http://bugs.python.org/issue4715
This patch by Antoine Pitrou optimizes the bytecode for conditional branches by
merging the following "POP_TOP" instruction into the conditional jump. For
example, the list comprehension "[x for x in l if not x]" produced the
following bytecode:
1 0 BUILD_LIST 0
3 LOAD_FAST 0 (.0)
>> 6 FOR_ITER 23 (to 32)
9 STORE_FAST 1 (x)
12 LOAD_FAST 1 (x)
15 JUMP_IF_TRUE 10 (to 28)
18 POP_TOP
19 LOAD_FAST 1 (x)
22 LIST_APPEND 2
25 JUMP_ABSOLUTE 6
>> 28 POP_TOP
29 JUMP_ABSOLUTE 6
>> 32 RETURN_VALUE
but after the patch it produces the following bytecode:
1 0 BUILD_LIST 0
3 LOAD_FAST 0 (.0)
>> 6 FOR_ITER 18 (to 27)
9 STORE_FAST 1 (x)
12 LOAD_FAST 1 (x)
15 POP_JUMP_IF_TRUE 6
18 LOAD_FAST 1 (x)
21 LIST_APPEND 2
24 JUMP_ABSOLUTE 6
>> 27 RETURN_VALUE
Notice that not only the code is shorter, but the conditional jump
(POP_JUMP_IF_TRUE) jumps right to the start of the loop instead of going through
the JUMP_ABSOLUTE at the end. "continue" statements are helped
similarly.
Furthermore, the old jump opcodes (JUMP_IF_FALSE, JUMP_IF_TRUE) have been
replaced by two new opcodes:
- JUMP_IF_TRUE_OR_POP, which jumps if true and pops otherwise
- JUMP_IF_FALSE_OR_POP, which jumps if false and pops otherwise
diff --git a/Lib/test/test_peepholer.py b/Lib/test/test_peepholer.py
index f3e240e..1543f3b 100644
--- a/Lib/test/test_peepholer.py
+++ b/Lib/test/test_peepholer.py
@@ -19,14 +19,14 @@
class TestTranforms(unittest.TestCase):
def test_unot(self):
- # UNARY_NOT JUMP_IF_FALSE POP_TOP --> JUMP_IF_TRUE POP_TOP'
+ # UNARY_NOT POP_JUMP_IF_FALSE --> POP_JUMP_IF_TRUE'
def unot(x):
if not x == 2:
del x
asm = disassemble(unot)
- for elem in ('UNARY_NOT', 'JUMP_IF_FALSE'):
+ for elem in ('UNARY_NOT', 'POP_JUMP_IF_FALSE'):
self.assert_(elem not in asm)
- for elem in ('JUMP_IF_TRUE', 'POP_TOP'):
+ for elem in ('POP_JUMP_IF_TRUE',):
self.assert_(elem in asm)
def test_elim_inversion_of_is_or_in(self):
@@ -64,13 +64,13 @@
self.assert_('LOAD_GLOBAL' not in disassemble(f))
def test_while_one(self):
- # Skip over: LOAD_CONST trueconst JUMP_IF_FALSE xx POP_TOP
+ # Skip over: LOAD_CONST trueconst POP_JUMP_IF_FALSE xx
def f():
while 1:
pass
return list
asm = disassemble(f)
- for elem in ('LOAD_CONST', 'JUMP_IF_FALSE'):
+ for elem in ('LOAD_CONST', 'POP_JUMP_IF_FALSE'):
self.assert_(elem not in asm)
for elem in ('JUMP_ABSOLUTE',):
self.assert_(elem in asm)