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

On April 25, 2017 8:03:20 PM GMT+02:00, Steve Ellcey <> wrote:
>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.

OK.  I briefly skimmed get_ref_base_and_extent and didn't see where it would make a difference.  Your patch should make that obvious (and allow for a much shorter testcase).


>Steve Ellcey

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