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A very (!) fast BrainFuck interpreter in Python

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Here is a BrainFuck example:

+++++ +++++             initialize counter (cell #0) to 10
[                       use loop to set the next four cells to 70/100/30/10
> +++++ ++              add  7 to cell #1
> +++++ +++++           add 10 to cell #2
> +++                   add  3 to cell #3
> +                     add  1 to cell #4
<<<< -                  decrement counter (cell #0)
> ++ .                  print 'H'
> + .                   print 'e'
+++++ ++ .              print 'l'
.                       print 'l'
+++ .                   print 'o'
> ++ .                  print ' '
<< +++++ +++++ +++++ .  print 'W'
> .                     print 'o'
+++ .                   print 'r'
----- - .               print 'l'
----- --- .             print 'd'
> + .                   print '!'
> .                     print '\n'

How to use the interpreter:

Hello World!

Speeding things up

With Pypy

If you try to run a long BrainFuck program like mandel.b, you will realize our interpreter is pretty slow.

./ examples/mandel.b
# wait 1h45

A first simple way of speeding things up is to use Pypy instead of CPython. You can use portable-pypy to get Pypy without compiling it yourself:

wget ''\
tar -xvjf 'pypy-5.6-linux_x86_64-portable.tar.bz2'
mv pypy-5.6-linux_x86_64-portable pypy-portable
# Only 1m30 now!
./pypy-portable/bin/pypy ./ ./examples/mandel.b

With a JIT

The interpreter is actually written in Rpython, so it can be statically compiled using the Pypy toolchain. Download the latest source of Pypy and uncompress it in a pypy-src folder.

wget ''
tar -xvjf 'pypy2-v5.6.0-src.tar.bz2'
mv pypy2-v5.6.0-src pypy-src

Then you can build from the Python script an executable binary bf-c:

# The compilation will take about 20s
python pypy-src/rpython/bin/rpython
# Mandelbrot now completes in 32s
./bf-c examples/mandel.b

You can rebuild the bf-c using --opt=jit to add a JIT to your BrainFuck interpreter:

# The compilation will take about 7m
python pypy-src/rpython/bin/rpython --opt=jit
# Mandelbrot now completes in about 5 seconds(!)
./bf-c examples/mandel.b

Let’s compare with a C implementation

I also looked for a fast BrainFuck interpreter, written in C. After compilation with gcc -O3 (6.2), running mandel.b take about 5 seconds to run, so it is in the same order of magnitude as the JIT version (without -O3, it takes 10 seconds).

gcc -O3 ./resources/bff4.c -o bff4
# About 5s
./bff4 < examples/mandel.b

Let’s compile the BrainFuck directly

To complete those numbers, I finally tested a Brainfuck to C translator, then compiled the C version of the mandel.b program. With -O3, the compiled mandel.b runs in a bit less than 1 second (without -O3, it takes 15 seconds).

gcc resources/brainfucc.c -o brainfucc
./brainfucc < examples/mandel.b > mandel.c
gcc -O3 mandel.c -o mandel
# 950ms


Here is a summary of the speed gain I could observe on Ubuntu 16.10 (core i7, 8Go of RAM), running mandel.b:

The JIT addition contains code from this amazing tutorial on JITs.

If the BrainFuck interpreter is a bit hairy to look at, you can check out the step_by_step folder to go from the simplest interpreter, then a bit better, then using only Rpython code, then with the JIT-specific code, then with some final optimizations.