add link to wikipedia test-driven development

[libreriscv.git] / HDL_workflow.mdwn

# HDL workflow

This section describes the workflow and some best practices for developing
the Libre-SOC hardware. We use nmigen, yosys and symbiyosys, and this
page is intended not just to help you get set up, it is intended to
help advise you of some tricks and practices that will help you become
effective team contributors.

It is particularly important to bear in mind that we are not just
"developing code", here: we are creating a "lasting legacy educational
resource" for other people to learn from, and for businesses and students
alike to be able to use, learn from and augment for their own purposes.

It is also important to appreciate and respect that we are funded under
NLNet's Privacy and Enhanced Trust Programme <http://nlnet.nl/PET>. Full
transparency, readability, documentation, effective team communication
and formal mathematical proofs for all code at all levels is therefore
paramount.

Therefore, we need not only to be "self-sufficient" (absolutely under no circumstances critically reliant on somebody else's servers **or protocols**) we also need to ensure that everything (including all communication such as the mailing list archives) are recorded, replicable, and accessible in perpetuity.  Use of slack or a "forum" either actively prevents or makes that much harder.

# Collaboration resources

The main message here: **use the right tool for the right job**.

* mailing list: general communication and discussion.
* bugtracker: task-orientated, goal-orientated *focussed* discussion.
* ikiwiki: document store, information store, and (editable) main website
* git repositories: code stores (**not binary or auto-generated output store**)
* ftp server (<http://ftp.libre-riscv.org>): large file store.

we will add an IRC channel at some point when there are enough people
to warrant having one (and it will be publicly archived)

note also the lack of a "forum" in the above list.  this is very deliberate.  forums are a serious distraction when it comes to technical heavily goal-orientated development. recent internet users may enjoy looking up the "AOL metoo postings" meme.

note also the complete lack of "social platforms". if we wanted to tell everybody how much better each of us are than anyone else in the team, how many times we made a commit (look at me, look at me, i'm so clever), and how many times we went to the bathroom, we would have installed a social media based project "management" system.

## Main contact method: mailing list

To respect the transparency requirements, conversations need to be
public and archived (i.e not skype, not telegram, not discord,
and anyone seriously suggesting slack will be thrown to the
lions).  Therefore we have a mailing list. Everything goes through
there. <http://lists.libre-riscv.org/mailman/listinfo/libre-riscv-dev>
therefore please do google "mailing list etiquette" and at the very
minimum look up and understand the following:

* This is a technical mailing list with complex topics. Top posting
  is completely inappropriate. Don't do it unless you have mitigating
  circumstances, and even then please apologise and explain ("hello sorry
  using phone at airport flight soon, v. quick reply: ....")
* Always trim context but do not cut excessively to the point where people
  cannot follow the discussion.  Especially do not cut the attribution
  ("On monday xxx wrote") of something that you are actually replying
  to.
* Use inline replies i.e. reply at the point in the relevant part of
  the conversation, as if you were actually having a conversation.
* Follow standard IETF reply formatting, using ">" for cascaded
  indentation of other people's replies.  If using gmail, please: SWITCH
  OFF RICH TEXT EDITING.
* Please for god's sake do not use "my replies are in a different
  colour". Only old and highly regarded people still using AOL are allowed
  to get away with that (such as Mitch).
* Start a new topic with a relevant subject line. If an existing
  discussion changes direction, change the subject line to reflect the
  new topic (or start a new conversation entirely, without using the
  "reply" button)
* DMARC is a pain on the neck. Try to avoid GPG signed messages. sigh.
* Don't send massive attachments. Put them online (no, not on facebook or
  google drive or anywhere else that demands privacy violations) and provide
  the link.  Which should not require any kind of login to access. ask the
  listadmin if you don't have anywhere suitable: FTP access can be arranged.

### Actionable items from mailing list

If discussions result in any actionable items, it is important not to
lose track of them. Create a bugreport, find the discussion in the
archives <http://lists.libre-riscv.org/pipermail/libre-riscv-dev/>,
and put the link actually in the bugtracker as one of the comments.

