llvm--emulator/README.org

#+TITLE: LLVM--emulator README
#+AUTHOR: Casper Freksen
#+OPTIONS: ^:nil

* Overview
  This application is an emulator/debugger for LLVM-- as defined in
  the Compiler course (dOvs) at Aarhus University.

  While I hope that you will find it useful, the tool is being
  provided AS IS, this means
  1) There is no guarantee that emulator emulates LLVM--
     correctly. Various liberties has been taken to simplify the
     emulations, e.g. values of type ~i64~ can be arbitrarily
     large. And there can, of course, be bugs in the application.
  2) Not all parts of LLVM-- has been implemented. While the main set
     of instructions and types has been implemented at this point, you
     might hit a "TODO: Not implemented yet" message.
  3) Many type annotations are being ignored. This means that
     ill-typed LLVM-- programs might be emulated without a
     problem. This can give a false sense of security, so always use a
     tool like ~clang~, if you want to check types of your generated code
  4) We, as TAs, do not give support for this emulator.
  5) *If your generated code works in this emulator, but not in clang,
     your generated code is incorrect!* We, as TAs, will *not* accept
     it.

* Requirements
  This application is written in Python. It was developed in Python
  3.6, but I expected to work for Python 3.5 and above. Note that
  Python 2 is not supported.

  Several Linux distributions have Python 2 as the default. You can
  usually use the commands ~python3~, ~pip3~, etc. to use Python 3 on
  such systems (assuming you have installed Python 3).

  The application uses some third party libraries as indicated in =requirements.txt=.

  This project is designed for Linux, though I do not expect there to
  be any issues on other platforms. As a consequence the commands
  given below are designed for a Linux shell. Lines beginning with a
  dollar sign ($) indicate commands you enter into your terminal,
  other lines are output from running those commands

** Getting Python 3
   To see if you already have Python 3 installed, try running (the
   lines beginning with $):
   #+BEGIN_SRC sh
     $ python --version
     Python 2.7.14
     $ python3 --version
     Python 3.6.3
   #+END_SRC
   If one of these commands gives a version number >= 3, you have
   Python 3 through that command (without ~--version~, of course). In
   the example above, I need to use the command ~python3~ in order to
   use the correct version of Python for this emulator.

   Otherwise, you could try to install Python 3 via your package
   manager. Depending on which Linux distribution you use, you could
   try something like
   #+BEGIN_SRC sh
     $ apt install python3
   #+END_SRC
   or
   #+BEGIN_SRC sh
     $ sudo apt install python3
   #+END_SRC

** Getting pip for Python 3
   You need pip to (easily) install Python packages. If your Linux
   distribution came with Python 2 and pip, the pip you have might not
   work for Python 3. Let us first check that:

   #+BEGIN_SRC sh
     $ pip --version
     pip 9.0.1 from /usr/lib/python2.7/site-packages (python 2.7)
     $ pip3 --version
     pip 9.0.1 from /usr/lib/python3.6/site-packages (python 3.6)
   #+END_SRC
   As with the Python version commands in the last section, I have to
   use ~pip3~. If I just used ~pip~ packages would be installed for
   Python 2, which I do not want.

   Pip should come with newer versions of Python. If you have a new
   version of Python, but no pip, you might need to install it
   seperately. You could try (depending on your Linux distribution)
   #+BEGIN_SRC sh
     $ apt install python3-pip
   #+END_SRC
   or
   #+BEGIN_SRC sh
     $ sudo apt install python3-pip
   #+END_SRC

* Installation
  Installation includes
  - Downloading and installing dependencies
  - Downloading the emulator
  - Storing the emulator libraries where your other Python libraries
    are
  - Adding a small script to easily start the emulator

** Pip way
   The recommended way to quickly install and use the emulator, is to
   install it via pip. The name on PyPI (where pip gets the software
   from) is =llvm-minusminus-emulator=[fn:0].

   Make sure that you have Python 3 and pip installed (see above).
   Then install the emulator by running

   #+BEGIN_SRC sh
     $ pip3 install llvm-minusminus-emulator
   #+END_SRC

   If all goes well, you are now ready to use the emulator.

[fn:0] I know the name is ugly, but Python packaging was not happy
about the double dash in =llvm--emulator=, and =llvm-emulator= makes
it sound like it covers the entire LLVM IR language.

** Git way
   To get the very latest version, you can do the following

   If you have not already, download the code.
   #+BEGIN_SRC sh
     $ cd path/to/folder/where/you/want/to/store/the/emulator
     $ git clone git@gitlab.com:cfreksen/llvm--emulator.git
   #+END_SRC

   To install the software, you can use ~pip~ on the folder containing =setup.py=:
   #+BEGIN_SRC sh
     $ cd path/to/folder/where/you//stored/the/emulator/llvm--emulator
     $ pip3 install .
   #+END_SRC

   You should now be ready to use the software.

