strict aliasing warning

[Silvius Rus]

Joe Buck wrote:
>
> If you first prove that there is no cross-type aliasing, then turn on
> -fstrict-aliasing, it seems to me that your alias sets won't change at
> all.  The reason is that if there is a change, it means that you
> eliminated an aliasing possibility based on the fact that it's not
> allowed because of cross-type aliasing, which you said you proved there
> is none of.
> There have been concerns expressed in the past that if gcc checks for
> strict aliasing violations, users will be upset when it misses some
> violations but does optimization that breaks an ill-formed program.
> I personally am not that worried about that; I think that it will help
> train users if they see warning messages about blatant violations, to
> the point where they will be less likely to create subtle, hard-to-detect
> violations.
>   
Yes, using the alias information produced by a complex algorithm may 
lead to errors as false negatives might sneak in.  That is OK when 
warning programmers, but will probably not be accepted as a 
preconditioner.  I was thinking more of a simplified check.  For 
instance, the most conservative and simplest would be verifying that 
there are no conversions to pointers to different types.  That will 
produce many false positives, but no false negatives.  However, if I 
understood you well, in such cases the complex algorithm will end up 
with the same alias sets with or without -fstrict-aliasing, so my 
conclusion is that it is probably not worth doing it.

Thank you,
Silvius