At some point in any discussion, the sudden realisation may dawn on one
or more people that this is an "actionable" discussion.  at that point
it may become better to use  <http://bugs.libre-riscv.org>
itself to continue the discussion rather than to keep on dropping copies
of links into the bugtracker.  The bugtracker sends copies of comments
*to* the list however this is 'one-way' (note from lkcl: because this
involves running an automated perl script from email, on every email,
on the server, that is a high security risk, and i'm not doing it. sorry.)

### Mailing list != editable document store

Also, please do not use the mailing list as an "information or document
store or poor-man's editor". We have the wiki for that.  Edit a page and
tell people what you did (summarise rather than drop the entire contents
at the list) and include the link to the page.

Or, if it is more appropriate, commit a document (or source code)
into the relevant git repository then look up the link in the gitweb
source tree browser and post that (in the bugtracker or mailing list)
See <http://git.libre-riscv.org>

### gmail "spam"ifying the list

see <https://blog.kittycooper.com/2014/05/keeping-my-mailing-list-emails-out-of-gmails-spam-folder/>

basically it is possible to select any message from the list, create a "filter" (under "More"),
and, on the 2nd dialog box, click the "never send this to Spam" option.

## Bugtracker

bugzilla. old and highly effective. sign up in the usual way. any
problems, ask on the list.

Please do not ask for the project to be transferred to github or other
proprietary nonfree service "because it's soooo convenient", as the
lions are getting wind and gout from overfeeding on that one.

## ikiwiki

Runs the main libre-riscv.org site (including this page). effective,
stunningly light on resources, and uses a git repository not a database.
That means it can be edited offline.

Usual deal: register an account and you can start editing and contributing
straight away.

Hint: to create a new page, find a suitable page that would link to it,
first, then put the link in of the page you want to create, as if the
page already exists.  Save that page, and you will find a questionmark
next to the new link you created.  click that link, and it will fire up a
"create new page" editor.

Hint again: the wiki is backed by a git repository.  Don't go overboard
but at the same time do not be afraid that you might "damage" or "lose"
pages.  Although it would be a minor pain, the pages can always be
reverted or edited by the sysadmins to restore things if you get in a tiz.

Assistance in creating a much better theme greatly appreciated.  e.g.
<http://www.math.cmu.edu/~gautam/sj/blog/20140720-ikiwiki-navbar.html>

## git

we use git. more on this below.  we also use gitolite3 running on a
dedicated server.  again, it is extremely effective and low resource
utilisation.  reminder: lions are involved if github is mentioned.

gitweb is provided which does a decent job. <http://git.libre-riscv.org>

## ftp server

<http://ftp.libre-riscv.org> is available for storing large files
that do not belong in a git repository, if we have (or ever need)
any.  images (etc.) if small and appropriate should go into the
wiki, however .tgz archives (etc.) and, at some point, binaries,
should be on the ftp server.

ask on the list if you have a file that belongs on the ftp server.

## server

as an aside: all this is "old school" and run on a single core 512MB
VM with only a 20GB HDD allocation. it costs only 8 GBP per month from
mythic-beasts and means that the project is in no way dependent on anyone
else - not microsoft, not google, not facebook, not amazon.

we tried gitlab. it didn't go well. please don't ask to replace the
above extremely resource-efficient services with it.

# Hardware

RAM is the biggest requirement. Minimum 16GB, the more the better (32
or 64GB starts to reach "acceptable" levels.  Disk space is not hugely
critical: 256GB SSD should be more than adequate.  Simulations and
FPGA compilations however are where raw processing power is a must.
High end Graphics Cards are nonessential.

What is particularly useful is to have hi-res screens (curved is
*strongly* recommended if the LCD is over 24in wide, to avoid eyeballs
going "prism" through longterm use), and to have several of them: the
more the better.  Either a DisplayLink UD160A (or more modern variant)
or simply using a second machine (lower spec hardware because it will
run editors) is really effective.

