relation.
*/
static __isl_give isl_map *path_along_delta(__isl_take isl_dim *dim,
__isl_take isl_basic_set *delta)
int i, k;
int is_id;
int *div_purity = NULL;
+ int impurity = 0;
if (!delta)
goto error;
if (!div_purity)
goto error;
- path = add_delta_constraints(path, delta, off, nparam, d, div_purity, 1);
- path = add_delta_constraints(path, delta, off, nparam, d, div_purity, 0);
+ path = add_delta_constraints(path, delta, off, nparam, d,
+ div_purity, 1, &impurity);
+ path = add_delta_constraints(path, delta, off, nparam, d,
+ div_purity, 0, &impurity);
+ if (impurity) {
+ isl_dim *dim = isl_basic_set_get_dim(delta);
+ delta = isl_basic_set_project_out(delta,
+ isl_dim_param, 0, nparam);
+ delta = isl_basic_set_add(delta, isl_dim_param, nparam);
+ delta = isl_basic_set_reset_dim(delta, dim);
+ if (!delta)
+ goto error;
+ path = isl_basic_map_extend_constraints(path, delta->n_eq,
+ delta->n_ineq + 1);
+ path = add_delta_constraints(path, delta, off, nparam, d,
+ NULL, 1, &impurity);
+ path = add_delta_constraints(path, delta, off, nparam, d,
+ NULL, 0, &impurity);
+ path = isl_basic_map_gauss(path, NULL);
+ }
is_id = empty_path_is_identity(path, off + d);
if (is_id < 0)
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