gh-130317: Fix strict aliasing in PyFloat_Pack8()

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• Fix strict aliasing in PyFloat_Pack8() and PyFloat_Pack4()
• Fix _testcapi.float_set_snan() on x86 (32-bit).

• Issue: struct (un)packing of half-precision nan floats is non-invertible #130317

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@skirpichev skirpichev self-requested a review

Sorry, I merged quickly my PR since I would like to fix the buildbots ASAP. Your (post-merge) review would still be welcomed :-)

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Follow-up: #133155 to fix the test.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot aarch64 Android 3.x (tier-3) has failed when building commit 02cd6d7.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1594/builds/2145) and take a look at the build logs.
4. Check if the failure is related to this commit (02cd6d7) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1594/builds/2145

Summary of the results of the build (if available):

Click to see traceback logs

remote: Enumerating objects: 13, done. remote: Counting objects: 7% (1/13) remote: Counting objects: 15% (2/13) remote: Counting objects: 23% (3/13) remote: Counting objects: 30% (4/13) remote: Counting objects: 38% (5/13) remote: Counting objects: 46% (6/13) remote: Counting objects: 53% (7/13) remote: Counting objects: 61% (8/13) remote: Counting objects: 69% (9/13) remote: Counting objects: 76% (10/13) remote: Counting objects: 84% (11/13) remote: Counting objects: 92% (12/13) remote: Counting objects: 100% (13/13) remote: Counting objects: 100% (13/13), done. remote: Compressing objects: 14% (1/7) remote: Compressing objects: 28% (2/7) remote: Compressing objects: 42% (3/7) remote: Compressing objects: 57% (4/7) remote: Compressing objects: 71% (5/7) remote: Compressing objects: 85% (6/7) remote: Compressing objects: 100% (7/7) remote: Compressing objects: 100% (7/7), done. remote: Total 7 (delta 6), reused 0 (delta 0), pack-reused 0 (from 0)  From https://github.com/python/cpython * branch main -> FETCH_HEAD Note: switching to '02cd6d709709e28c7a8fa62ce3620ea000b524d3'. You are in 'detached HEAD' state. You can look around, make experimental changes and commit them, and you can discard any commits you make in this state without impacting any branches by switching back to a branch. If you want to create a new branch to retain commits you create, you may do so (now or later) by using -c with the switch command. Example: git switch -c <new-branch-name> Or undo this operation with: git switch - Turn off this advice by setting config variable advice.detachedHead to false HEAD is now at 02cd6d70970 gh-130317: Fix strict aliasing in PyFloat_Pack8() (#133150) Switched to and reset branch 'main' configure: WARNING: no system libmpdecimal found; falling back to bundled libmpdecimal (deprecated and scheduled for removal in Python 3.15)configure: WARNING: pkg-config is missing. Some dependencies may not be detected correctly. In file included from ../../Python/ceval.c:1097: ../../Python/generated_cases.c.h:12162:13: warning: code will never be executed [-Wunreachable-code] 12162 | stack_pointer -= 3; |^~~~~~~~~~~~~ ../../Python/generated_cases.c.h:12155:13: warning: code will never be executed [-Wunreachable-code] 12155 | stack_pointer -= 4; |^~~~~~~~~~~~~ ../../Python/generated_cases.c.h:7467:13: warning: code will never be executed [-Wunreachable-code] 7467 | stack_pointer += -1; |^~~~~~~~~~~~~ 3 warnings generated. ld: warning: ignoring duplicate libraries: '-ldl'ld: warning: ignoring duplicate libraries: '-ldl'ld: warning: ignoring duplicate libraries: '-ldl'ld: warning: ignoring duplicate libraries: '-ldl'ld: warning: ignoring duplicate libraries: '-ldl'ld: warning: ignoring duplicate libraries: '-ldl' % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 100 192k 100 192k 0 0 1121k 0 --:--:-- --:--:-- --:--:-- 1121k % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 100 42455 100 42455 0 0 350k 0 --:--:-- --:--:-- --:--:-- 350k % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 100 5041k 100 5041k 0 0 27.8M 0 --:--:-- --:--:-- --:--:-- 61.5M % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 100 1252k 100 1252k 0 0 6304k 0 --:--:-- --:--:-- --:--:-- 6304k % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 100 635k 100 635k 0 0 3838k 0 --:--:-- --:--:-- --:--:-- 3838k ../../configure: line 4061: pkg-config: command not foundconfigure: WARNING: no system libmpdecimal found; falling back to bundled libmpdecimal (deprecated and scheduled for removal in Python 3.15)configure: WARNING: pkg-config is missing. Some dependencies may not be detected correctly. In file included from ../../Python/ceval.c:1097: ../../Python/generated_cases.c.h:12162:13: warning: code will never be executed [-Wunreachable-code] 12162 | stack_pointer -= 3; |^~~~~~~~~~~~~ ../../Python/generated_cases.c.h:12155:13: warning: code will never be executed [-Wunreachable-code] 12155 | stack_pointer -= 4; |^~~~~~~~~~~~~ ../../Python/generated_cases.c.h:7467:13: warning: code will never be executed [-Wunreachable-code] 7467 | stack_pointer += -1; |^~~~~~~~~~~~~ ../../Python/fileutils.c:460:1: warning: unused function 'decode_current_locale' [-Wunused-function] 460 | decode_current_locale(const char* arg, wchar_t **wstr, size_t *wlen, |^~~~~~~~~~~~~~~~~~~~~ ../../Python/fileutils.c:679:1: warning: unused function 'encode_current_locale' [-Wunused-function] 679 | encode_current_locale(const wchar_t *text, char **str, |^~~~~~~~~~~~~~~~~~~~~ 2 warnings generated. 3 warnings generated. ../../Modules/_localemodule.c:148:1: warning: unused function 'is_all_ascii' [-Wunused-function] 148 | is_all_ascii(const char *str) |^~~~~~~~~~~~ 1 warning generated. ../../Modules/_hacl/Lib_Memzero0.c:52:6: warning: "Your platform does not support any safe implementation of memzero -- consider a pull request!" [-W#warnings] 52 | #warning "Your platform does not support any safe implementation of memzero -- consider a pull request!" |^ ../../Modules/_hacl/Lib_Memzero0.c:40:10: warning: unused variable 'len_' [-Wunused-variable] 40 | size_t len_ = (size_t) len; |^~~~ 2 warnings generated. % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 100 8784 100 8784 0 0 123k 0 --:--:-- --:--:-- --:--:-- 124k % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 100 2894 100 2894 0 0 5824 0 --:--:-- --:--:-- --:--:-- 5834 % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0 curl: (56) The requested URL returned error: 429