dce.py

   1 import logging
   2 from core import *
   3
   4
   5 _log = logging.getLogger(__file__)
   6
   7
   8 def make_dead(insts, idx):
   9     org_inst = insts[idx]
  10     if org_inst.op == "DEAD":
  11         return
  12     if org_inst.side_effect():
  13         org_inst.dest = None
  14     else:
  15         dead = Inst(None, "DEAD", [])
  16         dead.addr = org_inst.addr
  17         insts[idx] = dead
  18         insts[idx].comments["org_inst"] = org_inst
  19
  20
  21 def dead_code_elimination_forward(bblock):
  22     """Try to perform eliminations using forward flow. This is reverse
  23     to the natural direction, and requires multiple passing over
  24     bblock to stabilize. Don't use it, here only for comparison."""
  25
  26     vars = bblock.defs()
  27     for v in vars:
  28         last = None
  29         for i, inst in enumerate(bblock.items):
  30             if v in inst.args:
  31                 last = None
  32             if inst.dest == v:
  33                 if last is not None:
  34                     make_dead(bblock.items, last)
  35                 last = i
  36         node = bblock.cfg[bblock.addr]
  37         live_out = node.get("live_out")
  38         if last is not None and live_out is not None:
  39             if v not in live_out:
  40                 make_dead(bblock.items, last)
  41
  42
  43 def dead_code_elimination_backward(bblock):
  44     node = bblock.cfg[bblock.addr]
  45     live = node.get("live_out")
  46     if live is None:
  47         _log.warn("BBlock %s: No live_out set, conservatively assuming all defined vars are live", bblock.addr)
  48         live = bblock.defs()
  49     live = live.copy()
  50
  51     changes = False
  52     for i in range(len(bblock.items) - 1,  -1, -1):
  53         inst = bblock.items[i]
  54         if isinstance(inst.dest, REG):
  55             if inst.dest in live:
  56                 live.remove(inst.dest)
  57             else:
  58                 make_dead(bblock.items, i)
  59                 changes = True
  60                 inst = bblock.items[i]
  61         live |= inst.uses()
  62
  63     return changes
  64
  65
  66 dead_code_elimination = dead_code_elimination_backward