How to generate variable labels for same component within a generate loop

[Weng Tianxiang]

Hi,
I want to know how to generate variable labels for same component
within a loop.

The following is the code I am writing:

component A port (
...
);
end component;

begin
for I in 5 to 0 generate
xxx : A port map (
...
);
end generate;

I don't know how to put variable I into the label xxx.

Thank you.

Weng

 

[Allan Herriman]

Hi,
I want to know how to generate variable labels for same component
within a loop.

The following is the code I am writing:

component A port (
...
);
end component;

begin
for I in 5 to 0 generate
xxx : A port map (
...
);
end generate;

I don't know how to put variable I into the label xxx.

You can't modify labels that way - they are fixed at compile time.

Note that the generated netlist *will* contain the value of "I"
somewhere in the instance name of A, so you can still uniquely
identify each instance. (Is that the problem you're trying to solve?)

From memory, VHDL doesn't actually specify how the instance names are
generated, but any practical tool will do what you want, albeit in a
way that may vary from tool to tool.
Verilog (the other popular HDL) does specify how the instance names
are generated, however my experience is that the tools may not conform
to the standard when using a generate loop (which is a relatively
recent addition to that language).

Regards,
Allan

 

[Weng Tianxiang]

Hi Allan,
Surprise to learn another hole in VHDL.

I don't have time reading full context of VHDL and hope there were some
methods to change labels in a generate loop.

Why cannot we add a '&' between a fixed label string and the loop
variable like this one:

A : for I in 5 to 0 generate
AB&I : ModuleA port map (
....
);
end generate;

When new lable is generated, '&' will be replaced with '_" so that
variable lables in the above example would be generated normally: AB_5,
AB_4, ... AB_0. This type of technique was used 50 years ago with
assembly language.

Currently during a generate loop with fixed labels, why doesn't a
compiler generate an error with information: same multiple labels are
generated!

Thank you.

Weng

 

[Allan Herriman]

Hi Allan,
Surprise to learn another hole in VHDL.

I don't have time reading full context of VHDL and hope there were some
methods to change labels in a generate loop.

Why cannot we add a '&' between a fixed label string and the loop
variable like this one:

A : for I in 5 to 0 generate
AB&I : ModuleA port map (
...
);
end generate;

When new lable is generated, '&' will be replaced with '_" so that
variable lables in the above example would be generated normally: AB_5,
AB_4, ... AB_0. This type of technique was used 50 years ago with
assembly language.

Currently during a generate loop with fixed labels, why doesn't a
compiler generate an error with information: same multiple labels are
generated!

Because the compiler does just what you wanted: it will create unique
labels like AB_5, AB_4, etc. It's just that the exact details are
compiler dependent, since they are not specified in the LRM. (IIRC -
someone please correct me if I'm wrong.)

Regards,
Allan

 

[Weng Tianxiang]

Hi Allan,
Thank you very much for your fastest response. Are you working today on
weekend?

Weng

 

[Christophe]

Hi everbody!

Allan Herriman a écrit :

Because the compiler does just what you wanted: it will create unique
labels like AB_5, AB_4, etc. It's just that the exact details are
compiler dependent, since they are not specified in the LRM. (IIRC -
someone please correct me if I'm wrong.)

Actually, the LRM (paragraph 12.4.2) specifies the elaboration of a
generate statement.
"[...]
For a generate statement with a for generation scheme, elaboration
consists of the elaboration of the discrete
range, followed by the generation of one block statement for each value
in the range. The block statements
all have the following form:
a) The label of the block statement is the same as the label of the
generate statement.
[..]
"

I don't reproduce the examples.

A note follows:
"NOTE-The repetition of the block labels in the case of a for
generation scheme does not produce multiple declarations
of the label on the generate statement. The multiple block statements
represented by the generate statement constitute
multiple references to the same implicitly declared label."


Finally, I don't see exactly why a variable labels should be needed. I
would think that the first goal is to discriminate multiple
instanciations of your component for, let's say, different
configurations. If so, I will quote a part (paragraph 4.2.3 GENERATE
Usage and Configuration) of the FAQ:
"
The generate statement (FAQ Part 4 - B.105) is a concurrent statement
(FAQ Part 4 - B.44) that contains other concurrent statements. Two
forms exist: for and if generate. An example which uses both is:
First: if i=0 generate
Q: adder port map (A(0), B(0), Cin, Sum(0), C(0));
end generate;
Second: for i in 1 to 3 generate
Q: adder port map (A(i), B(i), C(i-1), Sum(i), C(i));
end generate;

The components are addressed (e.g., for specification):

First.Q, Second(1).Q, Second(2).Q, and Second(3).Q

An external configuration specification might look like:
for First -- First.Q
for Q: adder use entity work.adder(architectureX);
end for;
end for;

for Second(1) -- Second(1).Q
for Q: adder use entity work.adder(architectureX);
end for;
end for;
for Second(2) -- Second(2).Q
for Q: adder use entity work.adder(architectureY);
end for;
end for;
for Second(3) -- Second(3).Q
for Q: adder use entity work.adder(architectureZ);
end for;
end for;
"
(see also LRM 1.3.1)

Regards,
Christophe