Re: [sv-bc] Type of a concat expression

Hi All.

Expressions are not declared and therefore should have no type.

The question is how an expression should be cast when used in
a statement. This is why the Std doesn't directly address
the "type" of an expression: It shouldn't!

For example, reg[3:0] X, Y;

type(X) or type(Y) makes sense because it is applied to the
declared name; but,

type(X & Y) doesn't make sense, except as the result of
an implied type-cast.

If I write Z <= Y & X;

the RHS expression can not be treated as a type, because the
expression will be adjusted in width according to the width
of Z.

Similarly, a concatenation is governed by width rules, but
it is an expression, not a declared type. However, the
components of a concatenation may have types (or may be, for
example, literals with implied types. This doesn't mean
that a concatenation "has" a type. The type of a concatenation,
is determined by how it is used.

In my opinion, type( {x,Y} ) should return a null (empty) string
"".

If SystemVerilog were more like C, then perhaps type( Z = {X,Y} )
might have a well-defined type (the type of Z).

On 06/15/2010 08:28 AM, Gordon Vreugdenhil wrote:
> Surya,
>
> The expression rules define the type of the expression. There
> are a few areas that are not well defined, but most are trivial
> to extrapolate from the defined behavior of concat (i.e. that
> width, size, and sign count). The main thing that isn't clear
> is selects -- i.e. should type(a[5:4]) == type(a[3:2])
> That does need to be clarified.
>
> Surya Pratik Saha wrote:
>> Hi Gordon,
>> I have looked at table 11-21. That is all related to bit length
>> calculation of expression, nothing related to type of expression. And
>> also sections 11.6 - 11.8 all talk about the evaluation rule of
>> expression, but not about the type. But if you have an expression
>> inside a type operator and used in comparison rule with another type
>> operator, it needs specific rule to check which is missing in LRM I
>> can see. For e.g. what is a type of a binary operator consisting of
>> two numbers:
>>
>> initial
>> if (type(100 + 2'b11) == type (int)) // Will it match or not?
>
> No - signedness and 2/4 state mismatch.
>
>> x = 1;
>> else
>> x = 2;
>>
>> Regarding 2-state/4-state, a logic can contain both 2-state and
>> 4-state values. So it is not clear whether following comparison will
>> match or not:
>>
>> typedef logic[1:0] mylogic;
>> type (type({1'b1, 1'b0}) == type (mylogic))
>
> Yes. Both types are unsigned, 4-state, vectors ranged 1:0.
>
> The LRM isn't quite as clear on the type of a sized, based
> literal; for an unsized it clearly says integer (i.e. 4-state)
> and since the LRM section starts with "There are two forms to
> express integer literal constants." I think it is fair to
> extrapolate that they are 4-state.
>
> BTW, this does imply that there are no natural 2-state numbers
> at all. In most cases this is immaterial, but in "type"
> expressions the difference can be noticed.
>
>
>> I think LRM should have some guidelines to clarify all.
>
> I don't think there is much to do. A few edge cases, yes. But
> the expression rules for sign, width, and state define the basic
> rule set.
>
> Gord
>
>
>>
>> Regards
>> Surya
>>
>>
>>
>> -------- Original Message --------
>> Subject: Re:[sv-bc] Type of a concat expression
>> From: Gordon Vreugdenhil <gordonv@model.com>
>> To: Surya Pratik Saha <spsaha@cal.interrasystems.com>
>> Cc: "sv-bc@eda.org" <sv-bc@eda.org>, "'Pradip Mukhopadhyay'"
>> <pradip@cal.interrasystems.com>, Subhankar Ghosh
>> <sghosh@cal.interrasystems.com>
>> Date: Monday, June 14, 2010 10:09:44 PM
>>> Have you looked at Table 11-21? And generally Section 11.6 - 11.8 in
>>> 1800-2009?
>>>
>>> If you have a specific question about a specific rule application,
>>> please
>>> do ask. But first, carefully work through Clause 11; there are many
>>> rules
>>> covering self-determined result types for expressions.
>>>
>>> The rules in SV are not trivial, but they are in the LRM.
>>>
>>> Gord.
>>>
>>>
>>> Surya Pratik Saha wrote:
>>>> Hi Shalom,
>>>> Thanks for your response. But I think we should have a list of all
>>>> self determined result type of an expression. For e.g. what is the
>>>> type of a binary expression?
>>>>
>>>> Regards
>>>> Surya
>>>>
>>>>
>>>>
>>>> -------- Original Message --------
>>>> Subject: Re:[sv-bc] Type of a concat expression
>>>> From: Bresticker, Shalom <shalom.bresticker@intel.com>
>>>> To: Surya Pratik Saha <spsaha@cal.interrasystems.com>, sv-bc@eda.org
>>>> <sv-bc@eda.org>
>>>> Cc: "'Pradip Mukhopadhyay'" <pradip@cal.interrasystems.com>,
>>>> Subhankar Ghosh <sghosh@cal.interrasystems.com>
>>>> Date: Monday, June 14, 2010 5:44:29 PM
>>>>> The LRM says (11.4.12)
>>>>>
>>>>> "The concatenation is treated as a packed vector of bits...One or
>>>>> more bits of a concatenation can be selected as if the
>>>>> concatenation were a packed array with the range [n-1:0]."
>>>>>
>>>>> and (11.8.1)
>>>>>
>>>>> "— Concatenate results are unsigned, regardless of the operands."
>>>>>
>>>>> Note that 'byte' is signed.
>>>>>
>>>>> Shalom
>>>>>
>>>>>
>>>>>
>>>>> ------------------------------------------------------------------------
>>>>>
>>>>> *From:* owner-sv-bc@eda.org [mailto:owner-sv-bc@eda.org] *On
>>>>> Behalf Of *Surya Pratik Saha
>>>>> *Sent:* Monday, June 14, 2010 3:04 PM
>>>>> *To:* sv-bc@eda.org
>>>>> *Cc:* 'Pradip Mukhopadhyay'; Subhankar Ghosh
>>>>> *Subject:* [sv-bc] Type of a concat expression
>>>>>
>>>>> Hi,
>>>>> What should be the return value of a "type ({b1, b2})"? LRM
>>>>> section 6.23 (Type operator) mentions following text:
>>>>> *The type operator applied to an expression shall represent the
>>>>> self-determined result type of that expression.
>>>>>
>>>>> *But I could not find anywhere self-determined result type defined
>>>>> of an expression.
>>>>> One of the standard simulators which supports type operator works
>>>>> in its own way.
>>>>>
>>>>> Here is a sample example:
>>>>> module top(input bit in1, input bit[31:0] in2, output
>>>>> bit [7:0] out1);
>>>>> bit [3:0] f0, f1; bit [3:0] l1;
>>>>> initial
>>>>> begin : blk
>>>>> if(type ({f0, f1}) == type (byte))
>>>>> out1 = 1;
>>>>> else out1 = 3;
>>>>> end
>>>>> endmodule
>>>>>
>>>>> -- Regards
>>>>> Surya
>>>>> -- This message has been scanned for viruses and
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>>>>
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>>
>>
>>
>>
>

-- 
      John Michael Williams
      Senior Adjunct Faculty
Silicon Valley Technical Institute
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