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Verilog-mode Faq

This is the Frequently Asked Questions (FAQ) for Verilog-mode. This FAQ is Copyright 2006-2008 by Michael McNamara and Wilson Snyder. You may redistribute this document in its entirety only, and the links to www.Verilog.com and www.Veripool.org must be retained.

Table of Contents

Obtaining, and general information

Where is the most up to date version of this FAQ?

The official released version of this document is the Verilog-Mode FAQ, from http://www.veripool.org/wiki/verilog-mode/Faq.

Where do I get Verilog-Mode?

The official released version of Verilog-Mode is distributed by Michael McNamara from his <A HREF="http://www.verilog.com">Verilog site</A>. You can also subscribe to get mailed updates.

Versions are also available from the Veripool under Verilog-mode.

How do I install Verilog-Mode?

See Installation.

Are there other Verilog Modes for Emacs?

The authors are only aware of one which have been maintained within the last half-dozen years.

Sun Yijiang maintains vlog-mode.el from http://sourceforge.net/projects/vlog-mode. His version supports GNU Emacs and he has started adding some Verilog 2001 support.

Competition is good; if you prefer vlog-mode.el, we'd like to hear back as to if there are features we could also add to in verilog-mode.el.

Can I release Verilog-Mode with my tool?

Yes. Verilog-Mode is released under the GNU General Public License. See the license for the full legal details, but fundamentally distributing it stand-alone with a commercial tool is no problem, you merely need to insure verilog-mode.el remains available to everyone. If you didn't make any changes, you're all set, else you'll need to post your version on a public website, or better, feed the changes back to the authors for inclusion in the base version.

Entering Verilog-Mode

Using viper, why when I load a verilog file does it loose viper?

You need to tell viper that it is ok with you for files in verilog to come up in verilog-mode and viper.

To do that, type 
M-x customize RET viper-misc

Then scroll down and find the item Vi State Mode List Left-Click on the triangle to open this up.

Scroll down through the blizzard of listed modes to the bottom. You should see: 
[INS] [DEL] Symbol: csh-mode
[INS] [DEL] Symbol: gnus-article-mode
[INS] [DEL] Symbol: mh-show-mode
[INS]
   [State]: this option has been set and saved.
Major modes that require Vi command state

Middle-click on the bare INS; you should get:

[INS] [DEL] Symbol: mh-show-mode
[INS] [DEL] Symbol: nil
[INS]
   [State]

Then left-click on nil, and replace the string 'nil' with verilog-mode You should get:

[INS] [DEL] Symbol: mh-show-mode
[INS] [DEL] Symbol: verilog-mode
[INS]
   [State]

Now middle-click on [State] and a pop up menu appears; select Set for Current Session and then also middle click again and select Save for Future Sessions.

Now if you are running viper, when you load a verilog file, it will start in viper mode.

Why do I get the message "File mode specification error:"?

Or, the similar messages:

File mode specification error:  (void-function define-skeleton)
File mode specification error: (file-error "Cannot open load file" "overlay")

You need skeleton.el, part of the base package for the first, or overlay.el, part of the fsf-compatibility package for the second, which are both XEmacs lisp packages, which you somehow have not installed, or have not updated.

Go to Tools -> Packages ->Add download site and pick a site (xemacs.org works for me)

Then select Tools -> Packages -> List and Install

Go to the bottom, and click on the line that says xemacs-base. to get the skeleton.el file. You will see something like:

                 Latest Installed
  Package name   Vers.  Vers.   Description
============================================================
  Sun             1.13   1.13   Support for Sparcworks.
     ...
  w3              1.18   1.18   A Web browser.
* xemacs-base     1.53   1.51   Fundamental XEmacs support.
- xemacs-devel    1.33   -----  Emacs Lisp developer support.
- xslt-process    1.03   -----  XSLT processing support.
  zenirc          1.09   1.09   ZENIRC IRC Client.
============================================================

For the overlay package, click on the line that says <font color="red">fsf-compat</font>. to get the overlay.el file. In this case you will see something like:

                 Latest Installed
  Package name   Vers.  Vers.   Description
============================================================
  Sun             1.13   1.13   Support for Sparcworks.
     ...

  w3              1.18   1.18   A Web browser.
* fsf-compat      1.12   ----   FSF EMacs compatibility files
  zenirc          1.09   1.09   ZENIRC IRC Client.
============================================================

When you click on it, the * changes to a +

Then type x, which will install the package.

