Injecting metering in Fizzy's parser

fizzy_metering.cpp

// ControlFrame new members:
struct ControlFrame
{
  // ...
  int gas;
  size_t metering_immediate_offset; 
};

void insert_metering_statement()
{
  code.instructions.emplace_back(Instr::i32_const);
  code.immediates.emplace_back(0); // cost immediate to be updated later
  code.instructions.emplace_bac(Instr::call);
  code.immdeidates.emplace_back(MeterFunctionIndex);
}

void update_metering_immediate(offset, cost)
{
  store(code.immediates.data() + offset, cost)
}
  

// Parser loop changes

// for each instruction, before switch:
  if (!frame.unreachable)
  	frame.gas += gas_table[instr];

// This is currently below the switch, it will have to be moved to before, 
// with adjusting offsets of jumps inside the switch
code.instructions.emplace_back(instr);
// Or without moving we could raise a flag inside the switch that would indicate,
// that metering statement needs to be inserted after current instruction

switch(instr)
{

case loop:
case block:
case if:
case else:
  if (!frame.unreachable)
    update_metering_immediate(frame.metering_immediate_offset, frame.gas);
  push control frame: gas = 0, metering_immediate_offset = immediates.size()
  insert_metering_statement();


case br:
case br_table:
case return:
case unreachable:
  if (!frame.unreachable)
    update_metering_immediate(frame.metering_immediate_offset, frame.gas);
  mark_frame_unreachable();


case br_if:
  if (!frame.unreachable)
	update_metering_immediate(frame.metering_immediate_offset, frame.gas);
  frame.gas = 0;
  frame.metering_immediate_offset = immediates.size();
  insert_metering_statement();


case end:
  if (!frame.unreachable)
    update_metering_immediate(frame.metering_immediate_offset, frame.gas);
  pop control frame
  if (!control_stack.empty())
    insert_metering_statement();
	  
}