verilator/internals.pod

# DESCRIPTION: DOCUMENT source run through perl to produce internals.txt file
# Use 'make internals.txt' to produce the output file

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=head1 NAME

Verilator Internals

=head1 INTRODUCTION

This file discusses internal and programming details for Verilator.  It's
the first for reference for developers and debugging problems.

See also the Verilator internals presentation at http://www.veripool.org.

=head1 ADDING A NEW FEATURE

Generally what would you do to add a new feature?

=over 4

File a bug (if there isn't already) so others know what you're working on.

Make a testcase in the test_regress/t/t_EXAMPLE format, see TESTING Below.

If grammar changes are needed, look at the git version of VerilogPerl's
src/VParseGrammar.y, as this grammar supports the full SystemVerilog
language and has a lot of back-and-forth with Verilator's grammar.  Copy
the appropriate rules to src/verilog.y and modify the productions.

If a new Ast type is needed, add it to V3AstNodes.h.

Now you can run "test_regress/t/t_{new testcase}.pl --debug" and it'll
probably fail but you'll see a test_regress/obj_dir/t_{newtestcase}/*.tree
file which you can examine to see if the parsing worked. See also the
sections below on debugging.

Modify the later visitor functions to process the new feature as needed.

=back

=head1 DEBUG OUTPUT/ TREE FILES

When you run with --debug there are two primary output file types placed into
the obj_dir, .tree and .dot files.

=head2 .dot output

Dot files are dumps of internal graphs in Graphviz
L<http://www.graphviz.org/> dot format.  When a dot file is dumped,
Verilator will also print a line on stdout that can be used to format the
output, for example:

    dot -Tps -o ~/a.ps obj_dir/Vtop_foo.dot

You can then print a.ps.  You may prefer gif format, which doesn't get
scaled so can be more useful with large graphs.

For dynamic graph viewing consider ZGRViewer
L<http://zvtm.sourceforge.net/zgrviewer.html>.  If you know of better
viewers let us know; ZGRViewer isn't great for large graphs.

=head2 .tree output

Tree files are dumps of the AST Tree and are produced between every major
algorithmic stage.  An example:

     NETLIST 0x90fb00 <e1> {0} w0
    1: MODULE 0x912b20 <e8822> {8} w0  top  L2 [P]
   *1:2: VAR 0x91a780 <e74#> {22} w70  out_wide [O] WIRE
    1:2:1: BASICDTYPE 0x91a3c0 <e73> {22} w70  [logic]

=over 4

"1:2:" indicates the hiearchy the VAR is op2p under the MODULE.

"VAR" is the AstNodeType.

"0x91a780" is the address of this node.

"<e74>" means the 74th edit to the netlist was the last modification to
this node.  A trailing # indicates this node changed since the last tree
dump was made.  You can gdb break on this edit; see below.

"{22}" indicates this node is related to line 22 in the source.

"w70" indicates the width is 70 bits.  sw70 would be signed 70 bits.

"out_wide" is the name of the node, in this case the name of the variable.

"[O]" are flags which vary with the type of node, in this case it means the
variable is an output.

=back

=head1 TESTING

To write a test see notes in the forum and in the verilator.txt manual.

Note you can run the regression tests in parallel; see the
test_regress/driver.pl script -j flag.

=head1 VISITOR FUNCTIONS

=head2 Passing Variables

There's three ways data is passed between visitor functions.

1. A visitor-class member variable.  This is generally for passing "parent"
information down to children.  m_modp is a common example.  It's set to
NULL in the constructor, where that node (AstModule visitor) sets it, then
the children are iterated, then it's cleared.  Children under an AstModule
will see it set, while nodes elsewhere will see it clear.  If there can be
nested items (for example an AstFor under an AstFor) the variable needs to
be save-set-restored in the AstFor visitor, otherwise exiting the lower for
will loose the upper for's setting.

2. User() attributes.  Each node has 4 ->user() number or ->userp() pointer
utility values (a common technique lifted from graph traversal packages).
A visitor first clears the one it wants to use by calling
AstNode::user#ClearTree(), then it can mark any node's user() with whatever
data it wants.  Readers just call nodep->user(), but may need to cast
appropriately, so you'll often see nodep->userp()->castSOMETYPE().  At the
top of each visitor are comments describing how the user() stuff applies to
that visitor class.  For example:

    // NODE STATE
    // Cleared entire netlist
    //   AstModule::user1p()     // bool. True to inline this module

This says that at the AstNetlist user1ClearTree() is called.  Each
AstModule's is user1() is used to indicate if we're going to inline it.

These comments are important to make sure a user#() on a given AstNode type
is never being used for two different purposes.

Note that calling user#ClearTree is fast, it doesn't walk the tree, so it's
ok to call fairly often.  For example, it's commonly called on every
module.

3. Parameters can be passed between the visitors in close to the "normal"
function caller to callee way.  This is the second "vup" parameter that is
ignored on most of the visitor functions.  V3Width does this, but it proved
more messy than the above and is deprecated.  (V3Width was nearly the first
module written.  Someday this scheme may be removed, as it slows the
program down to have to pass vup everywhere.)

=head1 DEBUGGING WITH GDB

The test_regress/driver.pl script accepts --debug --gdb to start Verilator
under gdb.  You can also use --debug --gdbbt to just backtrace and then
exit gdb.

To break at a specific edit number which changed a node (presumably to find
what made a <e####> line in the tree dumps):

   watch AstNode::s_editCntGbl==####

=head1 DISTRIBUTION

The latest version is available from L<http://www.veripool.org/>.

Copyright 2008-2009 by Wilson Snyder.  Verilator is free software; you can
redistribute it and/or modify it under the terms of either the GNU Lesser
General Public License Version 3 or the Perl Artistic License Version 2.0.

=cut

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