feat(wpcarro/scratch): Proof-of-concept register VM

I heard that register VMs might be slightly faster than stack VMs, and then it
occurred to me that I wouldn't know how to write a register VM if I tried. So I
wrote one (sort of).

Change-Id: I15309bca88f4b43f6e04957acedc90d9adf16673
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6902
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

users/wpcarro/scratch/compiler/register_vm.py

@@ -0,0 +1,161 @@
+# Silly proof-of-concept register VM.
+
+def compile_binary_op(op, ast):
+    result = []
+    for x in compile(ast[1]):
+        result.append(x)
+    result.append(PUSH_REG)
+    result.append(RES)
+    for x in compile(ast[2]):
+        result.append(x)
+    result.append(ASSIGN_REG_REG)
+    result.append(Y)
+    result.append(RES)
+    result.append(POP)
+    result.append(X)
+    result.append(op)
+    return result
+
+def compile(ast):
+    result = []
+
+    if ast[0] == 'CONST':
+        result.append(ASSIGN_REG_LIT)
+        result.append(RES)
+        result.append(ast[1])
+    elif ast[0] == 'ADD':
+        result += compile_binary_op(ADD, ast)
+    elif ast[0] == 'SUB':
+        result += compile_binary_op(SUB, ast)
+    elif ast[0] == 'MUL':
+        result += compile_binary_op(MUL, ast)
+    elif ast[0] == 'DIV':
+        result += compile_binary_op(DIV, ast)
+    elif ast[0] == 'RETURN':
+        result.append(RETURN)
+    else:
+        raise Exception('Cannot compile unknown AST node: {}'.format(ast[0]))
+
+    return result
+
+# opcodes
+ASSIGN_REG_LIT = 0x0
+ASSIGN_REG_REG = 0x1
+ADD = 0x2
+SUB = 0x3
+MUL = 0x4
+DIV = 0x5
+SWAP = 0x6
+RETURN = 0x7
+PUSH_REG = 0x8
+POP = 0x9
+
+# register indices
+X = 0x0
+Y = 0x1
+RES = 0x2
+
+registers = [0x0] * 8
+stack = []
+
+def reg_name(i):
+    if i == X: return 'x'
+    if i == Y: return 'x'
+    if i == RES: return 'res'
+
+def print_instructions(xs):
+    i = 0
+
+    while i < len(xs):
+        if xs[i] == ASSIGN_REG_LIT:
+            # print('ASSIGN_REG_LIT {} {}'.format(reg_name(xs[i + 1]), xs[i + 2]))
+            print('{} <- {}'.format(reg_name(xs[i + 1]), xs[i + 2]))
+            i += 3
+        elif xs[i] == ASSIGN_REG_REG:
+            # print('ASSIGN_REG_REG {} {}'.format(reg_name(xs[i + 1]), reg_name(xs[i + 2])))
+            print('{} <- ${}'.format(reg_name(xs[i + 1]), reg_name(xs[i + 2])))
+            i += 3
+        elif xs[i] == ADD:
+            print('add')
+            i += 1
+        elif xs[i] == SUB:
+            print('sub')
+            i += 1
+        elif xs[i] == MUL:
+            print('mul')
+            i += 1
+        elif xs[i] == DIV:
+            print('div')
+            i += 1
+        elif xs[i] == PUSH_REG:
+            print('push ${}'.format(reg_name(xs[i + 1])))
+            i += 2
+        elif xs[i] == POP:
+            print('{} <- pop'.format(reg_name(xs[i + 1])))
+            i += 2
+        else:
+            raise Exception('Cannot print instruction: {}'.format(xs[i]))
+
+def eval(instructions):
+    print_instructions(instructions)
+    ip = 0
+    cont = True
+    while ip < len(instructions):
+        if instructions[ip] == ASSIGN_REG_LIT:
+            r = instructions[ip + 1]
+            x = instructions[ip + 2]
+            registers[r] = x
+            ip += 3
+        elif instructions[ip] == ASSIGN_REG_REG:
+            r_dst = instructions[ip + 1]
+            r_src = instructions[ip + 2]
+            registers[r_dst] = registers[r_src]
+            ip += 3
+        elif instructions[ip] == ADD:
+            registers[RES] = registers[X] + registers[Y]
+            ip += 1
+        elif instructions[ip] == MUL:
+            registers[RES] = registers[X] * registers[Y]
+            ip += 1
+        elif instructions[ip] == SUB:
+            registers[RES] = registers[X] - registers[Y]
+            ip += 1
+        elif instructions[ip] == MUL:
+            registers[RES] = registers[X] * registers[Y]
+            ip += 1
+        elif instructions[ip] == DIV:
+            registers[RES] = registers[X] / registers[Y]
+            ip += 1
+        elif instructions[ip] == SWAP:
+            r1 = instructions[ip + 1]
+            r2 = instructions[ip + 2]
+            registers[r1], registers[r2] = registers[r2], registers[r1]
+            ip += 3
+        elif instructions[ip] == RETURN:
+            ip += 1
+            cont = False
+            return registers[RES]
+        elif instructions[ip] == PUSH_REG:
+            src = instructions[ip + 1]
+            stack.append(registers[src])
+            ip += 2
+        elif instructions[ip] == POP:
+            dst = instructions[ip + 1]
+            registers[dst] = stack.pop()
+            ip += 2
+        else:
+            raise Exception('Cannot eval instruction: {}'.format(instructions[ip]))
+    return registers[RES]
+
+def main():
+    ast = ['ADD',
+           ['MUL',
+            ['MUL', ['CONST', 2], ['CONST', 3]],
+            ['DIV', ['CONST', 5], ['CONST', 5]]],
+           ['ADD',
+            ['SUB', ['CONST', 10], ['CONST', 1]],
+            ['MUL', ['CONST', 2], ['CONST', 2]]]]
+
+    print('result: {}'.format(eval(compile(ast))))
+
+main()