condbranches-plus2.patch

http://bugs.python.org/issue4715

This patch by Antoine Pitrou optimizes the bytecode for conditional branches.
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Furthermore, the old jump opcodes (JUMP_IF_FALSE, JUMP_IF_TRUE) have been
replaced by two new opcodes: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

[Collin Winter]

LGTM