Then restart XEmacs and things should then work just fine.

Indentation

How do I indent a large region of code?

Typing TAB on every line can get old...

Use C-M-\ to indent a region (selected by setting the point at one end, and having the cursor at the other end, as usual). Perhaps a future version of the emacs mode will include functions that mimic some of C's extra bindings.

How do I reindent Verilog code from the command line?

You can repair the indentation of a Verilog file from the command line with the following command:

emacs --batch {filenames...} -f verilog-batch-indent

This will load the file, reindent, and save the file.

If your verilog-mode.el is not installed in a site-wide location, or you suspect you are getting the wrong version, try specifing the exact path to Verilog-Mode by adding -l {path}/verilog-mode.el after --batch.

Additional information is in Emacs under M-x describe-function verilog-batch-indent.

Why when others edit my code does it looks unindented?

This is a general problem sharing files between folks. It also occurs between folks using the same editor, as many editors allow one to set the tab width. The general solution is for you to add a write file hook that expands tabs to spaces.

Add the following to your .emacs file: (or init.el file for XEmacs 21 users.)

(add-hook 'verilog-mode-hook '(lambda ()
    (add-hook 'local-write-file-hooks (lambda()
       (untabify (point-min) (point-max))))))

This arranges so that any file in verilog mode (the "add-hook 'verilog-mode-hook" part) gets added to it's 'local-write-file-hooks' a call to the function 'untabify' with arguments that are the first and last character in the buffer. Untabify converts all tabs in the region to multiple spaces.

Why can't I insert tabs in some places in the file?

This is because tab is a electric key that causes reindentation. See another FAQ for how to disable this.

If you want to manually space something out, in general, in Emacs you can escape the special meaning of any key by first typing C-q, which quotes the next key.

How do I prevent tab from automatically indenting?

Set the verilog-tab-always-indent variable to nil. If your goal is minimal intrusion of magic keys, you'll probably also want to set verilog-auto-newline to nil.

Add to your .emacs file:

(add-hook 'verilog-mode-hook
      '(lambda ()
         (setq verilog-auto-newline nil)
         (setq verilog-tab-always-indent nil)
      )))

How do I prevent those // comments at the end of blocks?

Set verilog-auto-endcomments to nil:

(setq verilog-auto-endcomments nil)

Why does Verilog-Mode hang reading a huge file?

To debug the problem, type

M-x eval-expression RET
(setq debug-on-quit t)

Then load the file. After 10 seconds or whatever hit Ctrl-G to stop Emacs. It will show in the debugger what it's doing.

If you're using a older flavor of Emacs, most of the time it will stop somewhere in "fontification". Simply disable fontification (coloring) of larger files. Put into your .emacs:

(setq font-lock-maximum-size 100000)

Why do I not get any colors in huge files?

This is sort of the opposite of the last FAQ; any file exceeding the default size of 256,000 characters will not get font-locked. To override this, put into your .emacs:

(setq font-lock-maximum-size 2000000)

Alternatively, load the lazy-lock package. This will only highlight the region on the screen. To find it, use 

M-x find-library RET
lazy-lock
.

Language Support

Why does the signal "bit", "do", "const" get ignored?

They're keywords. Unfortunately the SystemVerilog committee made these into new keywords, but did not provide a way to make Verilog 1995 code forward compatible by leaving them as signal names until the Verilog 2005 standard, which we don't fully support yet. You need to rename your signals.

Can I use Verilog-Mode to do 'generate' like code?

There's no general way to do this with Verilog-Mode. Obviously you can use AUTOINST to simplify the code, then ifdef, but that isn't very clean.

When I've needed to do a true generate, I often read signals with Verilog-Perl, then write the text with printf's or the Perl Text::Template module. I then pass the output of this through Emacs Verilog-Mode --batch, and walla.

Movement

How can I jump the cursor to the file that defines a module?

Use \M-x verilog-goto-defun (C-c\C-d).

How can I invoke my compiler?

Use M-x compile, or M-x verilog-auto-save-compile (C-c C-s). This looks at the verilog-tool setting and chooses your linter, coverage, simulator or compiler. The verilog-linter is the default.

So, in your .emacs set reasonable defaults for all of them:

(setq verilog-tool 'verilog-linter)
(setq verilog-linter "vlint ...")
(setq verilog-coverage "coverage ...)
(setq verilog-simulator "verilator ... ")
(setq verilog-compiler "verilator ... "

Then, if a file needs a special setting, override it at the bottom of each Verilog file:

// Local Variables:
// verilog-linter:"vlint --local_options __FILE__" 
// End:

How do I go to the next error?