Also it is really recommended to have a UHD monitor (4k - 3840x2160),
or at least 2560x1200.  If given a choice, 4:3 aspect ratio is better
than 16:9 particularly when using several of them. However, caveat
(details below): please when editing do not assume that everyone will
have access to such high resolution screens.

# Operating System

First install and become familiar with Debian (ubuntu if you absolutely
must) for standardisation cross-team and so that toolchain installation
is greatly simplified.  yosys in particular warns that trying to use
Windows, BSD or MacOS will get you into a world of pain.

Only a basic GUI desktop is necessary: fvwm2, xfce4, lxde are perfectly
sufficient (alongside wicd-gtk for network management). Other more
complex desktops can be used however may consume greater resources.

# editors and editing

Whilst this is often a personal choice, the fact that many editors are
GUI based and run fullscreen with the entire right hand side *and* middle
*and* the majority of the left side of the hi-res screen entirely unused
and bereft of text leaves experienced developers both amused and puzzled.

At the point where such fullscreen users commit code with line lengths
well over 160 characters, that amusement quickly evaporates.

Where the problems occur with fullscreen editor usage is when a project
is split into dozens if not hundreds of small files (as this one is). At
that point it becomes pretty much essential to have as many as six to
eight files open *and on-screen* at once, without overlaps i.e. not in
hidden tabs, next to at least two if not three additional free and clear
terminals into which commands are regularly and routinely  typed (make,
git commit, nosetests3 etc).  Illustrated with the following 3840x2160
screenshot (click to view full image), where *every one* of those 80x70
xterm windows is *relevant to the task at hand*.

[[!img 2020-01-24_11-56.png size=640x ]]

(hint/tip: fvwm2 set up with "mouse-over to raise focus, rather than
additionally requiring a mouseclick, can save a huge amount of cumulative
development time here, switching between editor terminal(s) and the
command terminals).

Once this becomes necessary, it it turn implies that having greater
than 80 chars per line - and running editors fullscreen - is a severe
hindance to an essential *and highly effective* workflow technique.

Additionally, care should be taken to respect that not everyone will have
200+ column editor windows and the eyesight of a hawk. They may only have
a 1280 x 800 laptop which barely fits two 80x53 xterms side by side.
Consequently, having excessively long functions is also a hindrance to
others, as such developers with limited screen resources would need to
continuously page-up and page-down to read the code even of a single
function, in full.

This helps explain in part, below, why compliance with pep8 is enforced,
including its 80 character limit.  In short: not everyone has the same
"modern" GUI workflow or has access to the same computing resources as
you, so please do respect that.

More on this concept is
[here](https://www.linuxjournal.com/content/line-length-limits).
Note *very pointedly* that Linus Torvalds *specifically* states that
he does not want Linux kernel development to become the exclusive
domain of the "wealthy".  That means **no** to assumptions about
access to ultra-high resolution screens.

# Software prerequisites

Whilst many resources online advocate "sudo" in front of all root-level
commands below, this quickly becomes tiresome. run "sudo bash", get a
root prompt, and save yourself some typing.

* sudo bash
* apt-get install vim exuberant-ctags
* apt-get install build-essential
* apt-get install git python3.7 python3.7-dev python-nosetest3
* apt-get install graphviz xdot gtkwave
* apt-get install python3-venv
* apt-get install python-virtualenv  # this is an alternative to python3-venv
* return to user prompt (ctrl-d)

This will get you python3 and other tools that are needed. graphviz is
essential for showing the interconnections between cells, and gtkwave
is essential for debugging.

If you would like to save yourself a lot more typing, check out the
[dev-env-setup](https://git.libre-soc.org/?p=dev-env-setup.git;a=summary)
repository, examine the scripts there and use them to automate much of
the process below.

## git

Look up good tutorials on how to use git effectively.  There are so many
it is hard to recommend one. This is however essential. If you are not
comfortable with git, and you let things stay that way, it will seriously
impede development progress.

If working all day you should expect to be making at least two commits per
hour, so should become familiar with it very quickly.  If you are *not*
doing around 2 commits per hour, something is wrong and you should read
the workflow instructions below more carefully, and also ask for advice
on the mailing list.

