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 Furthermore, the old jump opcodes (JUMP_IF_FALSE, JUMP_IF_TRUE) have been replaced by two new opcodes: - JUMP_IF_TRUE_OR_POP, which jumps if true and pops otherwise - JUMP_IF_FALSE_OR_POP, which jumps if false and pops otherwise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/Include/opcode.h	Wed Feb 25 00:58:47 2009 +0000
+++ b/Include/opcode.h	Wed Feb 25 02:25:04 2009 +0000
@@ -96,9 +96,11 @@
 #define IMPORT_FROM	109	/* Index in name list */
 
 #define JUMP_FORWARD	110	/* Number of bytes to skip */
-#define JUMP_IF_FALSE	111	/* "" */
-#define JUMP_IF_TRUE	112	/* "" */
-#define JUMP_ABSOLUTE	113	/* Target byte offset from beginning of code */
+#define JUMP_IF_FALSE_OR_POP 111	/* Target byte offset from beginning of code */
+#define JUMP_IF_TRUE_OR_POP 112	/* "" */
+#define JUMP_ABSOLUTE	113	/* "" */
+#define POP_JUMP_IF_FALSE 114	/* "" */
+#define POP_JUMP_IF_TRUE 115	/* "" */
 
 #define LOAD_GLOBAL	116	/* Index in name list */