Rust编写的JavaScript引擎,该项目是一个试验性质的项目。
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# VM
## Architecture
![image](img/boa_architecture.png)
## Understanding the trace output
Once set up you can try some simple javascript in your test file. For example:
```js
let a = 1;
let b = 2;
```
Outputs:
```text
----------------------Compiled Output: '<main>'-----------------------
Location Count Opcode Operands
000001 0000 PushOne
000006 0001 DefInitLet 0000: 'a'
000008 0002 PushInt8 2
000013 0003 DefInitLet 0001: 'b'
Literals:
<empty>
Bindings:
0000: a
0001: b
Functions:
<empty>
------------------------------------------ VM Start ------------------------------------------
Time Opcode Operands Top Of Stack
386μs PushOne 1
6μs DefInitLet 0000: 'a' <empty>
1μs PushInt8 2 2
2μs DefInitLet 0001: 'b' <empty>
Stack:
<empty>
undefined
```
The above output contains the following information:
- The bytecode and properties of the function that will be executed
- `Compiled Output`: The bytecode.
- `Location`: Location of the instruction (instructions are not the same size).
- `Count`: Instruction count.
- `Opcode`: Opcode name.
- `Operands`: The operands of the opcode.
- `Literals`: The literals used by the bytecode (like strings).
- `Bindings`: Binding names used by the bytecode.
- `Functions`: Function names use by the bytecode.
- The code being executed (marked by `Vm Start` or `Call Frame`).
- `Time`: The amount of time that instruction took to execute.
- `Opcode`: Opcode name.
- `Operands`: The operands of the opcode.
- `Top Of Stack`: The top element of the stack **after** execution of instruction.
- `Stack`: The trace of the stack after execution ends.
- The result of the execution (The top element of the stack, if the stack is empty then `undefined` is returned).
### Comparing ByteCode output
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://udn.realityripple.com/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.
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:
```text
loc op
----- --
00000: GlobalOrEvalDeclInstantiation 0 #
main:
00005: One # 1
00006: InitGLexical "a" # 1
00011: Pop #
00012: Int8 2 # 2
00014: InitGLexical "b" # 2
00019: Pop #
00020: GetGName "dis" # dis
```