After using M-x compile, or M-x verilog-auto-save-compile (C-c C-s), you'll get the compile buffer. If errors are printed there, you can jump to the line number the message mentions with M-x next-error (C-x `). Or, place the cursor over the error message and press return.

If this does not work with your tool, the tool probably does not produce errors in a standard way. You'll need to tweak the verilog-error-regexp variable. This contains a regular expression which matches a error message and returns the file and line number.

Autos

How do I start using the autos for the first time?

There are two easy ways to get started. The first is to convert an existing file, and the second is covered in the next FAQ.

To convert an existing file to use the autos, use M-x verilog-auto-inject (C-c C-z). Then, expand them with M-x verilog-auto (C-c C-s).

What AUTOs should I use for a new file?

Here's a good template for a first file:

module Modname (/*AUTOARG*/);

   // Input/output
   //input signal;    // Comment on signal

   // Automatics
   /*AUTOWIRE*/
   /*AUTOREG*/

   // Body
   //statements, etc go here.

   // Linting
   wire _unused_ok = &amp;{1'b0,
               // Put list of unused signals here
               1'b0};
endmodule

You'd then add cells using AUTOINST:

   InstModule instName
     (/*AUTOINST*/);

(The newline before the open parenthesis is suggested for larger instantiations to make the lines look nicer.)

And add sensitivity blocks using AUTOSENSE (aka AS):

   always @(/*AS*/) begin   // or, @* if using Verilog-2001
      ...
   end

How do I make a Stub module?

A stub is a module with the same input/output as another module, but it simply ignores all the inputs and drives zeros for outputs. This is often useful for replacing modules that aren't needed for a simulation.

By using several Autos, the entire stub can be created for you:

module ModnameStub (/*AUTOARG*/);
   /*AUTOINOUTMODULE("Modname")*/

   /*AUTOWIRE*/
   /*AUTOREG*/

   /*AUTOTIEOFF*/

   wire _unused_ok = &amp;{1'b0,
               /*AUTOUNUSED*/
               1'b0};
endmodule

This presumes Modname.v already exists and you want to copy the entire I/O list from it. Otherwise, remove the AUTOINOUTMODULE and add the I/O list yourself.

How do I make a Testbench module?

A testbench for the purposes of this question is a module which instantiates another module for the purpose of testing it.

By using several Autos, most of the hookup for the testbench are created for you:

module ModnameTest;

   /*AUTOWIRE*/
   /*AUTOREGINPUT*/

   InstModule instName
     (/*AUTOINST*/);

   //==== Stimulus
   // You then put code here to set all of the inputs to the DUT.
   // The autos have created registers for all of the needed signals.

   //==== Stimulus
   // You then put code here to check all of the outputs from the DUT.
   // The autos have created wires for all of the needed signals.

endmodule

How do I update AUTOs from the command line?

Use the following command:

emacs --batch {filenames...} -f verilog-batch-auto

This will load the file, update the automatics, and re-save the file. The filenames need to be provided in a bottom-up order. For a utility to determine the hierarchy of a design, see vhier in Verilog-Perl.

If your verilog-mode.el is not installed in a site-wide location, or you suspect you are getting the wrong version, try specifing the exact path to Verilog-Mode by adding -l {path}/verilog-mode.el after --batch.

There are similar functions for deleting automatics using verilog-batch-delete-auto, injecting automatics with verilog-batch-inject-auto, and reindenting with verilog-batch-indent.

Additional information is in Emacs under M-x describe-function verilog-batch-auto, etc.

How do I tell the AUTOs what directories my files are in?

The cleanest way is to use standard Verilog-XL style flags at the bottom of your Verilog file:

// Local Variables:
// verilog-library-flags:("-y incdir1/ -y incdir2/")
// End:

You'll also often see files that do it in the way that old Verilog-Mode versions required:

// Local Variables:
// verilog-library-directories:("." "dir1" "dir2" ...)
// End:

If you find yourself adding the same flags to many files, you can create a file with all of your include directories in it, then point Emacs to it. All of your Verilog files would contain:

// Local Variables:
// verilog-library-directories:("-f ../../up_to_top/include/input.vc")
// End:

Then input.vc contains the list of flags:

-y incdir1
-y incdir2
...

