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Furthermore, the old jump opcodes (JUMP_IF_FALSE, JUMP_IF_TRUE) have been
replaced by two new opcodes: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, 16 insertions, 6 deletions

diff --git a/Doc/library/dis.rst b/Doc/library/dis.rst
index 7dc8a3c..856a9e4 100644
--- a/Doc/library/dis.rst
+++ b/Doc/library/dis.rst
@@ -595,16 +595,26 @@ the more significant byte last.
    Increments bytecode counter by *delta*.
 
 
-.. opcode:: JUMP_IF_TRUE (delta)
+.. opcode:: POP_JUMP_IF_TRUE (target)
 
-   If TOS is true, increment the bytecode counter by *delta*.  TOS is left on the
-   stack.
+   If TOS is true, sets the bytecode counter to *target*.  TOS is popped.
 
 
-.. opcode:: JUMP_IF_FALSE (delta)
+.. opcode:: POP_JUMP_IF_FALSE (target)
 
-   If TOS is false, increment the bytecode counter by *delta*.  TOS is not
-   changed.
+   If TOS is false, sets the bytecode counter to *target*.  TOS is popped.
+
+
+.. opcode:: JUMP_IF_TRUE_OR_POP (target)
+
+   If TOS is true, sets the bytecode counter to *target* and leaves TOS
+   on the stack.  Otherwise (TOS is false), TOS is popped.
+
+
+.. opcode:: JUMP_IF_FALSE_OR_POP (target)
+
+   If TOS is false, sets the bytecode counter to *target* and leaves
+   TOS on the stack.  Otherwise (TOS is true), TOS is popped.
 
 
 .. opcode:: JUMP_ABSOLUTE (target)