SET_SVAL only uses one, though, the fast_svalue pointer.
It is however true that reading from the argument passed after the set
might be optimized by gcc, I guess. Although that seems like a rather
odd optimization to do since the relevant code is right there.
Anyway, using an union is not an option since that would require all
code that directly reads type/subtype to change.
Interrestingly enough clang on my mac and all other compilers
accessible thorough gcc.godbolt.org does not warn about SET_SVAL
instances, nor does it produce non-working code.
Could you give this version of SET_SVAL a try, perhaps?
#define SET_SVAL(SVAL, TYPE, SUBTYPE, FIELD, EXPR) do { \
/* Set the type afterwards to avoid a clobbered \
* svalue in case EXPR throws. */ \
struct svalue * _sv_ptr=&( SVAL ); \
_sv_ptr->u.FIELD = (EXPR); \
*((ptrdiff_t*)&_sv_ptr->type) = TYPE_SUBTYPE(TYPE,SUBTYPE);\
} while(0)