Note reading a file of command flags with the -f argument is also supported by Verilog-XL, VCS, Verilator and most other Verilog related tools. Thus you can write a single input.vc with all of the directories specified and feed it to all of your tools.

How do I use environment variables for a filename, etc?

Emacs only expands $'s when you ask it to do so by using substitute-in-file-name. So, if you want to substitute $ENV into a Local Variables in the bottom of your file, you need something like:

// Local Variables:
// eval:(setq verilog-library-directories (list (substitute-in-file-name "$W") ))
// End:

In what order does Verilog-Mode search for modules?

It first searches the current file, then searches for the module.v in each directory you provided in the order you provided. If the module isn't found, it searches any libraries specified.

Generally it's a really really bad idea to have files with the same name in different directories... But you probably know that. :)

How do I make defaults common for my entire design team?

First, you may not want to. If you're distributing IP you're much better off using the Local Variables at the bottom of the file, and insuring all of your file paths are relative. That way your clients can modify the AUTOs without any tweaks.

That said, add the following to site-start.el in your global Emacs distribution:

(add-hook 'verilog-mode-hook '(lambda ()
                (setq verilog-auto-newline nil
                      verilog-tab-always-indent nil
                      verilog-auto-endcomments nil
                      verilog-compiler "verilator " 
                      ;; etc, etc...
                      )))

Alternatively, add the above to a group-start.el file and have all users add a group-startup to their .emacs files:

(when (file-exists-p "/path/to/group/group-start.el")
          (load-file "/path/to/group/group-start.el"))

AUTO problems

How do I use Verilog 2001 style port lists?

Place AUTOINPUT/AUTOOUTPUT etc inside the module () parenthesis.

Does anything help declare non-instance wires and regs?

No. AUTOWIRE and AUTOREG all assume that somewhere in your design you've declared the signal. This is based on the belief that there should be at least one point where you've declared the signal, and hopefully documented it on the same line.

Why does Emacs keep asking "Process `eval' or hook local variables in file?"

You need to put in your global .emacs

(setq enable-local-eval t)

Why doesn't Emacs save SystemVerilog .* expanded instantiations to disk?

When you compute autos, Verilog-Mode will find any SystemVerilog .* pins and expand them into the ports, so that you can debug your code more easily. By default it will then strip these inserted lines when saving the file. This allows downstream tools to see the original SystemVerilog syntax, and not require re-autoing when the submodules change.

If you want to save the expanded .* pins, add to the Local Variables at the bottom of your file.

// Local Variables:
// verilog-auto-star-save: t
// End:

Why does AUTOSENSE add `defines to the list?

Call it a misfeature. :-)

Take the case where you have

always @(/*AS*/)
   ...
   a = `b;

and b isn't defined. Is b a constant, or another signal? No way to tell, it's safest to put it in the always. Granted, b could simply be defined in the file. If so, you can tell Verilog-Mode to read defines in the current file, and any `includes using:

// Local Variables:
// eval:(verilog-read-defines)
// eval:(verilog-read-includes)
// End:

If you only use defines to represent constants, it's easier to just tell Verilog-Mode that fact, and it will exclude all of them:

// Local Variables:
// verilog-auto-sense-defines-constant: t
// End:

Why do the AUTOs treat SystemVerilog types as signals?

You need to tell Verilog-Mode how to recognize a type. All of your types should match some convention, a _t suffix is probably the most common. Then add to your files:

// Local Variables:
// verilog-typedef-regexp:"_t$" 
// End:

Why do the AUTOS ignore my ifdefs?

Verilog-Mode simply pretends all ifdefs don't exist. This is done because the permutations across multiple ifdefs quickly results in code that's way too messy. The work around is all the AUTOs ignore declarations that already exist, or are done before the AUTO itself.

For example:

   module foo (
`ifdef something
    things,
`endif
    /*AUTOARG*/);

   subfile subcell (
`ifdef something
    things,
`endif
    /*AUTOINST*/);

If your selecting modules, see the next FAQ.

If your ifdefing around signals for only debug, there's rarely a need to do so. Synthesis programs will remove irrelevant logic and ignore PLI calls.

An alternative technique to have a fake "mode" input wire, rather than a ifdef or parameter. This also prevents having to lint or run other translators in 2 different `define modes, thus reducing bugs. This also relies on your synthesis program's removal of irrelevant stuff. For example a unneeded input and output can always be present, and instead:

    input          FPGA;
    input         fpga_only_input;
    output [31:0] fpga_only_output;

    if (fpga_only_input &amp;&amp; FPGA) ...
    fpga_only_output = {32{FPGA}} &amp; {value_for_output}.

