Re: Violations of mandatory MISRA C:2012 Rule 19.1 in X86_64 build

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.07.2023 18:40, Roberto Bagnara wrote:
> Mandatory Rule 19.1 (An object shall not be assigned or copied to an
> overlapping object) is directly targeted at two undefined behaviors,
> one of which is the subject of 6.5.16.1p3, namely:
> 
>    If the value being stored in an object is read from another object
>    that overlaps in any way the storage of the first object, then the
>    overlap shall be exact and the two objects shall have qualified or
>    unqualified versions of a compatible type; otherwise, the behavior
>    is undefined.

I'd like to come back to this, for two reasons:

1) In the description of -fstrict-aliasing, gcc 13.2 doc says "Even with
-fstrict-aliasing, type-punning is allowed, provided the memory is accessed
through the union type." We even build with -fno-strict-aliasing, yet
misra/rules.rst has no mention at all of type punning being permitted.

2) With us using -fno-strict-aliasing, I wonder in how far e.g. commit
7225f13aef03 ("x86: avoid Misra Rule 19.1 violations") wasn't pointless, as
imo the "compatible types" part of the C spec clause then can be treated as
irrelevant.

To me both simply mean we're relying on another compiler extension that's
not exactly spelled out as such.

Opinions?

Jan