Worth noting: *this project does not use branches*. All code is committed
to master and we *require* that it be either zero-impact additions or that
relevant unit tests pass 100%.  This ensures that people's work does not
get "lost" or isolated and out of touch due to major branch diversion,
and that people communicate and coordinate with each other.

## yosys

Follow the source code (git clone) instructions here:
<http://www.clifford.at/yosys/download.html>

Or, alternatively, use the
[yosys-et-al](https://git.libre-soc.org/?p=dev-env-setup.git;a=blob;f=yosys-et-al;hb=HEAD)
script (which also installs symbiyosys and its dependencies)

Do not try to use a fixed revision of yosys (currently 0.9), nmigen is evolving
and frequently interacts with yosys.

## symbiyosys

Follow the instructions here:
<https://symbiyosys.readthedocs.io/en/latest/quickstart.html#installing>

You do not have to install all of those (avy, boolector can be left
out if desired) however the more that are installed the more effective
the formal proof scripts will be (less resource utilisation in certain
circumstances).

## nmigen

nmigen may be installed as follows:

* mkdir ~/src
* cd !$
* git clone https://github.com/nmigen/nmigen.git
* cd nmigen
* sudo bash
* python3 setup.py develop
* ctrl-d

testing can then be carried out with "python3 setup.py test"

## Softfloat and sfpy

These are a test suite dependency for the ieee754fpu library, and
will be changed in the future to use Jacob's [simple-soft-float](https://crates.io/crates/simple-soft-float)
library.  In the meantime, sfpy can be built as follows:

    git clone --recursive https://github.com/billzorn/sfpy.git
    cd sfpy
    cd SoftPosit
    git apply ../softposit_sfpy_build.patch
    git apply /path/to/ieee754fpu/SoftPosit.patch
    cd ../berkely-softfloat-3
    # Note: Do not apply the patch included in sfpy for berkely-softfloat, 
    # it contains the same changes as this one
    git apply /path/to/ieee754fpu/berkeley-softfloat.patch
    cd ..

    # prepare a virtual environment for building
    python3 -m venv .env

    # or, if you prefer the old way:
    #    virtualenv -p python3 .env

    # install dependencies
    source .env/bin/activate
    pip3 install --upgrade -r requirements.txt

    # build
    make lib -j8
    make cython
    make inplace -j8
    make wheel

    # install
    deactivate # deactivates venv, optional 
    pip3 install dist/sfpy*.whl

You can test your installation by doing the following:

    python3
    >>> from sfpy import *
    >>> Posit8(1.3)

It should print out `Posit8(1.3125)`

## qemu, cross-compilers, gdb

As we are doing POWER ISA, POWER ISA compilers, toolchains and
emulators are required.

Install powerpc64 gcc:

    apt-get install gcc-9-powerpc64-linux-gnu

Install qemu:

    apt-get install qemu-system-ppc

Install gdb from source.  Obtain the latest tarball, unpack it, then:

    cd gdb-9.1 (or other location)
    mkdir build
    cd build
     ../configure --srcdir=.. --host=x86_64-linux --target=powerpc64-linux-gnu
    make -j16
    make install

## Coriolis2

See [[coriolis2]] page, for those people doing layout work.

# Registering for git repository access

After going through the onboarding process and having agreed to take
responsibility for certain tasks, ask on the mailing list for git
repository access, sending in a public key (id_rsa.pub). If you do
not have one then generate it with ssh-keygen -t rsa. You will find it
in ~/.ssh

NEVER SEND ANYONE THE PRIVATE KEY.  By contrast the public key, on
account of being public, is perfectly fine to make... err... public.

