8000 Fix MSVC warning in frameobject.c by ammaraskar · Pull Request #20590 · python/cpython · GitHub
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Fix MSVC warning in frame 8000 object.c #20590

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Merged
merged 1 commit into from
Jun 4, 2020
Merged

Fix MSVC warning in frameobject.c #20590

merged 1 commit into from
Jun 4, 2020

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ammaraskar
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This should be a safe downcast as exceeding a size of 2^30 opcodes in a single code object is extremely unlikely and there's already some hard dependencies relying on 32-bit ints like struct _frame's f_lasti, f_iblock. See PEP 611 for proposals to solidify this limit and other research on existing limits.

While it would be possible to swap this out for a Py_ssize_t, it would be a much more involved refactoring as line numbers are still reliant on being ints.

@ammaraskar ammaraskar requested a review from pablogsal June 2, 2020 08:19
@pablogsal pablogsal merged commit 6e23a9c into python:master Jun 4, 2020
@vstinner
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vstinner commented Jun 4, 2020

This fix doesn't work, I get a warning using GCC 10 on Fedora 32:

gcc -pthread -c -Wno-unused-result -Wsign-compare -g -Og -Wall -O0   -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal  -I. -I./Include    -DPy_BUILD_CORE -o Objects/frameobject.o Objects/frameobject.c
In file included from ./Include/Python.h:61,
                 from Objects/frameobject.c:3:
Objects/frameobject.c: In function 'frame_setlineno':
./Include/pyport.h:313:35: warning: comparison of integer expressions of different signedness: 'long int' and 'long unsigned int' [-Wsign-compare]
  313 |     (assert((WIDE)(NARROW)(VALUE) == (VALUE)), (NARROW)(VALUE))
      |                                   ^~
Objects/frameobject.c:400:15: note: in expansion of macro 'Py_SAFE_DOWNCAST'
  400 |     int len = Py_SAFE_DOWNCAST(
      |               ^~~~~~~~~~~~~~~~

2^30 opcodes in a single code object is extremely unlikely

I'm not comfortable with relying on such assumption that things "should not happen in practice". I would prefer to implement a concrete runtime check in PyCode_NewWithPosOnlyArgs() to reject co_code if (size_t)PyBytes_GET_SIZE(f->f_code->co_code)/sizeof(_Py_CODEUNIT) > (size_t)INT_MAX is true. With that, you can replace Py_SAFE_DOWNCAST with (int) cast with a comment referring to PyCode_NewWithPosOnlyArgs() check.

@shihai1991
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Hm, I got this warning too when I use gcc9.3 on Red Hat 4.8.5-36.

@ammaraskar
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Aah sorry about that. I was trying to go with a narrow fix and didn't want to touch too many files. It looks like there is an assertion here https://github.com/python/cpython/blob/master/Python/ceval.c#L1328
but I agree with vstinner that this should probably just be a runtime check in codeobject. Let me open up a PR with that.

@ammaraskar ammaraskar mentioned this pull request Jun 4, 2020
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vstinner commented Jun 4, 2020

No problem :-) Follow-up PR: PR #20628.

@ammaraskar ammaraskar mentioned this pull request Jun 4, 2020
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arun-mani-j pushed a commit to arun-mani-j/cpython that referenced this pull request Jul 21, 2020
@ammaraskar @arun-mani-j
Fix MSVC warning in frameobject.c (pythonGH-20590)
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