New submission from Andrew Dalke:
Others have reported issues like #21074 where the peephole compiler generates
and discards large strings, and #30293 where it generates multi-MB integers and
stores them in the .pyc.
This is a different issue. The code:
def tuple20():
return ((((((((1,)*20,)*20,)*20,)*20,)*20,)*20,)*20,)*20
takes over four minutes (257 seconds) to compile on my machine. The seemingly
larger:
def tuple30():
return ((((((((1,)*30,)*30,)*30,)*30,)*30,)*30,)*30,)*30
takes a small fraction of a second to compile, and is equally fast to run.
Neither code generates a large data structure. In fact, they only needs about
1K.
A sampling profiler of the first case, taken around 30 seconds into the run,
shows that nearly all of the CPU time is spent in computing the hash of the
highly-nested tuple20, which must visit a very large number of elements even
though there are only a small number of unique elements. The call chain is:
Py_Main -> PyRun_SimpleFileExFlags -> PyAST_CompileObject -> compiler_body ->
compiler_function -> compiler_make_closure -> compiler_add_o -> PyDict_GetItem
and then into the tuple hash code.
It appears that the peephole optimizer converts the highly-nested tuple20 into
a constant value. The compiler then creates the code object with its co_consts.
Specifically, compiler_make_closure uses a dictionary to ensure that the
elements in co_consts are unique, and mapped to the integer used by LOAD_CONST.
It takes about 115 seconds to compute hash(tuple20). I believe the hash is
computed twice, once to check if the object is present, and the second to
insert it. I suspect most of the other 26 seconds went to computing the
intermediate constants in the tuple.
Based on the previous issues I highlighted in my first paragraph, I believe
this report will be filed under "Doctor, doctor, it hurts when I do this"/"Then
don't do it." I see no easy fix, and cannot think of how it would come about in
real-world use.
I point it out because in reading the various issues related to the peephole
optimizer there's a subset of people who propose a look-before-you-leap
technical solution of avoiding an optimization where the estimated result is
too large. While it does help, it does not avoid all of the negatives of the
peephole optimizer, or any AST-based optimizer with similar capabilities. I
suspect even most core developers aren't aware of this specific negative.
----------
components: Interpreter Core
messages: 294050
nosy: dalke
priority: normal
severity: normal
status: open
title: constant folding opens compiler to quadratic time hashing
versions: Python 2.7, Python 3.3, Python 3.4, Python 3.5, Python 3.6, Python 3.7
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue30416>
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