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Furthermore, the old jump opcodes (JUMP_IF_FALSE, JUMP_IF_TRUE) have been
replaced by two new opcodes: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

1 files changed, 5 insertions, 5 deletions

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 @@ def dis_single(line):
 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 @@ class TestTranforms(unittest.TestCase):
         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)