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The ICARUS Verilog Compilation System

Copyright 2000-2019 Stephen Williams

<details> <summary><h2>Table of Contents</h2></summary>
  1. What is ICARUS Verilog?
  2. Building/Installing Icarus Verilog From Source
  3. How Icarus Verilog Works
  4. Running iverilog
  5. Unsupported Constructs
  6. Nonstandard Constructs or Behaviors
  7. Credits
</details>

What is ICARUS Verilog?

Icarus Verilog is intended to compile ALL of the Verilog HDL, as described in the IEEE-1364 standard. Of course, it's not quite there yet. It does currently handle a mix of structural and behavioural constructs. For a view of the current state of Icarus Verilog, see its home page at https://steveicarus.github.io/iverilog/.

Icarus Verilog is not aimed at being a simulator in the traditional sense, but a compiler that generates code employed by back-end tools.

For instructions on how to run Icarus Verilog, see the iverilog man page.

Building/Installing Icarus Verilog from Source

If you are starting from the source, the build process is designed to be as simple as practical. Someone basically familiar with the target system and C/C++ compilation should be able to build the source distribution with little effort. Some actual programming skills are not required, but helpful in case of problems.

If you are building on Windows, see the mingw.txt file.

Compile Time Prerequisites

You can use:

apt install -y autoconf gperf make gcc g++ bison flex

You need the following software to compile Icarus Verilog from source on a UNIX-like system:

Compilation

<details> <summary><h4><a href="https://github.com/steveicarus/iverilog/releases">Compiling From Release</a></h4></summary>

Unpack the tar-ball, cd into the verilog-######### directory, and compile the source with the commands:

  ./configure
  make
</details> <details> <summary><h4>Compiling From GitHub</h4></summary>

If you are building from git, you have to run the command below before compiling the source. This will generate the "configure" file, which is automatically done when building from tarball.

  sh autoconf.sh
  ./configure
  make

Normally, this command automatically figures out everything it needs to know. It generally works pretty well. There are a few flags to the configure script that modify its behaviour:

	--prefix=<root>
		The default is /usr/local, which causes the tool suite to
		be compiled for install in /usr/local/bin,
		/usr/local/share/ivl, etc.

		I recommend that if you are configuring for precompiled
		binaries, use --prefix=/usr.  On Solaris systems, it is
		common to use --prefix=/opt.  You can configure for a non-root
		install with --prefix=$HOME.

	--enable-suffix
	--enable-suffix=<your-suffix>
	--disable-suffix
		Enable/disable changing the names of install files to use
		a suffix string so that this version or install can co-
		exist with other versions. This renames the installed
		commands (iverilog, iverilog-vpi, vvp) and the installed
		library files and include directory so that installations
		with the same prefix but different suffix are guaranteed
		to not interfere with each other.

	--host=<host-type>
		Compile iverilog for a different platform. You can use:
				x64_64-w64-mingw32 for building 64-bit Windows executables
				i686-w64-mingw32 for building 32-bit Windows executables
			Both options require installing the required mingw-w64 packages.
</details>

(Optional) Testing

To run a simple test before installation, execute

  make check

The commands printed by this run might help you in running Icarus Verilog on your own Verilog sources before the package is installed by root.

Installation

Now install the files in an appropriate place. (The makefiles by default install in /usr/local unless you specify a different prefix with the --prefix=<path> flag to the configure command.) You may need to do this as root to gain access to installation directories.

	make install

Uninstallation

The generated Makefiles also include the uninstall target. This should remove all the files that make install creates.

How Icarus Verilog Works

This tool includes a parser which reads in Verilog (plus extensions) and generates an internal netlist. The netlist is passed to various processing steps that transform the design to more optimal/practical forms, then is passed to a code generator for final output. The processing steps and the code generator are selected by command line switches.

Preprocessing

There is a separate program, ivlpp, that does the preprocessing. This program implements the `include and `define directives producing output that is equivalent but without the directives. The output is a single file with line number directives, so that the actual compiler only sees a single input file. See ivlpp/ivlpp.txt for details.

