An optimising compiler for brainfuck
bfc is an industrial grade compiler for brainfuck. It can:
- compile (and cross-compile) BF programs to executables
- optimise runtime speed
- optimise runtime memory usage
- optimise executable size
- show syntax errors with highlighting of the offending source code
- show warnings with highlighting of the offending source code
It is structured as follows:
BF source -> BF IR -> LLVM IR -> x86_32 Binary
Interested readers may enjoy my blog posts:
Table of Contents
Usage
You will need LLVM and Rust installed to compile bfc.
$ cargo build --release
Debug builds work, but large BF programs will take a long time
in speculative execution if bfc is compiled without optimisations. You
can disable this by passing --opt=0
or --opt=1
when running bfc.
$ target/release/bfc sample_programs/hello_world.bf
$ ./hello_world
Hello World!
By default, bfc compiles programs to executables that run on the current machine. You can explicitly specify architecture using LLVM target triples:
$ target/release/bfc sample_programs/hello_world.bf --target=x86_64-pc-linux-gnu
LLVM Version
LLVM 3.8+ is recommended, as there are known bugs with 3.7. Either download a prebuilt LLVM, or build it as follows:
$ wget http://llvm.org/pre-releases/3.8.0/rc1/llvm-3.8.0rc1.src.tar.xz
$ tar -xf llvm-3.8.0rc1.src.tar.xz
$ mkdir -p ~/tmp/llvm_3_8_build
$ cd ~/tmp/llvm_3_8_build
$ cmake -G Ninja /path/to/untarred/llvm
$ ninja
bfc depends on llvm-sys, which compiles against whichever
llvm-config
it finds.
$ export PATH=~/tmp/llvm_3_8_build:$PATH
$ cargo build --release
Running tests
$ cargo test
Portability
bfc considers cells to be single bytes, and arithmetic wraps
around. As a result, -
sets cell #0 to 255.
bfc provides 100,000 cells. Accessing cells outside of this range is explicitly undefined, and will probably segfault your program. bfc will generate a warning if it can statically prove out-of-range cell access.
bfc requires brackets to be balanced, so +[]]
is rejected, unlike
some BF interpreters.
Finally, bfc assumes input files are valid UTF-8.
Test programs
There are a few test programs in this repo, but http://www.hevanet.com/cristofd/brainfuck/tests.b is an excellent collection of small test BF programs and some more elaborate programs can be found at 1 and 2.
Diagnostics
bfc can report syntax errors and warnings with relevant line numbers and highlighting.
Note that some warning are produced during optimisation, so disabling optimisations will reduce warnings.
Optimisations
Peephole optimisations
bfc provides a range of peephole optimisations. We use quickcheck to ensure our optimisations are in the optimal order (by verifying that our optimiser is idempotent).
Combining Instructions
We combine successive increments/decrements:
Compile Combine
+++ => Increment 1 => Increment 3
Increment 1
Increment 1
If increments/decrements cancel out, we remove them entirely.
Compile Combine
+- => Increment 1 => # nothing!
Increment -1
We combine pointer increments:
Compile Combine
+++ => PointerIncrement 1 => PointerIncrement 2
PointerIncrement 1
We do the same thing for successive sets:
Combine
Set 1 => Set 2
Set 2
We combine sets and increments too:
Compile Known zero Combine
+ => Increment 1 => Set 0 => Set 1
Increment 1
We remove increments when there's a set immediately after:
Combine
Increment 1 => Set 2
Set 2
We remove both increments and sets if there's a read immediately after:
Combine
Increment 1 => Read
Read
We track the current cell position in straight-line code. If we can
determine the last instruction to modify the current cell, it doesn't
need to be immediately previous. For example, +>-<,
:
Combine
Increment 1 => PointerIncrement 1
PointerIncrement 1 Increment -1
Increment -1 PointerIncrement -1
PointerIncrement -1 Read
Read
Loop Simplification
[-]
is a common BF idiom for zeroing cells. We replace that with
Set
, enabling further instruction combination.
Compile Simplify
[-] => Loop => Set 0
Increment -1
Dead Code Elimination
We remove loops that we know are dead.
For example, loops at the beginning of a program:
Compile Known zero DCE
[>] => Loop => Set 0 => Set 0
DataIncrement 1 Loop
DataIncrement
Loops following another loop (one BF technique for comments is
[-][this, is+a comment.]
