Re: "Uninitialized array" warnings by c++ with -O2

[David Brown <david at westcontrol dot com>]

On 07/06/17 11:33, Andrew Haley wrote:
> On 07/06/17 10:15, Kirill Yu Berezin wrote:
>> My question is. Is this an expected behaviour or I must report a bug ?
> 
> It's not a bug: your code displays undefined behaviour: you're casting
> a pointer to struct udp_pseudo fields to an array of uint16_t.  This
> is never well-defined in C++, but if you really want to do this kind
> of thing, use -fno-strict-aliasing.
> 
> See also
> http://www.microhowto.info/howto/calculate_an_internet_protocol_checksum_in_c.html
> 

Two other ideas spring to mind.

Use the "may_alias" attribute on the udp_pseudo struct:

    struct udp_pseudo {
        uint32_t src;
        uint32_t dst;
         uint8_t  z;
         uint8_t  proto;
         uint16_t len;
    } __attribute__((may_alias));

This tells the compiler that this type may alias other types (such as
the uint8_t array).  I think you will also need a "may_alias" attribute
on the uint16_t pointer in calc_16bit_checksum_part as well.  You can
imagine "may_alias" attribute as giving you the effect of
-fno-strict-aliasing, but only for particular types.


The other is to make a union type that has your struct as a field, and
your uint8_t array as another field.  If you access everything through
the union, the compiler knows they alias.  Again, you would want to
include the uint16_t pointer in this solution in order to get all the
aliasing correct.