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1.5.1 The “brainf” language ¶

In this example we use libgccjit to construct an ahead-of-time compiler for an esoteric programming language that we shall refer to as “brainf”.

brainf scripts operate on an array of bytes, with a notional data pointer within the array.

brainf is hard for humans to read, but it’s trivial to write a parser for it, as there is no lexing; just a stream of bytes. The operations are:

Character	Meaning
>	idx += 1
<	idx -= 1
+	data[idx] += 1
-	data[idx] -= 1
.	output (data[idx])
,	data[idx] = input ()
[	loop until data[idx] == 0
]	end of loop
Anything else	ignored

Unlike the previous example, we’ll implement an ahead-of-time compiler, which reads .bf scripts and outputs executables (though it would be trivial to have it run them JIT-compiled in-process).

Here’s what a simple .bf script looks like:

[
  Emit the uppercase alphabet
]

cell 0 = 26
++++++++++++++++++++++++++

cell 1 = 65
>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++<

while cell#0 != 0
[
 >
 .      emit cell#1
 +      increment cell@1
 <-     decrement cell@0
]

Note: This example makes use of whitespace and comments for legibility, but could have been written as: ++++++++++++++++++++++++++ >+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++< [>.+<-] It’s not a particularly useful language, except for providing compiler-writers with a test case that’s easy to parse. The point is that you can use gcc_jit_context_compile_to_file() to use libgccjit as a backend for a pre-existing language frontend (provided that the pre-existing frontend is compatible with libgccjit’s license).