Refactor logic when exceeding the path count limit

Return Path_Count_Limit + 1 rather than 0 as a special value to indicate
that the number of paths in a decision binary diagram exceeds the path
count limit, to be able to distinguish such cases from static decisions.

Author: Jugst3r

tools/gnatcov/instrument-ada_unit.adb

@@ -8264,13 +8264,16 @@ package body Instrument.Ada_Unit is
                      Decision  : constant SCO_Id :=
                        Enclosing_Decision (Condition);
                   begin
-                     if Path_Count (Decision) /= 0
-                        and then not SC.Decision_Static
+                     --  If the number of paths in the decision binary diagram
+                     --  exceeds the path count limit, we do not instrument it.
+
+                     if Path_Count (Decision) > Get_Path_Count_Limit
+                       and then not SC.Decision_Static
                      then
+                        Set_Decision_SCO_Non_Instr_For_MCDC (Decision);
+                     else
                         Insert_Condition_Witness
                           (UIC, SC, Offset_For_True (Condition));
-                     else
-                        Set_Decision_SCO_Non_Instr_For_MCDC (Decision);
                      end if;
                   end;
                end loop;

tools/gnatcov/instrument-c.adb

@@ -3110,7 +3110,8 @@ package body Instrument.C is
                            Decision  : constant SCO_Id :=
                              Enclosing_Decision (Condition);
                         begin
-                           if Path_Count (Decision) = 0 then
+                           if Path_Count (Decision) > Get_Path_Count_Limit
+                           then
                               Set_Decision_SCO_Non_Instr_For_MCDC (Decision);
                            else
                               Insert_Condition_Witness

tools/gnatcov/instrument-common.adb

@@ -216,7 +216,9 @@ package body Instrument.Common is
 
          --  Warn if the number of paths exceeded the limit
 
-         if Path_Count = 0 and then Coverage.MCDC_Coverage_Enabled then
+         if Path_Count > Get_Path_Count_Limit
+           and then Coverage.MCDC_Coverage_Enabled
+         then
             Diagnostics.Report
               (Decision_Sloc,
                "Number of distinct paths in the decision exceeds the limit"

tools/gnatcov/sc_obligations-bdd.adb

@@ -301,7 +301,7 @@ package body SC_Obligations.BDD is
       BDD.Path_Count := Path_Count (BDD.Root_Condition);
    exception
       when Path_Limit_Error =>
-         BDD.Path_Count := 0;
+         BDD.Path_Count := Path_Count_Limit + 1;
    end Enumerate_Paths;
 
    ---------------

tools/gnatcov/sc_obligations.ads

@@ -538,9 +538,7 @@ package SC_Obligations is
    function Path_Count (SCO : SCO_Id) return Natural;
    --  Return count of paths through decision's BDD from root condition to
    --  any outcome. This should be a positive number for any decision
-   --  (at least 2, one for the True outcome, and one for the False outcome),
-   --  but this function can also return 0 when the number of paths exceeds
-   --  the limit set with Set_Path_Count_Limit.
+   --  (at least 2, one for the True outcome, and one for the False outcome).
 
    procedure Set_Path_Count_Limit (Limit : Natural);
    --  Set the path count limit beyond which BDD path enumeration is aborted