Create a file ~/.ssh/config with the following lines:

    Host git.libre-riscv.org
    Port 922

Wait for the Project Admin to confirm that the ssh key has been added
to the required repositories.  Once confirmed, you can clone any of the
repos at http://git.libre-riscv.org:

    git clone gitolite3@git.libre-riscv.org:REPONAME.git

Alternatively, the .ssh/config can be skipped and this used:

     git clone ssh://gitolite3@git.libre-riscv.org:922/REPONAME.git

# git configuration

Although there are methods online which describe how (and why) these
settings are normally done, honestly it is simpler and easier to open
~/.gitconfig and add them by hand.

core.autocrlf is a good idea to ensure that anyone adding DOS-formatted
files they don't become a pain.  pull.rebase is something that is greatly
preferred for this project because it avoids the mess of "multiple
extra merge git tree entries", and branch.autosetuprebase=always will,
if you want it, always ensure that a new git checkout is set up with rebase.

    [core]
        autocrlf = input
    [push]
        default = simple
    [pull]
        rebase = true
    [branch]
        autosetuprebase = always

# Checking out the HDL repositories

* mkdir ~/src
* cd !$
* git clone gitolite3@git.libre-riscv.org:nmutil.git
* git clone gitolite3@git.libre-riscv.org:ieee754fpu.git
* git clone gitolite3@git.libre-riscv.org:soc.git

In each of these directories, in the order listed, track down the
setup.py file, then, as root (sudo bash), run the following:

* python3 setup.py develop

The reason for using "develop" mode is that the code may be edited
in-place yet still imported "globally".  There are variants on this theme
for multi-user machine use however it is often just easier to get your
own machine these days.

The reason for the order is because soc depends on ieee754fpu, and
ieee754fpu depends on nmutil

If "python3 setup.py install" is used it is a pain: edit, then
install. edit, then install. It gets extremely tedious, hence why
"develop" was created.

# Development Rules

team communication:

* new members, add yourself to the [[about_us]] page and create yourself a home page using someone else's page as a template.
* communicate on the mailing list or the bugtracker an intent to take
  responsibility for a particular task.
* assign yourself as the bug's owner
* *keep in touch* about what you are doing, and why you are doing it.
* edit your home page regularly, particularly to track tasks so that they can be paid by NLNet.
* if you cannot do something that you have taken responsibility for,
  then unless it is a dire personal emergency please say so, on-list. we
  won't mind. we'll help sort it out.

regarding the above it is important that you read, understand, and agree
to the [[charter]] because the charter is about ensuring that we operate
as an effective organisation.  It's *not* about "setting rules and meting
out punishment".

## Coding

for actual code development

### Plan unit tests

* plan in advance to write not just code but a full test suite for
  that code.  **this is not optional**. large python projects that do not
  have unit tests **FAIL** (see separate section below).
* Prioritise writing formal proofs and a single clear unit test that is more like a "worked example".
  We receive NLNet funds for writing formal proofs, plus they
cover corner cases and take far less time to write

### Commit tested or zero-dependent code

* only commit code that has been tested (or is presently unused). other
  people will be depending on you, so do take care not to screw up.
  not least because, as it says in the [[charter]] it will be your
  responsibility to fix.  that said, do not feel intimidated: ask for help
  and advice, and you'll get it straight away.

### Commit often

* commit often. several times a day, and "git push" it.  this is
  collaboration. if something is left even overnight uncommitted and not
  pushed so that other people can see it, it is a red flag.  if you find
  yourself thinking "i'll commit it when it's finished" or "i don't want to
  commit something that people might criticise" *this is not collaboration*,
  it is making yourself a bottleneck.  pair-programming is supposed to help
  avoid this kind of thing however pair-programming is difficult to organise
  for remote collaborative libre projects (suggestions welcomed here)

### Absolutely no auto-generated output

* **do not commit autogenerated output**. write a shell script and commit
  that, or add a Makefile to run the command that generates the output, but
  **do not** add the actual output of **any** command to the repository.
  ever.  this is really important.  even if it is a human-readable file
  rather than a binary object file.
  it is very common to add pdfs (the result of running latex2pdf) or configure.in (the result of running automake), they are an absolute nuisance and interfere hugely with git diffs, as well as waste hard disk space *and* network bandwidth. don't do it.

