mem: Fix initial value problem with MemChecker

In highly loaded cases, reads might actually overlap with writes to the
initial memory state. The mem checker needs to detect such cases and
permit the read reading either from the writes (what it is doing now) or
read from the initial, unknown value.

This patch adds this logic.

src/mem/mem_checker.cc

@@ -195,6 +195,17 @@ MemChecker::ByteTracker::inExpectedData(Tick start, Tick complete, uint8_t data)
         }
         // Record non-matching, but possible value
         _lastExpectedData.push_back(last_obs.data);
+    } else {
+        // We have not seen any valid observation, and the only writes
+        // observed are overlapping, so anything (in particular the
+        // initialisation value) goes
+        // NOTE: We can overlap with multiple write clusters, here
+        if (!writeClusters.empty() && wc_overlap) {
+            // ensure that all write clusters really overlap this read
+            assert(writeClusters.begin()->start < complete &&
+                   writeClusters.rbegin()->complete > start);
+            return true;
+        }
     }
 
     if (_lastExpectedData.empty()) {