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 Furthermore, the old jump opcodes (JUMP_IF_FALSE, JUMP_IF_TRUE) have been replaced by two new opcodes: - JUMP_IF_TRUE_OR_POP, which jumps if true and pops otherwise - JUMP_IF_FALSE_OR_POP, which jumps if false and pops otherwise

--- a/Lib/opcode.py	Wed Feb 25 00:58:47 2009 +0000
+++ b/Lib/opcode.py	Wed Feb 25 02:25:04 2009 +0000
@@ -131,9 +131,11 @@
 name_op('IMPORT_FROM', 109)     # Index in name list
 
 jrel_op('JUMP_FORWARD', 110)    # Number of bytes to skip
-jrel_op('JUMP_IF_FALSE', 111)   # ""
-jrel_op('JUMP_IF_TRUE', 112)    # ""
-jabs_op('JUMP_ABSOLUTE', 113)   # Target byte offset from beginning of code
+jabs_op('JUMP_IF_FALSE_OR_POP', 111) # Target byte offset from beginning of code
+jabs_op('JUMP_IF_TRUE_OR_POP', 112)  # ""
+jabs_op('JUMP_ABSOLUTE', 113)        # ""
+jabs_op('POP_JUMP_IF_FALSE', 114)    # ""
+jabs_op('POP_JUMP_IF_TRUE', 115)     # ""
 
 name_op('LOAD_GLOBAL', 116)     # Index in name list