use std::atomic to protect concurrent access to the call_id_ static d…

…ata member

Protect concurrent access to the Blif::call_id_ using an atomic data type: it is cheaper than a lock and for a machine word it is sufficent.

Resolves rmp in #5981.

src/rmp/include/rmp/blif.h

@@ -38,6 +38,7 @@
 #include <stdio.h>
 #include <unistd.h>
 
+#include <atomic>
 #include <map>
 #include <set>
 #include <string>
@@ -90,7 +91,7 @@ class Blif
   std::string const1_cell_port_;
   std::map<std::string, std::pair<float, float>> requireds_;
   std::map<std::string, std::pair<float, float>> arrivals_;
-  static int call_id_;
+  static std::atomic<int> call_id_;
 };
 
 }  // namespace rmp

src/rmp/src/blif.cpp

@@ -62,7 +62,7 @@ using utl::RMP;
 
 namespace rmp {
 
-int Blif::call_id_ = 0;
+std::atomic<int> Blif::call_id_ = 0;
 
 Blif::Blif(Logger* logger,
            sta::dbSta* sta,