8000 Remove strict-aliasing rules errors on gcc 4.8.5 by corona10 · Pull Request #16714 · python/cpython · GitHub
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Remove strict-aliasing rules errors on gcc 4.8.5 #16714

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Merged
merged 1 commit into from
Oct 11, 2019
Merged

Remove strict-aliasing rules errors on gcc 4.8.5 #16714

merged 1 commit into from
Oct 11, 2019

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corona10
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@corona10 corona10 commented Oct 11, 2019 •
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Remove below error on
CentOS Linux release 7.6.1810
gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-36)

Objects/dictobject.c:1136:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
     PyDictKeyEntry *ep = &DK_ENTRIES(mp->ma_keys)[0];

@corona10 corona10 requested a review from methane as a code owner October 11, 2019 07:27
@corona10 corona10 requested a review from vstinner October 11, 2019 07:52
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methane commented Oct 11, 2019

I don't have gcc 4.8.5 environment but your patch looks good to me.

@methane methane merged commit c39d1dd into python:master Oct 11, 2019
@corona10 corona10 deleted the gcc_4_warning branch October 11, 2019 08:57
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@methane
Thanks for the review!

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@methane Can we open the backport patch for 3.8 3.7?

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methane commented Oct 11, 2019

@corona10 I think this is safe to backport. But I want to wait for buildbots before backporting.

@corona10 corona10 restored the gcc_4_warning branch October 11, 2019 10:23
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@methane Got it!

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"But I want to wait for buildbots before backporting."

Thank you!

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Thanks @corona10 for the PR, and @methane for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

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Thanks @corona10 for the PR, and @methane for merging it 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

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GH-16756 is a backport of this pull request to the 3.8 branch.

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Sorry, @corona10 and @methane, I could not cleanly backport this to 3.7 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker c39d1ddc012987e2159a997e27665d2d579c0ce0 3.7

miss-islington pushed a commit to miss-islington/cpython that referenced this pull request Oct 14, 2019
@corona10 @miss-islington
Fix strict-aliasing rules errors on gcc 4.8.5. (pythonGH-16714)
c3859ab 
(cherry picked from commit c39d1dd)

Co-authored-by: Dong-hee Na <donghee.na92@gmail.com>
miss-islington added a commit that referenced this pull request Oct 14, 2019
@miss-islington @corona10
Fix strict-aliasing rules errors on gcc 4.8.5. (GH-16714)
9a7c43b 
(cherry picked from commit c39d1dd)

Co-authored-by: Dong-hee Na <donghee.na92@gmail.com>
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Objects/dictobject.c:1136:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
     PyDictKeyEntry *ep = &DK_ENTRIES(mp->ma_keys)[0];

Hum, there are other places with similar code:

delitem_common():

    PyDictKeyEntry *ep;
    ...
    ep = &DK_ENTRIES(mp->ma_keys)[ix];

_PyDict_Pop_KnownHash:

    PyDictKeyEntry *ep;
    ...
    ep = &DK_ENTRIES(mp->ma_keys)[ix];

dict_equal:

        PyDictKeyEntry *ep = &DK_ENTRIES(a->ma_keys)[i];

Why only insert_to_emptydict() required to be modified?

I'm not sure that the magic DK_ENTRIES() macro respects strict aliasing.

#define DK_ENTRIES(dk) \
    ((PyDictKeyEntry*)(&((int8_t*)((dk)->dk_indices))[DK_SIZE(dk) * DK_IXSIZE(dk)]))

_dictkeysobject structure ends with:

    char dk_indices[];  /* char is required to avoid strict aliasing. */

_dictkeysobject ends with the following field, but the field is not declared in the structure:

PyDictKeyEntry dk_entries[dk_usable];

Since it's not declared, I'm not sure that the compiler is able to align it properly.

jacobneiltaylor pushed a commit to jacobneiltaylor/cpython that referenced this pull request Dec 5, 2019
@corona10
Fix strict-aliasing rules errors on gcc 4.8.5. (pythonGH-16714)
e8748ba
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