Both will be stripped when FPGA=0, and present when FPGA=1.

How do I ifdef select between modules?

Often the purpose of an ifdef is to select between two alternate implementations of a module with identical pinouts; perhaps a fast RAM and a slow gate RAM. Your first attempt may be something similar to:

`ifdef SUB_IS_A_FOOBAR
   foobar subcell (/*AUTOINST*/);
`else
   foobiz subcell (/*AUTOINST*/);
`endif

However, Verilog-mode ignores ifdefs. Assuming the pinout is the same you can use the define to instead select which version:

`ifdef SUB_IS_A_FOOBAR
 `define SUB_CELL  foobar
`else
 `define SUB_CELL  foobiz
`endif
   `SUB_CELL subcell (/*AUTOINST*/);

for this to work, you need to read the defines with the below at the bottom of your file. Verilog-mode will use the last definition of SUB_CELL to determine which one to pickup the pinlist from.

// Local Variables:
// eval:(verilog-read-defines)
// End:

Can I put delays into /*AUTORESET*/?

That is,

/*AUTORESET*/
foo <= #1 signal;

Do you really want to? You really shouldn't be using delays on your assignments, as they aren't necessary to prevent races, and slows down simulation. But if you must:

// Local Variables:
// verilog-assignment-delay: "#1 " 
// End:

Can AUTOASCIIENUM be changed to put translate_off pragmas around the code?

No. First of all, you'd be better of asking to wrap it "`ifdef synthesis" as that lets the tools pick which version of the code you want.

Second, there isn't one standard way that supports all tools.

Third, presuming you never use the value it generates at all (or only in $display's) there's no reason to disable translation, as the synthesis tool will rip it all out through its normal dead code optimization stage.

How do I remove outputs from AUTOOUTPUT?

Maybe you shouldn't be using AUTOOUTPUT? Consider listing your outputs manually; this insures your module's interface is documented and remains constant, even if other lower modules change.

With that warning given, on to the solutions. You have four choices, the last probably being the most used:

First, just ifdef fake outputs. Verilog-mode will see them, but no other tool will care. This is cleanest for signals you can list one-by-one, and are using Verilog 2001 port lists or when you want those listed to still appear in a AUTOARG.

`ifdef NEVER
        output a_out;   // Fake out Verilog-mode
        output b_out;   // Fake out Verilog-mode
`endif

Second alternative, simply create a fake module listing them as inputs. Since Verilog-Mode will then see them as inputs to a submodule, it won't output them.

`ifdef NEVER
  fake fake (// Inputs
        .fake(a_out),
        .fake(b_out),
        );
`endif

Third alternative, you can add them to verilog-auto-output-ignore-regexp using Local Variables:

/*
   Local Variables:
   eval:(setq verilog-auto-output-ignore-regexp (concat
   "^\\(" 
   "signal1_.*" 
   "\\|signal2_.*" 
   "\\)$" 
   )))
   End:
*/

Finally, you can again use verilog-auto-output-ignore-regexp, but use a AUTO_LISP. This gets around a Emacs limitation of 3000 characters in a Local Variable statement.

 /*AUTO_LISP(setq verilog-auto-output-ignore-regexp
             (verilog-regexp-words `(
        "q_single_reg_rddata_30" 
        )))*/

Here we've used verilog-regexp-words to convert a simple list of signal names to a regular expression. If you prefer, you can just specify a regular expression directly, perhaps as shown in the Local Variables alternative above.

Note AUTO_LISPs are evaluated during AUTO expansion multiple times instead of only when the file is loaded into Emacs. Thus it's a bit slower, but unlikely to be noticeable.

Why doesn't AUTOWIRE include the outputs from a submodule?

AUTOWIRE requires special comments in your instantiations to determine the direction of pins. Add // Input, // Output or // Inout comments inside each instantiation just before the relevant pins.

   foo foo ( // Outputs
        .bfm_output(bfm_output),
        ....)

Why doesn't AUTOWIRE create correct widths for AUTO_TEMPLATE signals?

You simply need to add [] to the name of the pin connection. This tells Verilog-Mode to put the bit vectors into the instantiation, where they can be read by AUTOWIRE.

