Calling comprehensions always makes CALL_PY_EXACT_ARGS miss
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Description
Consider the function
def f():
return [x for x in range(2)]
for i in range(20):
f() # warmup
from dis import dis
dis(f, adaptive=True)
dis(f)
1 0 RESUME_QUICK 0
2 2 LOAD_CONST 1 (<code object <listcomp> at 0x000001DA71743920, file "<stdin>", line 2>)
4 MAKE_FUNCTION 0
6 LOAD_GLOBAL_BUILTIN 1 (NULL + range)
18 LOAD_CONST 2 (2)
20 CALL_BUILTIN_CLASS 1
30 GET_ITER
32 CALL_PY_EXACT_ARGS 0
42 POP_TOP
44 LOAD_CONST 0 (None)
46 RETURN_VALUE
Disassembly of <code object <listcomp> at 0x000001DA71743920, file "<stdin>", line 2>:
2 0 RESUME_QUICK 0
2 BUILD_LIST 0
4 LOAD_FAST 0 (.0)
>> 6 FOR_ITER_RANGE 4 (to 18)
10 STORE_FAST__LOAD_FAST 1 (x)
12 LOAD_FAST 1 (x)
14 LIST_APPEND 2
16 JUMP_BACKWARD_QUICK 6 (to 6)
>> 18 RETURN_VALUE
That CALL_PY_EXACT_ARGS always misses at
DEOPT_IF(func->func_version != read_u32(cache->func_version), CALL);
because the function is created by MAKE_FUNCION so it will never have the same func_version as the previous time.
CALL_PY_EXACT_ARGS misses roughly 5% of the time in pyperformance, and this might decrease that a hair.
Maybe this is insignificant enough that it won't matter, but it would theoretically be nice to statically decide not to specialize these calls.
cc @markshannon
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
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Research direction
Start by tracing the CALL_PY_EXACT_ARGS specialization for functions created by MAKE_FUNCTION, focusing on the DEOPT_IF func_version check described in the issue. Use the disassembly example as the reproducer and compare pyperformance results; done means these comprehension calls are statically avoided or no longer miss unnecessarily, with no regression in the reported benchmark.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- performance
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 38/100