correct way of having a function pointer inside a GTY()-ed struct?

[Basile STARYNKEVITCH <basile at starynkevitch dot net>]

Hello All,

What is the politically correct way of having an ignored function 
pointer inside a GTY-ed struct?

Of course, the function pointer is obviously GTY ((skip))-ed.

In the MELT branch (file gcc/basilys.h rev144985) I have
// notice that FLEXIBLE_DIM is a macro expanding to empty

#define BASILYS_ROUTDESCR_LEN 96
#define BASILYS_ROUTADDR_LEN  (1 + (2*sizeof (basilysroutfun_t *))/ 
sizeof (long))

struct basilysroutine_st
GTY (())
{
 basilysobject_ptr_t discr;
 char routdescr[BASILYS_ROUTDESCR_LEN];
 long GTY ((skip)) routaddr[BASILYS_ROUTADDR_LEN];
 basilys_ptr_t routdata;
 unsigned nbval;
 basilys_ptr_t GTY ((length ("%h.nbval"))) tabval[FLEXIBLE_DIM];
};

/* unsafely set inside the basilysroutine_st pointed by Rptr the
  routine function pointer to Rout */
#define BASILYS_ROUTINE_SET_ROUTCODE(Rptr,Rout) do {            \
 (*(basilysroutfun_t**)((struct basilysroutine_st*)(Rptr))->routaddr)    \
    = (Rout);                                \
} while(0)


However, all invocations of the BASILYS_ROUTINE_SET_ROUTCODE macro gives 
still tons of warnings
warmelt-first-0.c:50068: warning: dereferencing type-punned pointer will 
break strict-aliasing rules
warmelt-first-0.c:50422: warning: dereferencing type-punned pointer will 
break strict-aliasing rules
warmelt-first-0.c:50434: warning: dereferencing type-punned pointer will 
break strict-aliasing rules
warmelt-first-0.c:50446: warning: dereferencing type-punned pointer will 
break strict-aliasing rules
warmelt-first-0.c:50458: warning: dereferencing type-punned pointer will 
break strict-aliasing rules
warmelt-first-0.c:50470: warning: dereferencing type-punned pointer will 
break strict-aliasing rules


Ideally, I would like to code

/* the signature of all MELT generated functions */
typedef basilys_ptr_t basilysroutfun_t (basilysclosure_ptr_t closp_,
                   basilys_ptr_t firstargp_,
                   const char xargdescr_[],
                   union basilysparam_un *xargtab_,
                   const char xresdescr_[],
                   union basilysparam_un *xrestab_);

#define BASILYS_ROUTDESCR_LEN 96

struct basilysroutine_st
GTY (())
{
 basilysobject_ptr_t discr;
 char routdescr[BASILYS_ROUTDESCR_LEN];
 basilysroutfun_t* GTY ((skip)) routfun;
 basilys_ptr_t routdata;
 unsigned nbval;
 basilys_ptr_t GTY ((length ("%h.nbval"))) tabval[FLEXIBLE_DIM];
};

/* unsafely set inside the basilysroutine_st pointed by Rptr the
  routine function pointer to Rout */
#define BASILYS_ROUTINE_SET_ROUTCODE(Rptr,Rout) do {            \
 ((struct basilysroutine_st*)(Rptr))->routfun)    \
    = (Rout);                                \
} while(0)

But last time I tried (more than a year ago) that didn't work because 
gengtype is unhappy with GTY((skip))-ed fields of a type it does not 
understand.

I could imagine that once plugin-s are politically correct (maybe in 
2011? but I still hope before) they also would like to have function 
pointers inside GTY-ed struct. So I believe my concern is potentially 
not only inside the MELT branch.

Regards.

--
Basile STARYNKEVITCH         http://starynkevitch.net/Basile/
email: basile<at>starynkevitch<dot>net mobile: +33 6 8501 2359
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*** opinions {are only mines, sont seulement les miennes} ***