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Date: Tue, 23 Mar 2021 16:00:39 -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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On Tuesday, March 23, 2021 at 4:31:05 AM UTC-5, Jeffrey R. Carter wrote:
> On 3/22/21 11:11 PM, John Perry wrote: 
> Interesting. With that optimization I would not expect this to differ noticeably 
> from a record without discriminants. 
> 
> I note that all your nested declarations could be declared constant. 
> 
> Use of Long_Float is completely non-portable. 

Thank you for these tips.

> If you want to look into this further, IIRC, there is an option that will output 
> a low-level, Ada-like description of the IR that the front end produces. You'll 
> have to look in the top-secret GNAT documentation to find it. 

Yep, that shows the problem. For the declaration that immediately follows "when Sphere =>" we get

|           B_2 : declare
|              [constraint_error when
|               objects__thing_typeD2 (obj.kind)
|                "discriminant check failed"]
|              eo : vectors__vector := vectors__Osubtract (obj.center,

and that function translates as

|     function objects__thing_typeD2 (kind : objects__object_type)
|       return boolean is
|     begin
|        if kind = objects__sphere then
|           return false;
|        else
|           return true;
|        end if;
|     end objects__thing_typeD2;

So I'm getting a completely useless warning.

It's even worse for "case Plane =>"; it checks the same thing twice!

|              if denom <= 0.0 then
|                 [constraint_error when
|                   objects__thing_typeD3 (obj.kind)
|                   "discriminant check failed"]
|                 [constraint_error when
|                   objects__thing_typeD3 (obj.kind)
|                   "discriminant check failed"]
|                 return (vectors__vector_dot (obj.norm, ray.start) +
|                   obj.offset) / (-denom);

Am I right in thinking this is a bug? It shouldn't even check it, let alone check it twice.

regards
john perry