isl_tab.c: to_row: allow pivoting in arbitrary direction

While undoing the allocation of a row, we may be dealing with
a row that was unbounded in both directions and is now in a column.
To move the column back to a row, we just pivot using any non-zero entry.

diff --git a/isl_tab.c b/isl_tab.c
index 4e759e6..3f6d5d1 100644 (file)
--- a/isl_tab.c
+++ b/isl_tab.c
@@ -494,6 +494,8 @@ static void pivot(struct isl_tab *tab, int row, int col)
 /* If "var" represents a column variable, then pivot is up (sgn > 0)
  * or down (sgn < 0) to a row.  The variable is assumed not to be
  * unbounded in the specified direction.
+ * If sgn = 0, then the variable is unbounded in both directions,
+ * and we pivot with any row we can find.
  */
 static void to_row(struct isl_tab *tab, struct isl_tab_var *var, int sign)
 {
@@ -502,8 +504,16 @@ static void to_row(struct isl_tab *tab, struct isl_tab_var *var, int sign)
        if (var->is_row)
                return;
 
-       r = pivot_row(tab, NULL, sign, var->index);
-       isl_assert(tab->mat->ctx, r >= 0, return);
+       if (sign == 0) {
+               for (r = tab->n_redundant; r < tab->n_row; ++r)
+                       if (!isl_int_is_zero(tab->mat->row[r][2 + var->index]))
+                               break;
+               isl_assert(tab->mat->ctx, r < tab->n_row, return);
+       } else {
+               r = pivot_row(tab, NULL, sign, var->index);
+               isl_assert(tab->mat->ctx, r >= 0, return);
+       }
+
        pivot(tab, r, var->index);
 }
 
@@ -1668,10 +1678,12 @@ static void perform_undo(struct isl_tab *tab, struct isl_tab_undo *undo)
                break;
        case isl_tab_undo_allocate:
                if (!undo->var->is_row) {
-                       if (max_is_manifestly_unbounded(tab, undo->var))
+                       if (!max_is_manifestly_unbounded(tab, undo->var))
+                               to_row(tab, undo->var, 1);
+                       else if (!min_is_manifestly_unbounded(tab, undo->var))
                                to_row(tab, undo->var, -1);
                        else
-                               to_row(tab, undo->var, 1);
+                               to_row(tab, undo->var, 0);
                }
                drop_row(tab, undo->var->index);
                break;