** Uninstalling
   To remove the emulator, just uninstall via pip:
   #+BEGIN_SRC sh
     $ pip3 uninstall llvm-minusminus-emulator
   #+END_SRC

* Usage
  If installing the emulator went well, a script (~llvm--emulator~)
  should have been added to your =bin= folder. This means that you can
  start the emulator (wherever you are), by running that script:
  #+BEGIN_SRC sh
    $ llvm--emulator --help
    usage: llvm--emulator [-h] [-a AUTO_PATH]

    A hacky LLVM-- emulator/debugger

    optional arguments:
      -h, --help            show this help message and exit
      -a AUTO_PATH, --auto AUTO_PATH
                            Automatically step through llvm in the given file
  #+END_SRC

  To automatically step through a LLVM file (and be quite verbose
  about it), you can use the ~-a~ (~--auto~) flag:
  #+BEGIN_SRC sh
    $ llvm--emulator -a path/to/your/file.ll
  #+END_SRC

  When running the emulator you might get messages like the following:
  #+BEGIN_SRC sh
    WARNING: Couldn't open 'parser.out'. [Errno 13] Permission denied: '/usr/lib/python3.6/site-packages/llvm_emulator/parser.out'
    WARNING: Couldn't create 'parsetab'. [Errno 13] Permission denied: '/usr/lib/python3.6/site-packages/llvm_emulator/parsetab.py'
  #+END_SRC
  This is because the script does not have permission to write files
  among your Python libraries. This is because the parser inside the
  emulator tries to cache its parsing table (think of =tiger.grm.sml=)
  where the parsing code is located. If does not have permission to do
  that, it still parses your LLVM code; it just needs to rebuild the
  parsing table next time you run the emulator. These warnings should
  be safe to ignore.

  I have tried to fix this issue without success, so hopefully you can
  live with a few warning messages.

** Example
   Let us say, that we have the following LLVM-- code in =some_file.ll=
   #+BEGIN_SRC llvm
     %Ttigermain = type { i64, i64, i64 }

     define i64 @tigermain (i64 %U_mainSL_8, i64 %U_mainDummy_9) {
         %t = alloca %Ttigermain
         %a = getelementptr %Ttigermain, %Ttigermain* %t, i32 0, i32 1
         store i64 9, i64* %a
         %r = load i64, i64* %a
         %s = add i64 100, %r
         %b = getelementptr %Ttigermain, %Ttigermain* %t, i32 0, i32 0
         store i64 %s, i64* %b
         ret i64 %s
     }
   #+END_SRC

   Then we run the emulator:
   #+BEGIN_SRC sh
     $ llvm--emulator -a some_file.ll
     Parsing some_file.ll
     Beginning execution of some_file.ll
     Heap after globals are allocated:
     [None]

     Evaluating alloca %Ttigermain
     alloca {i64, i64, i64}  -->  allocating 3 cells
     %t <- 1

     Evaluating getelementptr %Ttigermain, %Ttigermain* %t, i32 0, i32 1
     Gep formula: 1 + 0 * 3 + (1)
     %a <- 2

     Evaluating store i64 9, i64* %a
     heap[2] <- 9

     Evaluating load i64, i64* %a
     load heap[2]
     %r <- 9

     Evaluating add i64 100, %r
     add 100, 9
     %s <- 109

     Evaluating getelementptr %Ttigermain, %Ttigermain* %t, i32 0, i32 0
     Gep formula: 1 + 0 * 3 + 0
     %b <- 1

     Evaluating store i64 %s, i64* %b
     heap[1] <- 109

     Evaluating ret i64 %s
     Returning 109

     Stepping done!
     Final ssa_env: {'U_mainSL_8': 1234, 'U_mainDummy_9': 5678, 't': 1, 'a': 2, 'r': 9, 's': 109, 'b': 1}
     Final heap: [None, 109, 9, <<Garbage>>]
     Program resulted in 109 after 8 steps
   #+END_SRC

   This shows which values variables have as they are encountered as
   well as the order the instructions are evaluated.

** Alternatives
   If the =llvm--emulator= script does not work for you, you can
   inspect it in the =path/to/emulator/repository/bin/= folder
   (assuming you have the source code. See the section
   'Installation:Git Way', or look at the code online on
   [[https://gitlab.com/cfreksen/llvm--emulator]]). It should be clear
   enough what the script does, and if you know a bit of Python, you
   should be able to tweak it to your needs.

* Known Issues/Missing Features
  Here some of the known major issues/missing features are
  listed. This list might be updated, should the issues be fixed/the
  features implemented:

** Interactive mode
   There is currently no support for stepping through the code one key
   press at a time. Similarly, there is no support for inserting
   breakpoints, or looking up the current values in memory/registers
   via commands.

** Builtin functions
   When generating LLVM code from Tiger code, there can be several
   calls to functions defined in a file called =runtime.c=. Many of
   these functions are not implemented in the emulator. However, ~allocRecord~, ~initArray~, and ~print~ are so that will hopefully
   be enough for the majority of your LLVM programs.

* License
  The code in this project is licensed under GPLv3+. The full
  licensing text can be found in the ~LICENSE~ file, while a small but
  descriptive header is:
  #+BEGIN_QUOTE
  LLVM-- Emulator -- A simple hacky emulator and debugger for LLVM--
  Copyright © 2017  Casper Freksen

  This program is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program.  If not, see <http://www.gnu.org/licenses/>.
  #+END_QUOTE