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Re: Clarification on Gcc's strict aliasing rules
Francis Moreau <francis.moro@gmail.com> writes:
> I mean that I find quite dangerous that a program can subtly break (or
> behave differently) depending on the optimisation level.
I think it's important to say clearly that only an invalid program can
subtly behave differently. A valid program will work correctly at all
optimization levels.
> For example a user can rely on aliasing undefined behaviour (one reason
> is that is simply not aware of this fact), compile its program with no
> optimisation level, do some testings and find no problem so far. Then
> increase the level of optimisation and then the program start to behave
> incorrectly. I guess he's going to spend some time before discovering
> what's going on.
>
> Specially since no warnings can pop up with -O2 for example.
It is unfortunately very difficult to issue correct warnings for these
cases. That's why -Wstrict-aliasing is optional: it produces both false
positives and false negatives.
>> Would you be helped if there was an example (or two) of the basic
>> problem first?
>
> An exhaustive list would be better I think.
I don't know how to write an exhaustive list except in terms of
optimizations that are not performed.
Ian