Comment by jcranmer
2 days ago
Footnote 107 in C23, on page 75 in §6.5.2.3:
> If the member used to read the contents of a union object is not the same as the member last used to store a value in the object the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called type punning). This might be a non-value representation.
(though this footnote has been present as far back as C99, albeit with different numbers as the standard has added more text in the intervening 24 years).
The GCC developers disagree with your interpretation:
> Type punning via unions is undefined behavior in both c and c++.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118141#c13
I'm not sure tbh what's there to 'interpret' or how a compiler developer could misread that, the wording is quite clear.
It is an excerpt being taken out of context. Of course it is quite clear. Taking it out of context ignores everything else that the standard says. That interpretation is wrong as far as compiler authors are concerned.
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I wouldn't be surprised if Andrew Pinski was just wrong. It's anecdotal but my impression of him isn't very good.