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Reference counted garbage collection

Posted by Aardvajk, 23 September 2011 · 151 views

Bit too tired to write this up in detail, but I've got an interesting system working in the VM now where, as per ApochPiQ's advice last post, I'm now using reference counted garbage collection. A buffer is also tagged to say whether it points to a list of allocated buffers and if so, when it is freed it decrements the references of its members recursively, freeing them if needed.

I've implemented arbitrary dimensional arrays in the compiler and got everything working at run-time using a Copy On Write system that utilises the reference counting built into the VM. For what it is worth:




char[][][] c={ { "hello","world" },{ "hi","there" } };

char[][][] d=c;



out c[0][1],"\n";

out c[1][1],"\n";

out d[0][1],"\n";

out d[1][1],"\n";



c[0][1]="paul";

d[1][1]="froob";



out c[0][1],"\n";

out c[1][1],"\n";

out d[0][1],"\n";

out d[1][1],"\n";



Compiles to (deep breath):




    setrxi 2

    push 

    setrxi 4

    push 

    muli 

    pop 

    alloc 

    setrxi 1

    setbt 

    movar 

    push 

    movar 

    push 

    setrxi 2

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    setrxi 4

    push 

    muli 

    pop 

    alloc 

    setrxi 1

    setbt 

    movar 

    push 

    movar 

    push 

    setrxi string1

    push 

    axstm 8

    get 

    movra 

    pop 

    put 

    pop 

    addax 4

    movar 

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    axstm 8

    get 

    movra 

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    put 

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    get 

    movra 

    incref 

    axstm 8

    get 

    movra 

    pop 

    put 

    pop 

    addax 4

    movar 

    push 

    setrxi 2

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    muli 

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    alloc 

    setrxi 1

    setbt 

    movar 

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    setrxi string3

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    axstm 8

    get 

    movra 

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    put 

    pop 

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    push 

    setrxi string4

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    get 

    movra 

    pop 

    put 

    pop 

    axstm 4

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    put 

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    axstm 4

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    movra 

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    chkbi 4

    setrxi 4

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    push 

    axstm 12

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    movra 

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    chkbi 4

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    muli 

    addi 

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    movra 

    get 

    push 

    pop 

    outs 

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    push 

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    outs 

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    setrxi 0

    push 

    axstm 4

    get 

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    axstm 12

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    movra 

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    chkbi 4

    setrxi 4

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    pop 

    movra 

    get 

    push 

    pop 

    outs 

    setrxi string5

    push 

    pop 

    outs 

    axstm 4

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    push 

    setrxi 1

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    axstm 4

    get 

    push 

    axstm 12

    get 

    movra 

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    movra 

    get 

    push 

    setrxi 1

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    movra 

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    chkbi 4

    setrxi 4

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    muli 

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    movra 

    get 

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    outs 

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    push 

    pop 

    outs 

    axstm 8

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    detach 

    movar 

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    setrxi string6

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    axstm 12

    get 

    push 

    setrxi 0

    push 

    axstm 4

    get 

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    axstm 12

    get 

    movra 

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    chkbi 4

    setrxi 4

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    muli 

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    setrxi 1

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    get 

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    movra 

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    chkbi 4

    setrxi 4

    push 

    muli 

    addi 

    pop 

    movra 

    push 

    get 

    movra 

    decref 

    pop 

    movra 

    pop 

    put 

    push 

    pop 

    gc 

    axstm 4

    get 

    movra 

    detach 

    movar 

    axstm 4

    put 

    setrxi string7

    push 

    axstm 8

    get 

    push 

    setrxi 1

    push 

    axstm 4

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    movra 

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    chkbi 4

    setrxi 4

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    muli 

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    movra 

    get 

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    setrxi 1

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    axstm 4

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    push 

    axstm 12

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    movra 

    pop 

    chkbi 4

    setrxi 4

    push 

    muli 

    addi 

    pop 

    movra 

    push 

    get 

    movra 

    decref 

    pop 

    movra 

    pop 

    put 

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    gc 

    axstm 8

    get 

    push 

    setrxi 0

    push 

    axstm 4

    get 

    push 

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    chkbi 4

    setrxi 4

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    outs 

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    setrxi 1

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    axstm 4

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    chkbi 4

    setrxi 4

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    axstm 4

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    setrxi 0

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    addi 

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    movra 

    get 

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    setrxi 1

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    pop 

    outs 

    setrxi string5

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    pop 

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    setrxi 1

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    get 

    push 

    axstm 12

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    movra 

    pop 

    chkbi 4

    setrxi 4

    push 

    muli 

    addi 

    pop 

    movra 

    get 

    push 

    setrxi 1

    push 

    axstm 4

    get 

    push 

    axstm 12

    get 

    movra 

    pop 

    chkbi 4

    setrxi 4

    push 

    muli 

    addi 

    pop 

    movra 

    get 

    push 

    pop 

    outs 

    setrxi string5

    push 

    pop 

    outs 

    pop 

    movra 

    decref 

    pop 

    movra 

    decref 

    gc 

    exit 



-1 1 0 string5: 10b 

-1 5 0 string7: "froob"

-1 5 0 string1: "hello"

-1 2 0 string3: "hi"

-1 4 0 string6: "paul"

-1 5 0 string4: "there"

-1 5 0 string2: "world"



And correctly outputs what you would expect if deep copies of the arrays were being made. In fact the two arrays point to the same memory until the c[0][1]="paul" line, at which point a clone is created which c points to. When d[1][1]="froob" is executed, the same happens but as d only now has one reference, the cloning is skipped.

My god this stuff is hard to get right and to debug when it goes wrong. Wish my project was proceeding at the pace that Deyja's VM/compiler project was, but then I've written lots of these before and really wanted to see if I could get this nonsense to work.

This is an example of the VM running in debug mode, just to give you an idea of what I am staring at while going mad trying to work out what is broken.


Om 1.0 Copyright©2011 Aardvajk Software

REGISTERS:

    pc  : 5

    ir  : setrxi (11)

    st  : 1402

    rx.i: 2

    rx.f: 2.8026e-045

    ax  : 2004270470

    cx  : -1

ALLOCATED BLOCKS:

FREE BLOCKS:

2426: 1024

MEMORY SIZE: 3450

REGISTERS:

    pc  : 6

    ir  : push (21)

    st  : 1406

    rx.i: 2

    rx.f: 2.8026e-045

    ax  : 2004270470

    cx  : -1

ALLOCATED BLOCKS:

FREE BLOCKS:

2426: 1024

MEMORY SIZE: 3450

REGISTERS:

    pc  : 11

    ir  : setrxi (11)

    st  : 1406

    rx.i: 4

    rx.f: 5.60519e-045

    ax  : 2004270470

    cx  : -1

ALLOCATED BLOCKS:

FREE BLOCKS:

2426: 1024

MEMORY SIZE: 3450

ALLOCATED BLOCKS:

STACK:

1402: 2 (2)

1403: 0

1404: 0

1405: 0



Need to think about array resizing next really. Not much point having dynamic arrays in a language if they, er, aren't.




Part of the reason I'm moving so fast is that I'm writing the virtual machine in C#. My stack is a stack of Objects, my registers are Objects, and my 'classes' are an array of objects. I pay a performance penalty but I don't have to deal with figuring out the size of objects on the stack or do any sort of memory management.
Yeah, I think these are very different projects, although it's kind of fun that there are two VM/compiler projects running at a roughly similar development level in journal-land at the same time.

Mine is a complete curiosity. Being written purely for the masochistic joy of seeing if I can make this work.
Have you implemented virtual functions yet?
Hell no, not even implemented normal functions yet. :)

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