### Write commands that do tasks and commit those

* if the command needed to create any given autogenerated output is not
  currently in the list of known project dependencies, first consult on
  the list if it is okay to make that command become a hard dependency of
  the project (hint: java, node.js php and .NET commands may cause delays
  in response time due to other list participants laughing hysterically),
  and after a decision is made, document the dependency and how its source
  code is obtained and built (hence why it has to be discussed carefully)
* if you find yourself repeating commands regularly, chances are high
  that someone else will need to run them, too. clearly this includes
  yourself, therefore, to make everyone's lives easier including your own,
  put them into a .sh shell script (and/or a Makefile), commit them to
  the repository and document them at the very minimum in the README,
  INSTALL.txt or somewhere in a docs folder as appropriate.  if unsure,
  ask on the mailing list for advice.

###  Keep commits single-purpose

* edit files making minimal *single purpose* modifications (even if
  it involves multiple files. Good extreme example: globally changing
  a function name across an entire codebase is one purpose, one commit,
  yet hundreds of files. miss out one of those files, requiring multiple
  commits, and it actually becomes a nuisance).

### Run unit tests prior to commits

* prior to committing make sure that relevant unit tests pass, or that
  the change is a zero-impact addition (no unit tests fail at the minimum)

### Do not break existing code

* keep working code working **at all times**. find ways to ensure that this is the case. examples include writing alternative classes that replace existing functionality and adding runtime options to select between old and new code.

### Small commits with relevant commit message

* commit no more than around 5 to 10 lines at a time, with a CLEAR message
  (no "added this" or "changed that").
* if as you write you find that the commit message involves a *list* of
  changes or the word "and", then STOP. do not proceed: it is a "red flag"
  that the commit has not been properly broken down into separate-purpose
  commits. ask for advice on-list on how to proceed.

### Exceptions to small commit: atomic single purpose commit

* if it is essential to commit large amounts of code, ensure that it
  is **not** in use **anywhere** by any other code. then make a *small*
  (single purpose) followup commit which actually puts that code into use.

this last rule is kinda flexible, because if you add the code *and* add
the unit test *and* added it into the main code *and* ran all relevant
unit tests on all cascade-impacted areas by that change, that's perfectly
fine too.  however if it is the end of a day, and you need to stop and
do not have time to run the necessary unit tests, do *not* commit the
change which integrates untested code: just commit the new code (only)
and follow up the next day *after* running the full relevant unit tests.

### Why such strict rules?

the reason for all the above is because python is a dynamically typed language.
make one tiny change at the base level of the class hierarchy and the
effect may be disastrous.

it is therefore worth reiterating: make absolutely certain that you *only*
commit working code or zero-impact code.

therefore, if you are absolutely certain that a new addition (new file,
new class, new function) is not going to have any side-effects, committing
it (a large amount of code) is perfectly fine.

as a general rule, however, do not use this an an excuse to write code
first then write unit tests as an afterthought.  write *less* code *in
conjunction* with its (more basic) unit tests, instead. then, folliw up with
additions and improvements.

the reason for separating out commits to single purpose only becomes
obvious (and regretted if not followed) when, months later, a mistake
has to be tracked down and reverted.  if the commit does not have an
easy-to-find message, it cannot even be located, and once found, if the
commit confuses several unrelated changes, not only the diff is larger
than it should be, the reversion process becomes extremely painful.

### PEP8 format

* all code needs to conform to pep8.  use either pep8checker or better
  run autopep8.  however whenever committing whitespace changes, *make a
  separate commit* with a commit message "whitespace" or "autopep8 cleanup".
* pep8 REQUIRES no more than 80 chars per line. this is non-negotiable. if
  you think you need greater than 80 chars, it *fundamentally* indicates
  poor code design. split the code down further into smaller classes
  and functions.

### Docstring checker

* TBD there is a docstring checker.  at the minimum make sure to have
  an SPD license header, module header docstring, class docstring and
  function docstrings on at least non-obvious functions.

