C++: Add comment on why getGuardedUpperBound must have exactly one predecessor

Author: andersfugmann

cpp/ql/src/semmle/code/cpp/rangeanalysis/SimpleRangeAnalysis.qll

@@ -1540,6 +1540,13 @@ private float getGuardedUpperBound(VariableAccess guardedAccess) {
     RangeSsaDefinition def, StackVariable v, VariableAccess guardVa, Expr guard, boolean branch
   |
     def.isGuardPhi(v, guardVa, guard, branch) and
+    // If the basic block for the variable access being examined has
+    // more than one predecessor, the guard phi node could originate
+    // from one of the predecessors. This is because the guard phi
+    // node is attached to the block at the end of the edge and not on
+    // the actual edge. It is therefore possible to determine which
+    // edge the guard phi node belongs to. The predicate below ensures
+    // that there is one predecessor, albeit somewhat conservative.
     exists(unique(BasicBlock b | b = def.(BasicBlock).getAPredecessor())) and
     guardedAccess = def.getAUse(v) and
     result = max(float ub | upperBoundFromGuard(guard, guardVa, ub, branch))