Parse

The Verilog compiler starts by parsing the Verilog source file. The output of the parse is a list of Module objects in "pform". The pform (see pform.h) is mostly a direct reflection of the compilation step. There may be dangling references, and it is not yet clear which module is the root.

One can see a human-readable version of the final pform by using the -P <path> flag to the ivl subcommand. This will cause ivl to dump the pform into the file named <path>. (Note that this is not normally done, unless debugging the ivl subcommand.)

Elaboration

This phase takes the pform and generates a netlist. The driver selects (by user request or lucky guess) the root module to elaborate, resolves references and expands the instantiations to form the design netlist. (See netlist.txt.) Final semantic checks are performed during elaboration, and some simple optimizations are performed. The netlist includes all the behavioural descriptions, as well as gates and wires.

The elaborate() function performs the elaboration.

One can see a human-readable version of the final, elaborated and optimized netlist by using the -N <path> flag to the compiler. If elaboration succeeds, the final netlist (i.e., after optimizations but before code generation) will be dumped into the file named <path>.

Elaboration is performed in two steps: scopes and parameters first, followed by the structural and behavioural elaboration.

Scope Elaboration

This pass scans through the pform looking for scopes and parameters. A tree of NetScope objects is built up and placed in the Design object, with the root module represented by the root NetScope object. The elab_scope.cc file contains most of the code for handling this phase.

The tail of the elaborate_scope behaviour (after the pform is traversed) includes a scan of the NetScope tree to locate defparam assignments that were collected during scope elaboration. This is when the defparam overrides are applied to the parameters.

Netlist Elaboration

After the scopes and parameters are generated and the NetScope tree fully formed, the elaboration runs through the pform again, this time generating the structural and behavioural netlist. Parameters are elaborated and evaluated by now so all the constants of code generation are now known locally, so the netlist can be generated by simply passing through the pform.

Optimization

This is a collection of processing steps that perform optimizations that do not depend on the target technology. Examples of some useful transformations are

The actual functions performed are specified on the ivl command line by the -F flags (see below).

Code Generation

This step takes the design netlist and uses it to drive the code generator (see target.h). This may require transforming the design to suit the technology.

The emit() method of the Design class performs this step. It runs through the design elements, calling target functions as the need arises to generate actual output.

The user selects the target code generator with the -t flag on the command line.

Attributes

NOTE: The $attribute syntax will soon be deprecated in favour of the Verilog-2001 attribute syntax, which is cleaner and standardized.

The parser accepts, as an extension to Verilog, the $attribute module item. The syntax of the $attribute item is:

	$attribute (<identifier>, <key>, <value>);

The $attribute keyword looks like a system task invocation. The difference here is that the parameters are more restricted than those of a system task. The <identifier> must be an identifier. This will be the item to get an attribute. The <key> and <value> are strings, not expressions, that give the key and the value of the attribute to be attached to the identified object.

Attributes are [<key> <value>] pairs and are used to communicate with the various processing steps. See the documentation for the processing step for a list of the pertinent attributes.

Attributes can also be applied to gate types. When this is done, the attribute is given to every instantiation of the primitive. The syntax for the attribute statement is the same, except that the <identifier> names a primitive earlier in the compilation unit and the statement is placed in the global scope, instead of within a module. The semicolon is not part of a type attribute.

Note that attributes are also occasionally used for communication between processing steps. Processing steps that are aware of others may place attributes on netlist objects to communicate information to later steps.

Icarus Verilog also accepts the Verilog 2001 syntax for attributes. They have the same general meaning as with the $attribute syntax, but they are attached to objects by position instead of by name. Also, the key is a Verilog identifier instead of a string.

Running iverilog

The preferred way to invoke the compiler is with the iverilog(1) command. This program invokes the preprocessor (ivlpp) and the compiler (ivl) with the proper command line options to get the job done in a friendly way. See the iverilog(1) man page for usage details.

