[ECOS] Re: uSTL hello world

Jonathan Larmour jifl@jifvik.org
Tue Sep 29 15:36:00 GMT 2009

Bart Veer wrote:
>>>>>>"Jifl" == Jonathan Larmour <jifl@jifvik.org> writes:
>     <snip>
>     Jifl> Unfortunately we have another problem. I've looked at the
>     Jifl> output of our recent compilers, and I see that
>     Jifl> CYG_REFERENCE_OBJECT no longer appears to be effective :-(.
[snip failed attempts to resolve]
>     Jifl> Any other ideas? At least there are not many uses of
> I think the following may do the trick:
> ----------------------------------------------------------------------------
> // The unused attribute stops the compiler warning about the variable
> // not being used.
> // The used attribute prevents the compiler from optimizing it away.
> #define NEW_CYG_REFERENCE_OBJECT(__object__)                            \
>     CYG_MACRO_START                                                     \
>     static const void*  __cygvar_discard_me__                           \
>     __attribute__ ((unused, used)) = (const void*)&(__object__);        \
> ----------------------------------------------------------------------------
> I have tried it in a simple testcase at -O2. The compiler does not
> optimize away __cygvar_discard_me__ because the used attribute
> prevents that. Hence the referenced object gets pulled in during the
> link. Subsequently linker garbage collection eliminates
> __cygvar_discard_me__. If the referenced object is a C++ object then
> it will be preserved because of the KEEP(*.ctors) in the linker
> script. So, even though it seems peculiar to have a variable both used
> and unused, that combo appears to do what we want and should be fully
> portable.

That does seem very peculiar, but if it works....

> Looking at the gcc ChangeLog-2001, attribute(used) was added on
> 2001-10-18 so has probably been available since gcc 3.1 days. However
> there is a comment in cyg_type.h associated with CYGBLD_ATTRIB_USED
> claiming >= 3.3.2.

ISTR something about the behaviour of that attribute changing at some 
point- perhaps it was buggy before. We could ifdef CYG_REFERENCE_OBJECT 
based on GCC version easily enough.

> Let me know if it still does not solve the problem in al cases, and I
> can investigate further.

Now I think it's back to Uwe to verify that this, in combination with the 
stuff I mentioned before, does resolve his uSTL problems.


Even better if you can submit an associated patch.

--["No sense being pessimistic, it wouldn't work anyway"]-- Opinions==mine

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