Project Nayuki


Optimizing brainfuck compiler

Brainfuck Mandelbrot output

This Python script translates a brainfuck source file into a C/Java/Python source file. Along the way, it performs optimizations on the brainfuck program, described below. Afterward, you use your C/Java/Python compiler or interpreter on the newly generated source code.

Download: bfc.py

Usage: python bfc.py BrainfuckFile OutputFile.c/java/py

Full example:
python bfc.py mandelbrot.b.txt mandelbrot.c gcc -o mandelbrot mandelbrot.c ./mandelbrot

Tip: It can be tedious to compile and run a brainfuck program using this tool. For example for C, you might want to put all the commands on one line: python bfc.py myprog.b myprog.c && gcc -o myprog myprog.c && ./myprog. Alternatively, you can write a Makefile with rules for compiling, running, cleaning, etc.

Example and benchmark

mandelbrot.c in Notepad2

Benchmark:

Unoptimized Optimized
C, -O0 17.20 s 2.77 s
C, -O1 1.09 s 0.64 s
C, -O2 1.02 s 0.66 s
C, -O3 1.05 s 0.66 s
Java 41.40 s 19.20 s
Python 490.00 s 190.00 s

All benchmarks above were performed on an Intel Xeon E3-1270 CPU (single-threaded). The non-optimized source codes are not published because they are uninteresting. Numbers are reported to 3 significant figures. Software-wise, GCC 4.8.2, Oracle Java 1.7.0_51, CPython 2.7.6, and Linux 3.12.6 x86-64 were used.

Optimizations

I will use this notation: unsigned char *p points to the current cell (which corresponds to the C output from the script).

Fusing increments/decrements

A real computer can add by not just one, but by any number. So for example, we can translate +++ into p[0] += 3;.

Fusing left/right movements

Similarly, we can move the pointer by more than one – for example, we can translate >> into p += 2;.

Fusing movements into adds

We can add/subtract from cells other than the current one. For example, we can translate >++< into p[1] += 2;, which no longer contains pointer movements.

Postponing movements

In a block of brainfuck commands, if the block consists of only increments/decrements, left/right, and input/output, then we can keep track of the virtual movement and then perform it in one shot at the end of the block. For example, we can translate >+>-> into p[1]+=1; p[2]-=1; p+=3;. Note that we have to actually perform the movements before entering a loop/if and before exiting a loop/if.

Simple loops

A moment’s thought should make it clear that [-] translates to p[0] = 0;. But we can generalize even further than that. If the loop body has no subloops and no input/output, all the movements add up to 0, and all the increments/decrements at p[0] add up to −1, then we are basically running the loop body p[0] times. So for all the increments/decrements outside of p[0], some multiple of p[0] is added to that location. For example, we can translate [->+>---<<] into p[1]+=p[0]; p[2]-=p[0]*3; p[0]=0;.

Assign followed by add

Obviously, p[i] = 0; followed by p[i] += v; can be replaced by p[i] = v;. As well, p[i] = 0; followed by p[i] += p[j] * v; can be replaced by p[i] = p[j] * v;.

Complex loops

Extending the simple loop optimization, if a location other than the loop counter is used as the source for multiply-add and this location is cleared to 0 within the loop, then the loop can be turned into an if-statement. For example, we can optimize while(p[0]!=0){ p[0]--; p[1]+=2; p[2]+=p[3]*5; p[3]=0; } into if(p[0]!=0){ p[1]+=p[0]*2; p[0]=0; p[2]+=p[3]*5; p[3]=0; }.

Notes

  • Technically speaking, this script is called a source-to-source compiler, rather than a typical source-to-machine-code compiler.

  • To disable optimizations, simply remove the lines that call the optimize() function.

  • The generated source code uses a large but fixed-size array for the brainfuck memory, with no explicit bounds checking. Thus, the rare brainfuck program that uses a lot of memory will either crash precisely in the Java and Python versions, or exhibit undefined (arbitrary) behavior in the C version. This implementation choice is because I didn’t want to deal with the complexity of (for example) resizing the memory array to uphold the illusion of infinite memory for the brainfuck program.

  • For Java output, large brainfuck programs may exceed the class file format’s 64 KiB per method size limit. This can be fixed by modifying the Java code generator (in the Python script) to be able to split the brainfuck program into multiple methods based on a conservative estimate of the output bytecode size.