place: Fix placer validity checks

Signed-off-by: David Shah <davey1576@gmail.com>

common/place_sa.cc

@@ -177,13 +177,16 @@ static bool try_swap_position(Design *design, CellInfo *cell, BelId newBel,
     }
 
     chip.bindBel(newBel, cell->name);
+
     if (other != IdString()) {
-        if (!isValidBelForCell(design, other_cell, oldBel)) {
-            chip.unbindBel(newBel);
-            goto swap_fail;
-        } else {
-            chip.bindBel(oldBel, other_cell->name);
-        }
+        chip.bindBel(oldBel, other_cell->name);
+    }
+
+    if (!isBelLocationValid(design, newBel) || ((other != IdString() && !isBelLocationValid(design, oldBel)))) {
+        chip.unbindBel(newBel);
+        if (other != IdString())
+            chip.unbindBel(oldBel);
+        goto swap_fail;
     }
 
     cell->bel = newBel;

ice40/arch_place.cc

@@ -79,6 +79,27 @@ static bool logicCellsCompatible(const std::vector<const CellInfo *> &cells)
     return locals.size() <= 32;
 }
 
+bool isBelLocationValid(Design *design, BelId bel) {
+    const Chip &chip = design->chip;
+    if (chip.getBelType(bel) == TYPE_ICESTORM_LC) {
+        std::vector<const CellInfo *> cells;
+        for (auto bel_other : chip.getBelsAtSameTile(bel)) {
+            IdString cell_other = chip.getBelCell(bel_other, false);
+            if (cell_other != IdString()) {
+                const CellInfo *ci_other = design->cells[cell_other];
+                cells.push_back(ci_other);
+            }
+        }
+        return logicCellsCompatible(cells);
+    } else {
+        IdString cellId = chip.getBelCell(bel, false);
+        if (cellId == IdString())
+            return true;
+        else
+            return isValidBelForCell(design, design->cells.at(cellId), bel);
+    }
+}
+
 bool isValidBelForCell(Design *design, CellInfo *cell, BelId bel)
 {
     const Chip &chip = design->chip;

ice40/arch_place.h

@@ -30,6 +30,9 @@ NEXTPNR_NAMESPACE_BEGIN
 // such as conflicting set/reset signals, etc
 bool isValidBelForCell(Design *design, CellInfo *cell, BelId bel);
 
+// Return true whether all Bels at a given location are valid
+bool isBelLocationValid(Design *design, BelId bel);
+
 NEXTPNR_NAMESPACE_END
 
 #endif