).
Compile Annotate DCE
[>][>] => Loop => Loop => Loop
DataIncrement 1 DataIncrement 1 DataIncrement 1
Loop Set 0 Set 0
DataIncrement 1 Loop
DataIncrement 1
Loops where the cell has previously been set to zero:
Compile Simplify DCE
[-]>+<[] => Loop => Set 0 => Set 0
Increment -1 DataIncrement 1 DataIncrement 1
DataIncrement 1 Increment 1 Increment 1
Increment 1 DataIncrement -1 DataIncrement -1
DataIncrement -1 Loop
Loop
We remove redundant set commands after loops (often generated by loop annotation as above).
Remove redundant set
Loop => Loop
Increment -1 Increment -1
Set 0
We also remove dead code at the end of a program.
Remove pure code
Write => Write
Increment 1
Finally, we remove cell modifications that are immediately overwritten
by reads, e.g. +,
is equivalent to ,
.
Reorder with offsets
Given a sequence of instructions without loops or I/O, we can safely reorder them to have the same effect (we assume no out-of-bound cell access).
This enables us to combine pointer operations:
Compile Reorder
>+> => PointerIncrement 1 => Increment 1 (offset 1)
Increment 1 PointerIncrement 2
PointerIncrement 1
We also ensure we modify cells in a consistent order, to aid cache
locality. For example, >+<+>>+
writes to cell #1, then cell #0, then
cell #2. We reorder these instructions to obtain:
Increment 1 (offset 0)
Increment 1 (offset 1)
Increment 1 (offset 2)
PointerIncrement 2
Multiply-move loops
bfc can detect loops that perform multiplication and converts them to
multiply instructions. This works for simple cases like [->++<]
(multiply by two into the next cell) as well as more complex cases
like [>-<->>+++<<]
.
Cell Bounds Analysis
BF programs can use up to 100,000 cells, all of which must be zero-initialised. However, most programs don't use the whole range.
bfc uses static analysis to work out how many cells a BF program may use, so it doesn't need to allocate or zero-initialise more memory than necessary.
>><< only uses three cells
[>><<] uses three cells at most
[>><<]>>> uses four cells at most
[>] may use any number of cells, so we must assume 100,000
Speculative Execution
bfc executes as much as it can at compile time. For some programs (such as hello_world.bf) this optimises away the entire program to just writing to stdout. bfc doesn't even need to allocate memory for cells in this situation.
$ cargo run -- sample_programs/hello_world.bf --dump-llvm
@known_outputs = constant [13 x i8] c"Hello World!\0A"
declare i32 @write(i32, i8*, i32)
define i32 @main() {
entry:
%0 = call i32 @write(i32 0, i8* getelementptr inbounds ([13 x i8]* @known_outputs, i32 0, i32 0), i32 13)
ret i32 0
}
Infinite Loops
bfc sets a maximum number of execution steps, avoiding infinite loops
hanging the compiler. As a result +[]
will have +
executed (so our
initial cell value is 1
and []
will be in the compiled output.
Runtime Values
If a program reads from stdin, speculation execution stops. As a
result, >,
will have >
executed (setting the initial cell pointer
to 1) and ,
will be in the compiled output.
Loop Execution
If loops can be entirely executed at compile time, they will be removed from the resulting binary. Partially executed loops will be included in the output, but runtime execution can begin at an arbitrary position in the loop.
For example, consider +[-]+[+,]
. We can execute +[-]+
entirely, but [+,]
depends on runtme values. The
compiled output contains [+,]
, but we start execution at the
,
(continuing execution from where compile time execution had to
stop).
License
GPLv2 or later license. Sample programs are largely written by other authors and are under other licenses.
Other projects optimising BF
There are also some interesting other projects for optimising BF programs:
- https://code.google.com/p/esotope-bfc/wiki/Optimization
- http://www.nayuki.io/page/optimizing-brainfuck-compiler
- http://mearie.org/projects/esotope/bfc/
- http://calmerthanyouare.org/2015/01/07/optimizing-brainfuck.html
- http://xn--2-umb.com/10/brainfuck-using-llvm
- https://github.com/stedolan/bf.sed (simple optimisations, but compiles directly to asm)
- https://github.com/matslina/bfoptimization
- Platonic Ideal Brainfuck Interpeter (src) (even has a profiler!)
- https://github.com/rmmh/beefit - using LuaJIT