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Furthermore, the old jump opcodes (JUMP_IF_FALSE, JUMP_IF_TRUE) have been
replaced by two new opcodes: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

Include/opcode.h

@@ -96,9 +96,11 @@ extern "C" {
 #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 */