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Furthermore, the old jump opcodes (JUMP_IF_FALSE, JUMP_IF_TRUE) have been
replaced by two new opcodes: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/Include/opcode.h b/Include/opcode.h
index 063ab6c..2b1c59d 100644
--- a/Include/opcode.h
+++ b/Include/opcode.h
@@ -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 */