### Clear code commenting and docstrings

* make liberal but not excessive use of comments.  describe a group of
  lines of code, with terse but useful comments describing the purpose,
  documenting any side-effects, and anything that could trip you or other
  developers up.  unusual coding techniques should *definitely* contain
  a warning.

### Only one class per module (ish)

* unless they are very closely related, only have one module (one class)
  per file. a file only 25 lines long including imports and docstrings
  is perfectly fine however don't force yourself. again, if unsure,
  ask on-list.

### File and Directory hierarchy

* *keep files short and simple*. see below as to why
* create a decent directory hierarchy but do not go mad. ask for advice
  if unsure

### No import star!

* please do not use "from module import \*". it is extremely bad practice,
  causes unnecessary resource utilisation, makes code readability and
  tracking extremely difficult, and results in unintended side-effects.

example: often you want to find the code from which a class was imported.
nirmally you go to the top of the file, check the imports, and you know
exactly which file has the class because of the import path.  by using
wildcards, you have absolutely *no clue* which wildcard imported which
class or classes.

example: sometimes you may accidentally have duplicate code maintained
in two or more places.  editing one of them you find, puzzlingly, that
the code behaves in some files with the old behaviour, but in others it
works.  after a nassive amount of investigation, you find that the working
files happen to have a wildcard import of the newer accidental duplicate
class **after** the wildcard import of the older class with exactly the
same name.  if you had used explicit imports, you would have spotted
the double import of the class from two separate locations, immediately.

really.  don't.  use.  wildcards.

### Keep file and variables short but clear

* try to keep both filenames and variable names short but not ridiculously
  obtuse. an interesting compromise on imports is "from ridiculousfilename
  import longsillyname as lsn", and to assign variables as well: "comb =
  m.d.comb" followed by multiple "comb += nmigen_stmt" lines is a good trick
  that can reduce code indentation by 6 characters without reducing clarity.

Additionally, use comments just above an obtuse variable in order to
help explain what it is for.  In combination with keeping the the module
itself short, other readers will not need to scroll back several pages
in order to understand the code.

Yes it is tempting to actually use the variables as
self-explanatory-comments and generally this can be extremely good
practice.  the problem comes when the variable is so long that a function
with several parameters csn no longer fit on a single line, and takes
up five to ten lines rather than one or two. at that point, the length
of the code is adversely affected and thus so is readability by forcing
readers to scroll through reams of pages.

it is a tricky balance: basically use your common sense, or just ask
someone else, "can you understand this code?"

### Reasons for code structure

regarding code structure: we decided to go with small modules that are
both easy to analyse, as well as fit onto a single page and be readable
when displayed as a visual graph on a full UHD monitor.  this is done
as follows:

* using the capability of nmigen (TODO crossref to example) output the
  module to a yosys ilang (.il) file
* in a separate terminal window, run yosys
* at the yosys prompt type "read_ilang modulename.il"
* type "show top" and a graphviz window should appear. note that typing
  show, then space, then pressing the tab key twice will give a full list
  of submodules (one of which will be "top")

you can now fullsize the graphviz window and scroll around.  if it looks
reasonably obvious at 100% zoom, i.e the connections can be clearly
related in your mind back to the actual code (by matching the graph names
against signals and modules in the original nmigen code) and the words are
not tiny when zoomed out, and connections are not total incomprehensible
spaghetti, then congratulations, you have well-designed code. If not,
then this indicates a need to split the code further into submodules
and do a bit more work.

The reasons for doing a proper modularisation job are several-fold:

* firstly, we will not be doing a full automated layout-and-hope
  using alliance/coriolis2, we will be doing leaf-node thru tree node
  half-automated half-manual layout, finally getting to the floorplan,
  then revising and iteratively adjusting.
* secondly, examining modules at the gate level (or close to it) is just
  good practice.  poor design creeps in by *not* knowing what the tools
  are actually doing (word to experienced developers: yes, we know that
  the yosys graph != final netlist).
* thirdly, unit testing, particularly formal proofs, is far easier on
  small sections of code, and complete in a reasonable time.

