91 lines
3.1 KiB
Markdown
91 lines
3.1 KiB
Markdown
# VM
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## Architecture
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## Understanding the trace output
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Once set up you can try some simple javascript in your test file. For example:
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```js
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let a = 1;
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let b = 2;
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```
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Outputs:
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```text
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----------------------Compiled Output: '<main>'-----------------------
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Location Count Opcode Operands
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000001 0000 PushOne
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000006 0001 DefInitLet 0000: 'a'
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000008 0002 PushInt8 2
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000013 0003 DefInitLet 0001: 'b'
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Literals:
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<empty>
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Bindings:
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0000: a
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0001: b
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Functions:
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<empty>
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------------------------------------------ VM Start ------------------------------------------
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Time Opcode Operands Top Of Stack
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386μs PushOne 1
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6μs DefInitLet 0000: 'a' <empty>
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1μs PushInt8 2 2
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2μs DefInitLet 0001: 'b' <empty>
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Stack:
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<empty>
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undefined
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```
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The above output contains the following information:
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- The bytecode and properties of the function that will be executed
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- `Compiled Output`: The bytecode.
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- `Location`: Location of the instruction (instructions are not the same size).
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- `Count`: Instruction count.
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- `Opcode`: Opcode name.
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- `Operands`: The operands of the opcode.
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- `Literals`: The literals used by the bytecode (like strings).
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- `Bindings`: Binding names used by the bytecode.
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- `Functions`: Function names use by the bytecode.
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- The code being executed (marked by `Vm Start` or `Call Frame`).
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- `Time`: The amount of time that instruction took to execute.
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- `Opcode`: Opcode name.
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- `Operands`: The operands of the opcode.
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- `Top Of Stack`: The top element of the stack **after** execution of instruction.
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- `Stack`: The trace of the stack after execution ends.
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- The result of the execution (The top element of the stack, if the stack is empty then `undefined` is returned).
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### Comparing ByteCode output
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If you wanted another engine's bytecode output for the same JS, SpiderMonkey's bytecode output is the best to use. You can follow the setup [here](https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Introduction_to_the_JavaScript_shell). You will need to build from source because the pre-built binarys don't include the debugging utilities which we need.
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I named the binary `js_shell` as `js` conflicts with NodeJS. Once up and running you should be able to use `js_shell -f tests/js/test.js`. You will get no output to begin with, this is because you need to run `dis()` or `dis([func])` in the code. Once you've done that you should get some output like so:
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```text
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loc op
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----- --
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00000: GlobalOrEvalDeclInstantiation 0 #
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main:
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00005: One # 1
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00006: InitGLexical "a" # 1
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00011: Pop #
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00012: Int8 2 # 2
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00014: InitGLexical "b" # 2
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00019: Pop #
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00020: GetGName "dis" # dis
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```
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