Re: Signed vs Natural/32-bits vs 31 bits

[Mark H Johnson <mark_h_johnson@raytheon.com>]

skidmarks wrote:

> I'm using gcc under Cygwin and gcc with Mingw and am getting the
> following warning:
> 
>  476.       function D_Datum is new Unchecked_Conversion(Datum_Type,
> AIL_List.Datum_Type);
>             |
>       >>> warning: types for unchecked conversion have different sizes
>       >>> warning: size of "DATUM_TYPE" is 32, size of "DATUM_TYPE" is
> 31
>       >>> warning: 1 trailing bits of source will be ignored
> 
> I understand what the message means but what is the operational
> effect? What does 'ignored' mean in this context?

To answer the specific question, "ignored" means "ignored". I should 
really go look at my notes but if I recall, GNAT (or the newer GCC's) 
doing unchecked conversion will do something like...
  - ignore unused bits, value will be truncated
  - source value will be extended with unspecified bits
  - sign extend
  - zero extend
depending on the conditions when the number of source / destination bits 
do not match. These conditions basically are (in the same order...)
  - destination size < source size
  - destination size > source size & not a number
  - destination size > source size & a signed number
  - destination size > source size & an unsigned number
I generally consider a warning like this to be an error instead and fix 
the offending code.

Your subject (and not the body) makes me curious, why do you need an 
Unchecked_Conversion between signed and natural numbers? Is there 
something wrong with doing a type cast or is there something else involved?

   --Mark