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bpo-42246: Eliminate jumps to exit blocks by copying those blocks. by markshannon · Pull Request #23251 · python/cpython · GitHub

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bpo-42246: Eliminate jumps to exit blocks by copying those blocks. - #23251

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markshannon:compiler-eliminate-jump-to-exits
Nov 12, 2020
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bpo-42246: Eliminate jumps to exit blocks by copying those blocks.#23251
markshannon merged 5 commits into
python:masterfrom
markshannon:compiler-eliminate-jump-to-exits

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markshannon commented Nov 12, 2020 •
edited by bedevere-bot
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More yak-shaving for PEP 626.

This PR:

  • Marks all blocks that exit the scope or that do not fall through to the b_next block.
  • Removes the a_reverse_postorder array in the assembler, as it is just a copy of the b->b_next chain.
  • Eliminates unconditional jumps to short exit blocks, by copying the exit blocks onto the end of its predecessor.

The net effect of this is that:

def f(c):
    if c:
        A
    else:
        B

now compiles to:

  2           0 LOAD_FAST                0 (c)
              2 POP_JUMP_IF_FALSE       12

  3           4 LOAD_GLOBAL              0 (A)
              6 POP_TOP
              8 LOAD_CONST               0 (None)
             10 RETURN_VALUE

  5     >>   12 LOAD_GLOBAL              1 (B)
             14 POP_TOP
             16 LOAD_CONST               0 (None)
             18 RETURN_VALUE

instead of

  2           0 LOAD_FAST                0 (c)
              2 POP_JUMP_IF_FALSE       10

  3           4 LOAD_GLOBAL              0 (A)
              6 POP_TOP
              8 JUMP_FORWARD             4 (to 14)

  5     >>   10 LOAD_GLOBAL              1 (B)
             12 POP_TOP
        >>   14 LOAD_CONST               0 (None)
             16 RETURN_VALUE

This has two effects

  1. Means that f_lineno can be determined from f_lasti even after an implicit return. This is needed for PEP 626.
  2. Reduces the number of bytecodes executed at the cost of a minor increase in bytecode size, and might result in a tiny speedup. I haven't measured it, as I expect the change to be too small to meaningful, or even measurable.

https://bugs.python.org/issue42246

markshannon force-pushed the compiler-eliminate-jump-to-exits branch from 1930bc7 to 0d2af11 Compare November 12, 2020 16:02
markshannon merged commit cc75ab7 into python:master Nov 12, 2020

bedevere-bot commented Nov 12, 2020

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@markshannon: Please replace # with GH- in the commit message next time. Thanks!

adorilson pushed a commit to adorilson/cpython that referenced this pull request Mar 13, 2021
bpo-42246: Eliminate jumps to exit blocks by copying those blocks. (p…
441e23d 
…ython#23251)

* Compiler: eliminate jumps to short exit blocks by copying.
tekknolagi mentioned this pull request Jun 12, 2023
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