forked from M-Labs/artiq
1
0
Fork 0
artiq/artiq/compiler/transforms/dead_code_eliminator.py

84 lines
2.9 KiB
Python

"""
:class:`DeadCodeEliminator` is a dead code elimination transform:
it only basic blocks with no predecessors as well as unused
instructions without side effects.
"""
from .. import ir
class DeadCodeEliminator:
def __init__(self, engine):
self.engine = engine
def process(self, functions):
for func in functions:
self.process_function(func)
def process_function(self, func):
# defer removing those blocks, so our use checks will ignore deleted blocks
preserve = [func.entry()]
work_list = [func.entry()]
while any(work_list):
block = work_list.pop()
for succ in block.successors():
if succ not in preserve:
preserve.append(succ)
work_list.append(succ)
to_be_removed = []
for block in func.basic_blocks:
if block not in preserve:
block.is_removed = True
to_be_removed.append(block)
for insn in block.instructions:
insn.is_removed = True
for block in to_be_removed:
self.remove_block(block)
modified = True
while modified:
modified = False
for insn in func.instructions():
# Note that GetLocal is treated as an impure operation:
# the local access validator has to observe it to emit
# a diagnostic for reads of uninitialized locals, and
# it also has to run after the interleaver, but interleaver
# doesn't like to work with IR before DCE.
if isinstance(insn, (ir.Phi, ir.Alloc, ir.GetAttr, ir.GetElem, ir.Coerce,
ir.Arith, ir.Compare, ir.Select, ir.Quote, ir.Closure,
ir.Offset)) \
and not any(insn.uses):
insn.erase()
modified = True
def remove_block(self, block):
# block.uses are updated while iterating
for use in set(block.uses):
if use.is_removed:
continue
if isinstance(use, ir.Phi):
use.remove_incoming_block(block)
if not any(use.operands):
self.remove_instruction(use)
elif isinstance(use, ir.SetLocal):
# setlocal %env, %block is only used for lowering finally
use.erase()
else:
assert False
block.erase()
def remove_instruction(self, insn):
for use in set(insn.uses):
if use.is_removed:
continue
if isinstance(use, ir.Phi):
use.remove_incoming_value(insn)
if not any(use.operands):
self.remove_instruction(use)
else:
assert False
insn.erase()