C++: Exclude BlockVar computation for "large" vars

cpp/ql/src/semmle/code/cpp/dataflow/internal/FlowVar.qll

@@ -490,7 +490,8 @@ module FlowVar_internal {
   private SubBasicBlock getAReachedBlockVarSBB(TBlockVar start) {
     exists(Variable v |
       start = TBlockVar(result, v) and
-      variableLiveInSBB(result, v)
+      variableLiveInSBB(result, v) and
+      not largeVariable(v, _, _)
     )
     or
     exists(SubBasicBlock mid, SubBasicBlock sbbDef, Variable v |
@@ -503,6 +504,16 @@ module FlowVar_internal {
     )
   }
 
+  /**
+   * Holds if `v` may have too many combinations of definitions and reached
+   * blocks for us the feasibly compute its def-use relation.
+   */
+  private predicate largeVariable(Variable v, int liveBlocks, int defs) {
+    liveBlocks = strictcount(SubBasicBlock sbb | variableLiveInSBB(sbb, v)) and
+    defs = strictcount(SubBasicBlock sbb | exists(TBlockVar(sbb, v))) and
+    liveBlocks * defs > 1000000
+  }
+
   /**
    * Holds if a value held in `v` at the start of `sbb` (or after the first
    * assignment, if that assignment is to `v`) might later be read.