Make sure setOldestVersion has work to do in bench

Bench.cpp

@@ -787,13 +787,21 @@ template <class ConflictSet_> void benchConflictSet(const std::string &name) {
       ++iter;
     }
 
+    while (version < kMvccWindow) {
+      auto v = ++version;
+      for (auto &w : writes) {
+        w.writeVersion = v;
+      }
+      cs.addWrites(writes.data(), writes.size());
+    }
+
     bench.run(name + " (point writes)", [&]() {
       auto v = ++version;
       for (auto &w : writes) {
         w.writeVersion = v;
       }
       cs.addWrites(writes.data(), writes.size());
-      cs.setOldestVersion(std::max<int64_t>(version - kMvccWindow, 0));
+      cs.setOldestVersion(version - kMvccWindow);
     });
   }
 
@@ -817,7 +825,7 @@ template <class ConflictSet_> void benchConflictSet(const std::string &name) {
         w.writeVersion = v;
       }
       cs.addWrites(writes.data(), writes.size());
-      cs.setOldestVersion(std::max<int64_t>(version - kMvccWindow, 0));
+      cs.setOldestVersion(version - kMvccWindow);
     });
   }
 
@@ -841,7 +849,7 @@ template <class ConflictSet_> void benchConflictSet(const std::string &name) {
         w.writeVersion = v;
       }
       cs.addWrites(writes.data(), writes.size());
-      cs.setOldestVersion(std::max<int64_t>(version - kMvccWindow, 0));
+      cs.setOldestVersion(version - kMvccWindow);
     });
   }
 }

ConflictSet.cpp

@@ -1743,6 +1743,9 @@ struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
   }
 
   void setOldestVersion(int64_t oldestVersion) {
+    if (oldestVersion <= this->oldestVersion) {
+      return;
+    }
     this->oldestVersion = oldestVersion;
     Node *prev = firstGeq(root, removalKey).n;
     // There's no way to erase removalKey without introducing a key after it