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Date: Mon, 22 Mar 2021 15:11:24 -0700 (PDT)
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Subject: Re: Performance of records with variant parts
From: John Perry <john.perry@usm.edu>
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Xref: reader02.eternal-september.org comp.lang.ada:61646
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On Monday, March 22, 2021 at 2:31:46 PM UTC-5, Jeffrey R. Carter wrote:
> On 3/22/21 6:49 PM, John Perry wrote: 
> > Obj is an "in" parameter to a function. Do you need more than that?
> Yes. 
> 
> What compiler and optimization option are you using? I would expect any compiler 
> (except GNAT at -O0) to optimize away any discriminant checks on direct accesses 
> to variant components in the branches of the case statement. 
> 
> Is Obj passed to any subprograms from the branches of the case statement? Are 
> any variant components accessed outside of the case statement?

The compiler on the computer I'm using at the moment is GNAT Community 2020 (20200429-93) on x86_64-redhat-linux. I've also tested on MacOS, but I think that one is Community 2019. Both exhibit identical behavior.

Current optimization is -O3 -funroll-loops -gnatn, but the behavior remains consistent.

The full subprogram as it currently stands is below. As indicated in the OP, the *only* differences between this code and the non-variant code occurs lie in the variance of Thing_Type and the field names.

|  function Object_Intersect( obj: in Thing_Type; ray: in Ray_Type )
|                            return Long_Float
|  is
|  begin
|     case obj.kind is
|     when Sphere =>
|        declare
|           eo: Vector := obj.center - ray.start;
|           v: Long_Float := eo * ray.dir;
|        begin
|           if v > 0.0 then
|              declare
|                 disc: Long_Float := obj.radius2 - ( eo * eo - v * v );
|              begin
|                 if disc > 0.0 then
|                    return v - Sqrt(disc);
|                 end if;
|              end;
|           end if;
|        end;
|     when Plane =>
|        declare
|           denom: Long_Float := obj.norm * ray.dir;
|        begin
|           if denom <= 0.0 then
|              return ( obj.norm * ray.start + obj.offset ) / ( -denom );
|           end if;
|        end;
|        when None =>
|           return far_away;
|     end case;
|     return Far_Away;
|  end Object_Intersect;

john perry