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http://bugs.python.org/issue4715

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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Furthermore, the old jump opcodes (JUMP_IF_FALSE, JUMP_IF_TRUE) have been
replaced by two new opcodes: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
This commit is contained in:
Jeffrey Yasskin 2009-02-25 02:25:04 +00:00
parent 0a68b01d64
commit 9de7ec7868
9 changed files with 195 additions and 129 deletions
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8
Python/opcode_targets.h
View file

@ -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,