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Date: Mon, 13 Sep 2010 09:47:22 -0400
From: "Peter C. Chapin" <chapinp@acm.org>
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Subject: Re: Question about package Interfaces.
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Date: 2010-09-13T09:47:22-04:00
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On 2010-09-12 23:54, Robert A Duff wrote:

> Unsigned_8'Size = 8.  But that does not mean that all objects
> of subtype Unsigned_8 are 8 bits, nor that X'Size = 8
> (where "X: Unsigned_8;").  For example, an object could
> be stored in a 64-bit register.
> 
> A packed array A of Unsigned_8 will have A'Component_Size = 8,
> and A(I)'Size = 8 for all I.

So I have to explicitly ask for a packed array in order to get this
guarantee? Is that true even if I declare a component type with an
explicit representation clause requesting an 8 bit size? I guess probably.

>> The reference manual does say, "For shifting, zero bits are shifted
>> in..." However, it has recently come to my attention that some
>> processors, such as Intel architecture machines, have shift instructions
>> that mask the shift count. For example, shifting a 32 bit operand by 32
>> bits results in an actual shift of 0 bits. Does that mean
>> implementations for such a system can't compile Interfaces.Shift_Left
>> into a single instruction but instead have to force "unlimited" shift
>> semantics on top of a hardware facility that doesn't do it that way?
> 
> Yes, in general.  However, if the compiler knows that
> the shift count is in a certain range it could optimize
> to a single machine instruction, even on when the hardware
> does something silly (as x86 does).

Okay, good to know. It's another argument in favor of sensible user
defined subtypes.

Thanks

Peter