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Programming Lessons from Tarot: 1 - qcode

To execute a scheme program with tarot, we compile it into “qcode” then load and execute it in the virtual machine. This document describes qcode. It’s a lot like bytecode except instead being byte-based it’s 64 bit (qword) based.

Virtual machine

For the virtual machine to execute compiled scheme it needs to implement a runtime platform meaning: symbol table, datatypes, garbage collection, … Every scheme object is represented in the VM as a tagged 64 bit word. The tag says whether it’s a boolean, number, character, cons cell, vector or a file port. Once you take the tag off you either get the raw data for the number, char or you get a pointer into the heap.

I wanted to be able to easily refer to constant data in compiled code so instead of using a bytecode format, what about making it a sequence of 64 bit words? This makes the interpreter loop really simple: just fetch a qword, execute it and its execution might fetch more qwords as immediate. There is no “decoding” of complicated byte sequences. The previous scheme compiler I wrote used a horrible bytecode system that required lots of decoding, unescaping.. it was nasty. This new one is so much cleaner, I’m really happy with it.

From qcode.h you can see there is only 30 different instructions but we encode them using a whole 64 bit word, capable of expressing 18446744073709551616 different instructions. A lot of waste! I was worried that such waste would be a disaster for speed and make it very very slow - luckily it doesn’t!

bf test

Brainfuck interpreters are usually based on bytecode, so to test the idea about using 64 bit words I took (A) a standard brainfuck interpreter (B) A faster brainfuck interpreter that does a quick assembly pass during loading, to point open and close brackets to each other.

Then I modified them both to operate on 64 bit words instead of bytes, if we call these (A’) and (B’). I ran the brainfuck mandelbrot render program in them. A’ is a tiny bit slower than A. A is much much much slower than B’. B’ is a tiny bit slower than B. A' > A >>>>>>>>>>>>>>> B' > B .

This gave a strong indication that using 64 bit words will be fine, only causing a tiny impact on speed compared against a much more compact encoding.

qcode serialization and loading

The compiler doesn’t actually emit a sequence of 64 bit words though. It emits a “tokens” which the virtual machine can read in and translate into 64 bit words before they get executed. When it seeds string data it allocates that and loads a single pointer to it into the code section! The loading stage also resolves global variables. Like brainfuck resolving []’s at load time, instead of every single time program flow reaches them this is an enormous speedup!

(Actually I forgot to do the global variable resolution at loading time, so originally the compiler was a lot slower. 1 min 30 to bootstrap. When this was found and fixed - by making that happen at load time - it cut the time down to one second! I cut this down to half a second later on by rewriting the parser.)

The serialized form is just text tokens separated by \0 the nul byte. Since strings can never contain a nul byte one magic aspect of using this format is that we do not have to escape or unescape strings! It’s really simple to work with:

Here’s an example of a scheme function:

( define ( not b ) ( if b #f #t ))

and the qcode it gets compiled into:

stack-grow^@1^@allocate-closure^@0^@7^@clo-set-loc^@0 ^@var-loc^@0^@set-glo^@not^@halt^@var-loc^@0^@branch ^@3^@datum-false^@jump^@1^@datum-true^@ret^@

( ^@ represents the nul byte which is a non printing character)

Interpreter

Finally here is the loader and the VM interpreter loop: interpreter.c 0xC0FFEEEEEEEEEEEE and 0xDEADBEEFDEADBEEF are stack canaries.

I used a computed goto table instead of a switch/case because this paper says it’s much better for the branch predictor: The Structure and Performance of Efficient Interpreters.

edit update feb 2019. This paper Branch Prediction and the Performance of Interpreters - Don’t Trust Folklore suggests that the computed goto trick is not a big optimization anymore. So a basic switch/case might be just fine.

accuracy of indirect branch prediction is no longer critical for interpreters. We further compare the characteristics of these interpreters and analyze why the indirect branch is less important than before

Conclusions