[Bug lto/47247] Linker plugin specification makes it difficult to handle COMDATs

["hubicka at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47247

--- Comment #6 from Jan Hubicka <hubicka at gcc dot gnu.org> 2011-01-10 17:54:51 UTC ---
The plugin specification says that once the COMDAT is marked PREVAILING, it has
to be output.
  "Any symbol marked PREVAILING_DEF must be defined in one object file added to
the link after WPA is done, or an undefined symbol error will result. Any
symbol marked PREVAILING_DEF_IRONLY may be left undefined (provided all
references to the symbol have been removed), and the linker will not issue an
error."

GCC will optimize it out only if it is set PREVAILING_IRONLY, but I think we
conlcuded earlier that this can not be used for symbols visible to dynamic
linking (and GCC would use this info to bring the COMDAT symbol static). 

So I am convinced that with current API it is impossible and we need something
like PREVAILING_IRONLY_EXPORTED or so for the case of symbol that is not used
in current DSO but is exported. GCC can then use the knowledge of what COMDATs
can be taken away.