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Re: Alias analysis and zero-sized arrays vs. flexible arrays


On Tue, 2017-04-25 at 12:53 +0200, Richard Biener wrote:

> > int foo() {
> >    int i,j;
> >    for (i = 0; i < m; i++) {
> >         a->o[i] = sizeof(*a);
> >         b = ((struct r *)(((char *)a) + a->o[a->n]));
> >         for (j = 0; j < 10; j++) {
> >                 b->slot[j].b = 0;
> in case b->slot[j].b aliases a->o[i] or a->o[a->n]
> you invoke undefined behavior becuase you violate
> strict aliasing rules.  I don't know why there's a
> difference between -DFLEX and -UFLEX but your
> code is buggy even if it works in one case.
> 
> Richard.

Should this work if I use -fno-strict-alias?  Even with that option I
get different code with a zero-sized array vs. a flexible array.
I have a patch to get_ref_base_and_extent that changes the behaviour
for zero-length arrays and I will submit it after I have tested it.

Steve Ellcey
sellcey@cavium.com


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