EXAMPLE: Hello World

Example: Compiling "hello.vl"

// ------------------------ hello.vl ----------------------------

module main();

initial
  begin
    $display("Hello World");
    $finish ;
  end

endmodule

// --------------------------------------------------------------

Ensure that iverilog is on your search path, and the vpi library is available.

To compile the program:

	iverilog hello.vl

(The above presumes that /usr/local/include and /usr/local/lib are part of the compiler search path, which is usually the case for gcc.)

To run the generated program:

	./a.out

You can use the -o switch to name the output command to be generated by the compiler. See the iverilog(1) man page.

Unsupported Constructs

Icarus Verilog is in development - as such it still only supports a (growing) subset of Verilog. Below is a description of some of the currently unsupported Verilog features. This list is not exhaustive and does not account for errors in the compiler. See the Icarus Verilog web page for the current state of support for Verilog, and in particular, browse the bug report database for reported unsupported constructs.

Nonstandard Constructs or Behaviors

Icarus Verilog includes some features that are not part of the IEEE1364 standard, but have well-defined meaning, and also sometimes gives nonstandard (but extended) meanings to some features of the language that are defined. See the "extensions.txt" documentation for more details.

Builtin system functions

Certain of the system functions have well-defined meanings, so can theoretically be evaluated at compile-time, instead of using runtime VPI code. Doing so means that VPI cannot override the definitions of functions handled in this manner. On the other hand, this makes them synthesizable, and also allows for more aggressive constant propagation. The functions handled in this manner are:

Implementations of these system functions in VPI modules will be ignored.

Preprocessing Library Modules

Icarus Verilog does preprocess modules that are loaded from libraries via the -y mechanism. However, the only macros defined during the compilation of that file are those that it defines itself (or includes) or that are defined in the command line or command file.

Specifically, macros defined in the non-library source files are not remembered when the library module is loaded. This is intentional. If it were otherwise, then compilation results might vary depending on the order that libraries are loaded, and that is too unpredictable.

It is said that some commercial compilers do allow macro definitions to span library modules. That's just plain weird.

Width in %t Time Formats

Standard Verilog does not allow width fields in the %t formats of display strings. For example, this is illegal:

	$display("Time is %0t", $time);

Standard Verilog instead relies on the $timeformat to completely specify the format.

Icarus Verilog allows the programmer to specify the field width. The %t format in Icarus Verilog works exactly as it does in standard Verilog. However, if the programmer chooses to specify a minimum width (i.e., %5t), then for that display Icarus Verilog will override the $timeformat minimum width and use the explicit minimum width.

vpiScope Iterator on vpiScope Objects

In the VPI, the normal way to iterate over vpiScope objects contained within a vpiScope object, is the vpiInternalScope iterator. Icarus Verilog adds support for the vpiScope iterator of a vpiScope object, that iterates over everything the is contained in the current scope. This is useful in cases where one wants to iterate over all the objects in a scope without iterating over all the contained types explicitly.

Time 0 Race Resolution

Combinational logic is routinely modelled using always blocks. However, this can lead to race conditions if the inputs to the combinational block are initialized in initial statements. Icarus Verilog slightly modifies time 0 scheduling by arranging for always statements with ANYEDGE sensitivity lists to be scheduled before any other threads. This causes combinational always blocks to be triggered when the values in the sensitivity list are initialized by initial threads.

Nets with Types

Icarus Verilog supports an extended syntax that allows nets and regs to be explicitly typed. The currently supported types are logic, bool and real. This implies that logic and bool are new keywords. Typical syntax is:

	wire real foo = 1.0;
	reg logic bar, bat;

... and so forth. The syntax can be turned off by using the -g2 flag to iverilog, and turned on explicitly with the -g2x flag to iverilog.

Credits

Except where otherwise noted, Icarus Verilog, ivl, and ivlpp are Copyright Stephen Williams. The proper notices are in the head of each file. However, I have early on received aid in the form of fixes, Verilog guidance, and especially testing from many people. Testers, in particular, include a larger community of people interested in a GPL Verilog for Linux.