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I have a VHDL function that returns a std_logic_vector representation of a record and I want the length of that std_logic_vector. I am able to use the length attribute directly on the function. Why does this produce a warning in ModelSim? Am I inviting subtle problems? Googling the warning text did not turn up anything I understood to be helpful.

my_function_returns_slv(my_record)'length;

** Warning: ../src/my.vhd(line#): (vcom-1515) Prefix of predefined attribute "length" is function call "my_function_returns_slv"

I've written the function to assemble the output by concatenating std_logic_vector representations of the record elements. The length of the record is fixed at compile time, but I do not want to hard code the length. I need the length to create signals for using the function output. So I can't just call 'length on the output of the function (ex: call 'length on a signal holding the function output) because it is not possible to declare an unconstrained signal to hold the output. I could write a similar function to calculate the length of the std_logic_vector, but that would add some significant code, especially for the number of records I have. Should I accept the ModelSim warning and carry on? Should I deal with the extra code from writing functions to assemble the bit width of my records? Is there a better solution altogether?

Helpful record pack/unpack subprograms I am making use of:

http://www.eda-twiki.org/twiki/pub/P1076/RecordReflectionToSlv/standard_functions.vhd

Thanks!

Scott Watson
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2 Answers2

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Using the 'length attribute directly on a function can be seen as just taking another part of the function result than the primary output, thus from a conceptual point of view there should be nothing wrong with that.

So I would accept the ModelSim warning, but also take it as an indication that the tool is worried about the construction, so I would check that my other tools, e.g. synthesis tools and code checkers, accept this use of an attribute directly on a function call.

Appears that you can avoid the ModelSim warning by making a function like:

function len(slv : std_logic_vector) return natural is
begin
  return slv'length;
end function;

and then this won't result in a ModelSim warning:

signal MY_LEN : natural := len(slv_not(CONST));

So being able to avoid the warning using such encapsulation confirms that the warning is a little flaky in the first place.

Morten Zilmer
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    Whether or not `my_function_returns_slv(my_record)'length` is legal stems from a typo transcribing the 'length attribute prefix requirement to the -1987 LRM from the Ada 83 LRM. See [Issue Report 1005 (-1993)](http://web.archive.org/web/20120217010956/http://www.eda.org/isac/IRs-VHDL-93/IR1005.txt) - the typos never fixed, the rationale indicating they shouldn't need to be the meaning matches and "appropriate for an array object" is not defined anywhere, while "for an array type" is. – user1155120 Oct 27 '16 at 10:32
  • Thank you @user1155120! I'm just starting to get acquainted with the LRM and the standards process. This historical information is very interesting to me. – Scott Watson Oct 27 '16 at 20:40
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    Thank you @Morten! Encapsulating the 'length call with a function does remove the warning in my environment as well. I had been thinking I needed to constrain a signal to assign to. What a creative solution! I thought I would also report for posterity that my synthesis tool (Quartus) does not flag my_func()'length with a warning – Scott Watson Oct 27 '16 at 20:43
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    Neither does ghdl. – user1155120 Oct 27 '16 at 21:37
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"I need the length to create signals for using the function output. So I can't just call 'length on the output of the function (ex: call 'length on a signal holding the function output) because it is not possible to declare an unconstrained signal to hold the output."

A fun work around for sizing signals is:

constant MY_CONST : std_logic_vector := my_function_returns_slv(my_record) ;
signal MySig : std_logic_vector(MY_CONST'range) := MY_CONST ;

We have an LCS for VHDL-2017 that allows signals to be unconstrained and get their constraints from the initialization.

Jim Lewis
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