memory: fix address range overlap with zero-sized resources.

Author: Jean-François Nguyen

nmigen_soc/memory.py

@@ -171,7 +171,7 @@ def _compute_addr_range(self, addr, size, step=1, *, alignment, extend):
         if not isinstance(size, int) or size < 0:
             raise ValueError("Size must be a non-negative integer, not {!r}"
                              .format(size))
-        size = self._align_up(size, alignment)
+        size = self._align_up(max(size, 1), alignment)
 
         if addr > (1 << self.addr_width) or addr + size > (1 << self.addr_width):
             if extend:

nmigen_soc/test/test_memory.py

@@ -104,6 +104,11 @@ def test_add_resource_extend(self):
                          (0x10000, 0x10001))
         self.assertEqual(memory_map.addr_width, 17)
 
+    def test_add_resource_size_zero(self):
+        memory_map = MemoryMap(addr_width=1, data_width=8)
+        self.assertEqual(memory_map.add_resource("a", size=0), (0, 1))
+        self.assertEqual(memory_map.add_resource("b", size=0), (1, 2))
+
     def test_add_resource_wrong_address(self):
         memory_map = MemoryMap(addr_width=16, data_width=8)
         with self.assertRaisesRegex(ValueError,