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Date: Fri, 26 Jun 2015 06:20:26 -0700 (PDT)
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Subject: Fixed-point question
From: Patrick Noffke <patrick.noffke@gmail.com>
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Xref: news.eternal-september.org comp.lang.ada:26484
Date: 2015-06-26T06:20:26-07:00
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Why are C1 and C2 different?

with Ada.Text_IO;

procedure FP_Test is
   type Fixed_Type is delta 1.0 / 180.0 range 0.0 .. 65535.0 / 180.0;
   N : constant := 0.2;
   D : constant := 4.5 / 330.0;  -- 0.013636

   C1 : constant Fixed_Type := Fixed_Type (N / D);
   C2 : constant Fixed_Type := N / D;

begin
   Ada.Text_IO.Put_Line ("C1 := " & C1'Img);
   Ada.Text_IO.Put_Line ("C2 := " & C2'Img);
end FP_Test;

$ ./fp_test 
C1 :=  14.664
C2 :=  17.000

I am using GNAT on Fedora 21 (x86_64) and GNAT 2014 for ARM-ELF (Linux-hosted).  The output above is on Fedora.  In the ARM processor (Cortex-M4), C1 is 14.667 and C2 is 18.

It appears that for ARM, the compiler is computing C2 as Fixed_Type (N) / Fixed_Type (D), or 0.2 / (2 / 180) = 0.2 / 0.01111 = 18.

I'm not sure why C2 is 17 on the Fedora PC.

Thanks,
Patrick