TinyVM 1.0 released; adds 16 lines of code, registers, a VM stack and more

https://github.com/GenTiradentes/tinyvm/tree/v1.0

by davedx • 15 years ago

121 22 15 years ago

joeyespo• 15 years ago

This is one of those projects I've always wanted to do myself, but never actually spent the time to get it right. Looks good.

jws• 15 years ago

Most files have a last commit message of: Nothing left can be taken away. That pretty much sums up the target for this VM.

Instruction dispatching and malloc handling were discussed here last month in http://news.ycombinator.com/item?id=2722383 (Instruction dispatch is now a switch† instead of chain of if statements, malloc() results are still unchecked.)

␄

† The switch was slower in my benchmarks last time, I don't have time to check this time.

jsherer• 15 years ago

How many of the previous discussion points / issues have been changed in this release? Just the switch?

sitkack• 15 years ago

TVM has 9 registers, modeled after x86 registers

maaku3• 15 years ago

But in hardware at least, the x86 registers have been found quite limiting on software and performance. That's why AMD64 doubles the number of general purpose registers. Does anyone know if there is research into the effects of register counts on VMs?

carbonica4• 15 years ago

> But in hardware at least, the x86 registers have been found quite limiting on software and performance.

While the doubled register count makes a big difference - especially with the new x86-64 calling convention - register renaming and hidden registers do a lot on x86 to mitigate the lack of general purpose registers. You don't need more than 8 registers to get the benefits of having more than 8.

Someone• 15 years ago

If you can map all VM registers to CPU registers, the interpreter will be way simpler.

If you have more VM registers than CPU registers, you have to write code to load and store your VM registers.

If you have more CPU registers than VM registers, you will have to write code to undo some of the register spilling done by the compiler (if you want your VM to use the hardware to the fullest.)

So, the optimum (from a viewpoint of 'makes implementation easier') depends on the target architecture.

Of course, a generic VM cannot know how many registers (and what kinds; uniform register files are not universally present) the target CPU will have.

That is a reason that choosing either zero (i.e. a stack machine) or infinitely many (i.e. SSA) are popular choices fo VMs: for the first, you know for sure your target will not have fewer registers than your VM, for the second, that it will not have more.

If you choose any other number of VM registers, a fast generic VM will have to handle both cases.

Alternatively, of course, one can target a specific architecture, and have the VM be fairly close to the target; Google's NaCl is (was? I think they changed things a bit) an extreme example. I have not checked the code, but this, I think, is similar.

maaku3• 15 years ago

This appears to be very heavily influenced by x86. Why would a VM built in 2011 use an ISA stuck in the 80's?

EDIT: I'm not trolling, I'm seriously wondering. There's plenty of literature out there on the advantages of three-operand instructions, large register spaces, and RISC-type instructions. Was there a reason that x86 was used as a basis?

aninteger• 15 years ago

I'm not the author but I guess there would be a few reasons:

1. Familiarity with x86 assembly. It's probably the assembly language that the majority of people who write assembly language know. 6502 probably being 2nd place

2. Easier to port small x86 assembly programs

3. Even if it's stuck in the 80's it's the most widely used assembly language and there are dedicated online communities of people still writing x86 asm today in 2011.

xyzzyz• 15 years ago

2. Easier to port small x86 assembly programs

I seriously doubt that people will do it. If people write assembly, they mostly do it either because of speed, or because it's impossible to do in any other way. None of the reason is relevant in context of VM assembly, which is usually meant to be a compiler's target.

carbonica4• 15 years ago

The instruction set is vastly simpler than x86, but it does inherit some limitations for no reason. x86 had 8 named registers for so long because the mod_rm byte in x86 only has 3 bits for each operand. Adding the REX prefix byte in x86=64 complicates things a bit, but isn't too tough.

This VM doesn't have a particularly compact in-memory encoding - it uses an entire `int` for each argument! They could double the number of registers and the entire register set would fit in a cache line (except for EIP) at 4 bytes apiece. I guess it is nice that all 9 fit in one line now, but I doubt that was considered.

sb• 15 years ago

Well, at least in the area of virtual machines and interpreters, the usual argument goes like this:

A stack-based virtual machine architecture has compact code representation (only bytecodes) where operands are pushed onto and popped of the corresponding argument stack. Register based VM-architectures require you to encode source and destination registers into the operations. IIRC, for Java bytecode, going from a stack-based representation to a register-based virtual machine grew the code size by more than 40%. But on the other hand interpretation got more efficient, since you have less instructions overall and thus fewer instruction dispatches. If you want more details I'll gladly point you to the excellent and canonical reference for this: Shi, Casey, Ertl and Gregg: "Virtual machine showdown: Stack versus registers." TACO http://dl.acm.org/citation.cfm?id=1328195.1328197 (There is also the journal article's predecessor from VEE05, https://www.usenix.org/events/vee05/full_papers/p153-yunhe.p...)

diminish• 15 years ago

i ll make a de-pull request, to remove more.

swah• 15 years ago

I enjoyed your comment, but will downvote you because I was downvoted when doing similar jokes.

RodgerTheGreat• 15 years ago

If you find TinyVM interesting, one of my own projects might be worth checking out: https://github.com/JohnEarnest/Mako/blob/master/MakoVM.java

It's a stack-oriented VM, similarly designed to be as simple as possible, but my goal was to create something like an idealized game console. Thus, I have basic graphics support and gamepad input. The project includes a compiler for a dialect of Forth and a big pile of example code.

chrisb• 15 years ago

In a slightly similar vein, you might enjoy looking at my Dot Net Anywhere project: https://github.com/chrisdunelm/DotNetAnywhere/tree/master/dn... (or http://www.dotnetanywhere.org for an easy to use zip download that will load and compile directly on Windows)

It is an almost complete .NET2.0 CIL interpreter, so it can run your exe's compiled from C#/VB/etc...

I'm not sure it has much practical application, but it was certainly fun to write, and I learnt a huge amount about interpreters and how .NET works.

PS Lots of bugs, a memory leak, an incomplete base class library, etc... so don't try to use this for anything serious!

DanielHimmelein• 15 years ago

Some more resources about virtual machines and a little TinyVM overview: http://himmele.blogspot.com/2011/08/virtual-machines.html

spitfire• 15 years ago

It's nice to see the huge if/then waterfall has been replaced with a switch statement. That was quite the code smell there.

I'm still as impressed as when I first saw it. Small, simple and elegant. Neat little project.

parbo• 15 years ago

If you want to program a VM and have some fun at the same time, do the 2006 ICFP contest: http://www.boundvariable.org/task.shtml

You have to implement a VM and run the provided program. This will give you access to the actual problem set.

TinyVM 1.0 released; adds 16 lines of code, registers, a VM stack and more

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