/* InstModule AUTO_TEMPLATE ( .signal (signal[]), ); */

What does AUTOWIRE "can't merge into single bus" mean?

When there are multiple submodules that output the same signal, AUTOWIRE needs to merge those outputs into a single bus. For example, if one instantiation outputs a[1:0], and the second instantiation outputs a[3:2], then AUTOWIRE needs to declare wire a[3:0].

This error message means that it cannot determine how to declare that vector. Usually this is because you used parameters or something complicated in the instantiations. You'll need to declare that wire yourself.

How do I use AUTO_TEMPLATE to tie off inputs to zero?

Use a LISP format template, and the lisp variable vl-width, which contains the width of the port.

/* InstModule AUTO_TEMPLATE (
    .\(.*\)_test ({@"vl-width"{1'b0}}),
);
*/

How do I use AUTO_TEMPLATE to lower case all signals?

Use a lisp expression, and the lisp function "downcase".

/* InstModule AUTO_TEMPLATE (
   .IN (@"(downcase vl-name)"[]),
*/

If you're trying the reverse, namely to upcase your signal names, did you consider lower case is more readable by 15% or so than all upper case?

How do I use AUTO_TEMPLATE to include the instantiation name for pin?

Yet another lisp expression:

/* InstModule AUTO_TEMPLATE (
     .a(@"vl-cell-name"_in[]),
     .b(@"vl-cell-name"_out[]),
     );*/

InstModule u_a0 (/*AUTOINST*/
     // Inouts
     .a    (u_a0_in[bitsa:0]),     // Templated
     .b    (u_a0_out[bitsb:0]));     // Templated
InstModule u_a1 (/*AUTOINST*/
     // Inouts
     .a    (u_a1_in[bitsa:0]),     // Templated
     .b    (u_a1_out[bitsb:0]));     // Templated

Oh, but what if I didn't want the u_?

/* InstModule AUTO_TEMPLATE (
     .a(@"(substring vl-cell-name 2)"_in[]),
     .b(@"(substring vl-cell-name 2)"_out[])
     );*/

InstModule u_a0 (/*AUTOINST*/
   // Inouts
   .a    (a0_in[bitsa:0]),     // Templated
   .b    (a0_out[bitsb:0]));     // Templated

Substring is very useful in templates. All of your cell names need to be the same length however. Often you can simply pad the names by adding zeros, for example use u_00 ... u_15, rather than u_0 ... u_15.

How do I have AUTO_TEMPLATE use the second number in a instance name?

The standard @ sign in a template by default returns the first number in a instance name, so if you want a earlier number, you have three main choices.

If you only need the second digit, you can define the @ sign to come from the second digits in the module:

/* InstModule AUTO_TEMPLATE ".*[0-9]+.*\([0-9]\)" (
                             .a (in_@),
*/

Next easiest is to use @"(substring vl-cell-name ...) to extract the relevant digits. See the examples above.

The most flexible is to define your own function to do the relevant extraction, then call it. For example:

/* AUTO_LISP(defun getparam2 (strg)
    (string-match "[^0-9]*[0-9]+[^0-9]*\\([0-9]+\\)" strg)
    (match-string 1 strg)) */

/* InstModule AUTO_TEMPLATE (
    .in (@"(getparam2 vl-cell-name)"),
    );
    */

How do I use AUTO_TEMPLATE to connect bytes to instances?

This is for when you want the first instance to get a[7:0], the second a[15:8], and so on.

Use a lisp template and a little math.

/* InstModule AUTO_TEMPLATE (
     .a(@in[@"(+ (* 8 @) 7)":@"(* 8 @)"]),
     );*/

InstModule u_a0 (/*AUTOINST*/
     .a    (in[7:0]));     // Templated
InstModule u_a1 (/*AUTOINST*/
     .a    (in[15:8]));     // Templated
InstModule u_a2 (/*AUTOINST*/
     .a    (in[23:16]));     // Templated
InstModule u_a3 (/*AUTOINST*/
     .a    (in[31:24]));     // Templated

How do I use AUTOINST with Synplify syn_prune attributes?

Synplify documentation suggests placing attributes just before the final semicolon of instance names. Instead place the comment before the list of ports, which works just as well, and has the additional advantage of being close to the instantiated module name (instead of potentially pages lower if there's many pins.) Synplify has been notified of this issue, and is likely to change their documentation.

InstModule u_a0 /*synthesis syn_noprune=1*/
  (/*AUTOINST*/
     .a    (a));

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