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Furthermore, the old jump opcodes (JUMP_IF_FALSE, JUMP_IF_TRUE) have been
replaced by two new opcodes: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, 4 insertions, 4 deletions

diff --git a/Python/opcode_targets.h b/Python/opcode_targets.h
index c66271756e6..043e42a30a5 100644
--- a/Python/opcode_targets.h
+++ b/Python/opcode_targets.h
@@ -110,11 +110,11 @@ static void *opcode_targets[256] = {
 	&&TARGET_IMPORT_NAME,
 	&&TARGET_IMPORT_FROM,
 	&&TARGET_JUMP_FORWARD,
-	&&TARGET_JUMP_IF_FALSE,
-	&&TARGET_JUMP_IF_TRUE,
+	&&TARGET_JUMP_IF_FALSE_OR_POP,
+	&&TARGET_JUMP_IF_TRUE_OR_POP,
 	&&TARGET_JUMP_ABSOLUTE,
-	&&_unknown_opcode,
-	&&_unknown_opcode,
+	&&TARGET_POP_JUMP_IF_FALSE,
+	&&TARGET_POP_JUMP_IF_TRUE,
 	&&TARGET_LOAD_GLOBAL,
 	&&_unknown_opcode,
 	&&_unknown_opcode,