llvm--emulator/README.org

LLVM–emulator README

• Overview
• Requirements
  • Getting Python 3
  • Getting pip for Python 3
• Installation
  • Pip way
  • Git way
  • Uninstalling
• Usage
  • Example
  • Alternatives
• Known Issues/Missing Features
  • Interactive mode
  • Builtin functions
• License

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 $):

  $ python --version
  Python 2.7.14
  $ python3 --version
  Python 3.6.3

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

  $ apt install python3

or

  $ sudo apt install python3

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:

  $ 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)

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)

  $ apt install python3-pip

or

  $ sudo apt install python3-pip

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-emulator¹.

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

  $ pip3 install llvm-minusminus-emulator

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

Git way

To get the very latest version, you can do the following

If you have not already, download the code.

  $ cd path/to/folder/where/you/want/to/store/the/emulator
  $ git clone git@gitlab.com:cfreksen/llvm--emulator.git

To install the software, you can use pip on the folder containing setup.py:

  $ cd path/to/folder/where/you//stored/the/emulator/llvm--emulator
  $ pip3 install .

You should now be ready to use the software.

Uninstalling

To remove the emulator, just uninstall via pip:

  $ pip3 uninstall llvm-minusminus-emulator

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:

  $ 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

To automatically step through a LLVM file (and be quite verbose about it), you can use the -a (--auto) flag:

  $ llvm--emulator -a path/to/your/file.ll

When running the emulator you might get messages like the following:

  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'

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

  %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
  }

Then we run the emulator:

  $ 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

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:

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/>.

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¹

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.