Re: [PATCH] Fix LTO type mismatch warning on transparent union

[Richard Biener <richard.guenther@gmail.com>]

> Am 30.05.2024 um 13:46 schrieb Eric Botcazou <botcazou@adacore.com>:
> 
> 
>> 
>> Do function pointers inter-operate TBAA wise for this case and would this
>> possibly An issue?
> 
> Do you mean in LTO mode?  I must say I'm not sure of the way LTO performs TBAA
> for function pointers: does it require (strict) matching of the type for all
> the parameters of the pointed-to function types?  

Yes, I think in terms of how we compute canonical types.  These pointer to derived types prove difficult for c23 as well (even without considering cross language interoperability and LTO).

> If so, then I guess it could
> theoretically assign different alias sets to compatible function pointers when
> one of them happens to point to the function type of a function imported with
> the transparent union gap, with some problematic fallout when objects of these
> function pointers happen to be indirectly modified in the program...
> 
> Note that there is an equivalent bypass based on common_or_extern a few lines
> below in the function (although I'm not sure if it's problematic TBAA-wise).

I’d have to check.  I think the diagnostic at hand tries to diagnose possible call ABI issues, so not sure why it mentions strict aliasing.  Or maybe I misremember.

A patch like yours would be OK if this is really just about the ABI issue.

Richard 

> --
> Eric Botcazou
> 
>