[Bug tree-optimization/103964] [9/10/11/12 Regression] OVS miscompilation since r0-92313-g5006671f1aaa63cd

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103964

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Ilya Maximets from comment #5)
> (In reply to Richard Biener from comment #4)
> > the IVOPTs reference is likely due to the fact that while IVOPTs uses
> > uintptrs to create the base pointer the TARGET_MEM_REF contained arithmetic
> > itself is still considered pointer arithmetic (like also here the embedded
> > MEM_REF pointer offsetting) and the base "pointer" cannot be a non-pointer
> > to disable that behavior.
> 
> Does this mean that this is UB and the GCC itself relies on a certain result
> of it?

If GCC through optimizations introduces UB in a code which wasn't there in the
user's code, then it would be a GCC bug and something the compiler needs to
fix.

> Maybe there is a way to not treat a &pos->elem as a pointer arithmetic?
> Maybe there should be one?  I mean, compilers allows users to perform
> computations with offsets of structure fields where the base pointer
> is NULL, and NULL obviously doesn't point to any valid object.  I'm not
> sure if it's an UB or not, but constructions like '&((struct s
> *)NULL)->field'
> are very common.

&((struct s *)NULL)->field is not valid in C or C++, but for many years the
offsetof macro which is valid in those has been defined like that and various
projects occassionally still use the above, so GCC supports those as an
extension (poor man's offsetof).  See e.g. spots with comments like
Cope with user tricks that amount to offsetof.
etc. in GCC sources.  That doesn't change anything about this case, the poor
man's offsetof is folded into a constant very early (well, with variable
offsets in array refs in there could also into an expression, but still
integral expression, the pointer arithmetics is gone from there).

What is the reason why OVS (and kernel) doesn't use 2 variables, one for the
iterator that is a pointer to the prev/next structure only and one assigned
e.g. in the condition from the iterator that is used only when it isn't the
start?
At least if targetting C99 and above (or C++) one can declare one of those
iterators in the for loop init expression...