InitObjectSizing < P1076

P1076 Web>Vhdl2019CollectedRequirements>InitObjectSizing (2020-02-17, JimLewis) (raw view)EditAttach
---+ Inferring Constraints from Initial Values for Signals and Variables

%TOC%

---++ Proposal Information
   * Who Updates: Main.RyanHinton, Main.JimLewis, ...
   * Date Proposed: 2013-05-01
   * Date Last Updated: 2013-05-01
   * Priority:
   * Complexity:
   * Focus: General Language
---++ Language Change Specification Link

[[LCS2016_019]]
---++ Summary

Currently constant subtype constraints can be inferred from initial values. However, variables and signals must be of fully-constrained subtypes before the initial value is considered. This proposal relaxes this constraint for convenience.

---++ Requirements

Allow signals and variables that are given fully constrained initial values to have unconstrained or partially constrained subtypes. The subtype constraints in this situation come from the initial value.

---++ Related Issues: None = General
---++ Competing Issues: None at this time

---++ Use cases

This feature is particularly handy with the new fixed-point types. Currently the following is legal.
<verbatim>
  constant a    : sfixed := to_sfixed(1.56723984, 1, -12);
  constant b    : sfixed := to_sfixed(-0.56263673, 0, -4);
  constant prod : sfixed := a * b;</verbatim>

Main.RyanHinton uses this to infer sizes and ranges of intermediate calculations from unconstrained ports.
<verbatim>
entity 
  port (
    a_port : sfixed;
    b_port : sfixed;
...
  
  constant prod_proto : sfixed := a_port * b_port;  -- don't care about actual value, just subtype
  variable prod       : prod_proto'subtype;  -- use constant just to get the appropriate subtype
...

  prod := a_port * b_port;</verbatim>

Often I will have a sequence of 8 or more "prototype" constants depending on ports and earlier constants representing a chain of calculations. But I have to create a bunch of dummy constants. It's a pain to maintain.
---++ Proposal: Part 1

With the proposed change, the above simplifies as follows.
<verbatim>
entity 
  port (
    a_port : sfixed;
    b_port : sfixed;
...

  -- NO DUMMY CONSTANT NEEDED  
  variable prod       : sfixed := a_port * b_port;
  -- the initial value may not be meaningful, but the subtype of the 
  -- result is used to constrain the variable type.

...

  prod := a_port * b_port;</verbatim>

The change looks small here, but it adds up with long processing chains (e.g. heavy pipelining).

I agree we should do this, particularly for testbenches, however, for RTL code it has an implication of some flavor of power up value. As a result, I propose Part 2 be considered in conjunction with part 1. -- Main.JimLewis - 2013-05-02
---++ Proposal: Part 2

-- Main.JimLewis - 2013-05-02

Extend 'range and 'subtype to work with an expression.

Rationale: 'range and 'subtype will not initialize a signal and as a result, for hardware will not have any power on implications.
<verbatim>entity 
 port (
    a_port : sfixed;
    b_port : sfixed;
...

  variable prod  : sfixed( (a_port * b_port)'range ) ;</verbatim>

In addition, 'range and 'subtype use a range and a subtype as a first class object. If a predefined function can do this, then so should a user defined function.
---++ Arguments FOR

The following is ok:
<verbatim>
constant MY_SLV : std_logic_vector := "0011" ;
signal MySlvSig : std_logic_vector(MY_SLV'range) ;</verbatim>

--- Main.JimLewis -- 2013-04-24, interpreted from reflector email by Main.RyanHinton

(Comment by Main.RyanHinton:) In other words, if a signal can get its subtype (via a range constraint) from a constant that gets its subtype constraint from an expression, why not allow the signal to get its subtype directly from an expression? In yet other words, there are no static-ness issues here. Or if there are, they're no different from the constant case.

---++ Arguments AGAINST

---++ Supporters

-- Main.RyanHinton - 2013-05-02 -- [[Main.BrentHahoe][Brent Hayhoe]] -2013-15-02 -- Main.JimLewis - 2013-05-02 -- Main.KevinJennings - 2013-05-09 (Proposal 2 approach)

_Add your signature here to indicate support for this proposal._
Attachments
I	Attachment	Action	Size	Date	Who	Comment
docx	LRM_Mods_Inferring_Constraints.docx	manage	10.6 K	2016-10-14 - 15:29	KevinJennings
pdf	LRM_Mods_Inferring_Constraints.pdf	manage	192.1 K	2016-10-14 - 15:29	KevinJennings
Topic revision: r9 - 2020-02-17 - 15:34:34 - JimLewis
P1076