Tracing JIT

[giann]

• Profile loops: super simple right now, a simple counter
• When hits threshold, compile the AST node into a native function with no arguments
• Patch the bytecode to replace the loop with the function call

Example:

| Say this triggers compilation
for (int i = 0; i < 100; i = i + 1) {
    std.print(">{i}");
}

Bytecode:

006 |   OP_HOTSPOT  30 125  -> Will check here if loops deserves to be compiled
008 |   OP_GET_LOCAL    3
009 |   OP_CONSTANT 4 100
010 |   OP_LESS 
011 |   OP_JUMP_IF_FALSE    11 + 16 -> 28
012 |   OP_POP  
013 |   OP_JUMP 13 + 6 -> 20
014 |   OP_GET_LOCAL    3
015 |   OP_CONSTANT 5 1
016 |   OP_ADD_I    
017 |   OP_SET_LOCAL    3
018 |   OP_POP  
019 |   OP_LOOP 19 - 14 -> 6
020 008 OP_GET_GLOBAL   1
021 |   OP_CONSTANT 6 >
022 |   OP_GET_LOCAL    3
023 |   OP_TO_STRING    
024 |   OP_ADD_STRING   
025 |   OP_CALL 1 0
026 |   OP_POP  
027 007 OP_LOOP 27 - 14 -> 14
028 |   OP_POP  
029 |   OP_POP

Once compiled, we patch the bytecode to call the hotspot function rather than doing the loop:

006 |   OP_JUMP 6 + 21 -> 28  -> We jump directly at the hotspot call
007 |   OP_VOID 
008 |   OP_GET_LOCAL    3
009 |   OP_CONSTANT 4 100
010 |   OP_LESS 
011 |   OP_JUMP_IF_FALSE    11 + 16 -> 28
012 |   OP_POP  
013 |   OP_JUMP 13 + 6 -> 20
014 |   OP_GET_LOCAL    3
015 |   OP_CONSTANT 5 1
016 |   OP_ADD_I    
017 |   OP_SET_LOCAL    3
018 |   OP_POP  
019 |   OP_LOOP 19 - 14 -> 6
020 008 OP_GET_GLOBAL   1
021 |   OP_CONSTANT 6 >
022 |   OP_GET_LOCAL    3
023 |   OP_TO_STRING    
024 |   OP_ADD_STRING   
025 |   OP_CALL 1 0
026 |   OP_POP  
027 007 OP_LOOP 27 - 14 -> 14
028 |   OP_CONSTANT 7 native: 0x100d3b720 -> Push the hotspot function
029 |   OP_HOTSPOT_CALL                       -> Call it

We keep the dead bytecode to avoid patching other OP_JUMP outside of this particular loop.