Indefinite Studies

Here, you can use “virtual machine” as in “Java Virtual Machine”, not as in virtualization. We will play with the virtual machine described in the paper Tracing the Meta-Level: PyPy’s Tracing JIT Compiler by C.F. Bolz, A. Cuni, M. Fijalkowski and A. Rigo (it’s a great read by the way).

PyPy is a fascinating project, too complex to describe here. Among other things, PyPy can take any interpreter written in a subset of Python, translate it to C, and automatically generate a JIT compiler for this language. Does it sound too good to be true? Let’s try this.

• grab PyPy source code
• create the interpreter in pypy/translator/goal/target-toy.py with the following code:

</pre>
<pre style="text-align: justify;">import os, sys
import autopath
import py

# these are the opcodes for the interpreted language
JUMP_IF_A  = 1
MOV_A_R    = 2
MOV_R_A    = 3
ADD_R_TO_A = 4
DECR_A     = 5
RETURN_A   = 6

<em>from pypy.rlib.jit import JitDriver tlrjitdriver = JitDriver(greens = ['pc', 'bytecode'], reds = ['a', 'regs'])</em>

# the main interpreter loop
def interpret(bytecode, a):
   regs = [0] * 256
   pc = 0
   while True:
<em> tlrjitdriver.jit_merge_point(bytecode=bytecode, pc=pc, a=a, regs=regs) </em>       opcode = bytecode[pc]
       pc += 1
       if opcode == JUMP_IF_A:
           target = bytecode[pc]
           pc += 1
           if a:
<em> if target<pc: tlrjitdriver.can_enter_jit(bytecode=bytecode, pc=target, a=a, regs=regs) </em>               pc = target
       elif opcode == MOV_A_R:
           n = bytecode[pc]
           pc += 1
           regs[n] = a
       elif opcode == MOV_R_A:
           n = bytecode[pc]
           pc += 1
           a = regs[n]
       elif opcode == ADD_R_TO_A:
           n = bytecode[pc]
           pc += 1
           a += regs[n]
       elif opcode == DECR_A:
           a -= 1
       elif opcode == RETURN_A:
           return a

# __________  Entry point  __________
def entry_point(argv):
    # the program we want to interpret
    # it computes the square of its argument
    bytecode = [
        MOV_A_R,    0, # i = a
        MOV_A_R,    1, # copy of ’a’
        # 4:
        MOV_R_A,    0, # i--
        DECR_A,
        MOV_A_R,    0,
        MOV_R_A,    2, # res += a
        ADD_R_TO_A, 1,
        MOV_A_R,    2,
        MOV_R_A,    0, # if i!=0: goto 4
        JUMP_IF_A,  4,
        MOV_R_A,    2,
        RETURN_A
    ]
    result = interpret(bytecode, int(argv[1]))
    print result
    return 0

def jitpolicy(driver):
    from pypy.jit.metainterp.policy import JitPolicy
    return JitPolicy()

# _____ Define and setup target ___
def target(*args):
    return entry_point, None

# main function, if this script is called from the command line
if __name__ == '__main__':
    entry_point(sys.argv)</pre>
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