Performance of records with variant parts

[John Perry <john.perry@usm.edu>]

Hello all

I encountered a significant penalty when using a record with variant parts in gnat community edition, on both MacOS and Linux. Suppose I have the following. (Kind of long, sorry.)

| type Object_Type is ( Sphere, Plane, None );
| type Surface_Type is ( Shiny, Checkerboard );
| type Thing_Type(kind: Object_Type := None; surface: Surface_Type := Shiny)
| is record
|    case kind is
|       when None =>
|          null;
|       when Sphere =>
|          radius2: Long_Float := 0.0;
|          center: Vector;
|       when Plane =>
|          offset: Long_Float := 0.0;
|          norm: Vector;
|    end case;
| end record;

Noticing that the fields are actually the same types, but different names, I also tried this:

| type Thing_Type is record
|    Kind: Object_Type := None;
|    Surface: Surface_Type := Shiny;
|    Size: Long_Float := 0.0;
|    Pos: Vector;
| end record;

This second version doesn't seem a good idea, but compiled output runs 15-30% faster. As far as I can tell***, it's related to this test which gets called ~3 million times:

|       case obj.kind is
|       when Sphere =>
|          -- do stuff
|       when Plane =>
|          -- do other stuff
|       when None =>
|          null;
|       end case;

Is there a reason that the record with variant parts runs so much slower? I can make the complete source available if need be.

john perry

***"as far as I can tell": gprof  stated at one point that this procedure sucks up most of the time; it's one of only two places where the discriminant is evaluated, and it's the only one that gets called A LOT; and this one change makes for the difference in performance.