Problem report: Struct aliasing problem causes Thread_Ready_Chain corruption in 4.6.99.3

[Joel Sherrill]

Peer Stritzinger wrote:
> On 12/5/06, Kim Barrett <kab at irobot.com> wrote:
>   
>> Some further web
>> searches turned up the "may_alias" type attribute. I think that attribute
>> would solve the rtems chain problem, without the nasty (and I think not
>> actually portable in general) union approach.
>>     
>
> That sounds for me a very intersting feature for a transition period.
> If everything
> that might still cause trouble is declared with __attribute__ ((may_alias))
> strict-aliasing could be turned on again without having to rewrite the whole
> code.
>
> Then the data structures could be cleaned up one by one.
>
> And this approach would also be save for inlined stuff in contrast to
> a file by file specification in some Makefile machinery.
>
> For ease of reference here the explanation out of gcc-4.1.0 manual:
>
> `may_alias'
>      Accesses to objects with types with this attribute are not
>      subjected to type-based alias analysis, but are instead assumed to
>      be able to alias any other type of objects, just like the `char'
>      type.  See `-fstrict-aliasing' for more information on aliasing
>      issues.
>
>      Example of use:
>
>           typedef short __attribute__((__may_alias__)) short_a;
>
>           int
>           main (void)
>           {
>             int a = 0x12345678;
>             short_a *b = (short_a *) &a;
>
>             b[1] = 0;
>
>             if (a == 0x12345678)
>               abort();
>
>             exit(0);
>           }
>
>      If you replaced `short_a' with `short' in the variable
>      declaration, the above program would abort when compiled with
>      `-fstrict-aliasing', which is on by default at `-O2' or above in
>      recent GCC versions.
>   

I like this since it lets us mark the places known to alias and not have to
immediately redesign them.

Would this have to placed on Chain_node_struct, Chain_Control, and 
Chain_Node
types?

And most importantly, does it fix your known miscompilation?

--joel
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