## Special warning / alert to vim users!

Some time around the beginning of 2019 some bright spark decided that
an "auto-recommend-completion-of-stuff" option would be a nice, shiny
idea to enable by default from that point onwards.

This incredibly annoying "feature" results in tabs (or spaces) being
inserted "on your behalf" when you press return on one line, for your
"convenience" of not needing to type lots of spaces/tabs just to get
to the same indentation level.

Of course, this "feature", if you press return on one line in edit
mode and then press "escape", leaves a bundle-of-joy extraneous
whitespace **exactly** where you don't want it, and didn't ask for it,
pooped all over your file.

Therefore, *please*: **before** running "git commit", get into the
habit of always running "git diff", and at the very minimum
speed-skim the entire diff, looking for tell-tale "red squares"
(these show up under bash diff colour-syntax-highlighting) that
inform you that, without your knowledge or consent, vim has
"helpfully" inserted extraneous whitespace.

Remove them **before** git committing because they are not part
of the actual desired code-modifications, and committing them
is a major and constant distraction for reviewers about actual
important things like "the code that actually *usefully* was
modified for that commit"

This has the useful side-effect of ensuring that, right before
the commit, you've got the actual diff right in front of you
in the xterm window, on which you can base the "commit message".

## Unit tests

For further reading, see the wikipedia page on
[Test-driven Development](https://en.wikipedia.org/wiki/Test-driven_development)

This deserves its own special section.  It is extremely important to
appreciate that without unit tests, python projects are simply unviable.
Python itself has over 25,000 individual tests.

This can be quite overwhelming to a beginner developer, especially one
used to writing scripts of only 100 lines in length.

Thanks to Samuel Falvo we learned that writing unit tests as a formal
proof is not only shorter, it's also far more readable and also, if
written properly, provides 100% coverage of corner-cases that would
otherwise be overlooked or require tens to hundreds of thousands of
tests to be run.

No this is not a joke or even remotely hypothetical, this is an actual
real-world problem.

The ieee754fpu requires several hundreds of thousands of tests to be
run (currently needing several days to run them all), and even then we
cannot be absolutely certain that all possible combinations of input have
been tested.  With 2^128 permutations to try with 2 64 bit FP numbers
it is simply impossible to even try.

This is where formal proofs come into play.

Samuel illustrated to us that "ordinary" unit tests can then be written
to *augment* the formal ones, serving the purpose of illustrating how
to use the module, more than anything.

However it is appreciated that writing formal proofs is a bit of a
black art.  This is where team collaboration particularly kicks in,
so if you need help, ask on the mailing list.

## Don't comment out unit tests: add them first (as failures) and fix code later

Unit tests serve an additional critical purpose of keeping track of code
that needs to be written.  In many cases, you write the unit test *first*,
despite knowing full well that the code doesn't even exist or is completely
broken.  The unit test then serves as a constant and important reminder
to actually fix (or write) the code.

Therefore, *do not* comment out unit tests just because they "don't work".
If you absolutely must stop a unit test from running, **do not delete it**.
Simply mark it with an appropriate
["skip" decorator](https://docs.python.org/3/library/unittest.html#skipping-tests-and-expected-failures),
preferably with a link to a URL in the [bugtracker](http://bugs.libre-riscv.org)
with further details as to why the unit test should not be run.

# TODO Tutorials

Find appropriate tutorials for nmigen and yosys, as well as symbiyosys.

* Robert Baruch's nmigen tutorials look really good:
  <https://github.com/RobertBaruch/nmigen-tutorial>
* Although a verilog example this is very useful to do
  <https://symbiyosys.readthedocs.io/en/latest/quickstart.html#first-step-a-simple-bmc-example>
* This tutorial looks pretty good and will get you started
  <http://blog.lambdaconcept.com/doku.php?id=nmigen:nmigen_install> and
  walks not just through simulation, it takes you through using gtkwave
  as well.
* There exist several nmigen examples which are also executable
  <https://github.com/m-labs/nmigen/tree/master/examples/> exactly as
  described in the above tutorial (python3 filename.py -h)