https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91287

--- Comment #34 from Xiong Hu XS Luo <luoxhu at cn dot ibm.com> ---
(In reply to rguent...@suse.de from comment #32)
> On Mon, 5 Aug 2019, luoxhu at cn dot ibm.com wrote:
> 
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91287
> > 
> > --- Comment #31 from Xiong Hu XS Luo <luoxhu at cn dot ibm.com> ---
> > (In reply to rguent...@suse.de from comment #30)
> > > On Fri, 2 Aug 2019, luoxhu at cn dot ibm.com wrote:
> > > 
> > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91287
> > > > 
> > > > --- Comment #28 from Xiong Hu XS Luo <luoxhu at cn dot ibm.com> ---
> > > > (In reply to Richard Biener from comment #24)
> > > > > Btw, this is controlled by symtab_node::output_to_lto_symbol_table_p 
> > > > > which
> > > > > has
> > > > > 
> > > > >   /* FIXME: Builtins corresponding to real functions probably should 
> > > > > have
> > > > >      symbol table entries.  */
> > > > >   if (TREE_CODE (decl) == FUNCTION_DECL && fndecl_built_in_p (decl))
> > > > >     return false;
> > > > > 
> > > > > we could try to do sth like
> > > > > 
> > > > >   if (TREE_CODE (decl) == FUNCTION_DECL
> > > > >       && (fndecl_built_in_p (decl, BUILT_IN_MD)
> > > > >           || (fndecl_built_in_p (decl, BUILT_IN_NORMAL)
> > > > >               && !associated_internal_fn (decl))))
> > > > >     return false;
> > > > > 
> > > > > but that would still leave us with too many undefineds I guess
> > > > > (gcc_unreachable for one).
> > > > > 
> > > > > We do not currently track builtins that do have a library 
> > > > > implementation
> > > > > (whether that it is used in the end is another thing, but less 
> > > > > important).
> > > > > 
> > > > > What we definitely can do is put a whitelist above like via the 
> > > > > following
> > > > > which also catches the case of definitions of builtins.
> > > > > 
> > > > > Index: gcc/symtab.c
> > > > > ===================================================================
> > > > > --- gcc/symtab.c        (revision 273968)
> > > > > +++ gcc/symtab.c        (working copy)
> > > > > @@ -2375,10 +2375,24 @@ symtab_node::output_to_lto_symbol_table_
> > > > >       first place.  */
> > > > >    if (VAR_P (decl) && DECL_HARD_REGISTER (decl))
> > > > >      return false;
> > > > > +
> > > > >    /* FIXME: Builtins corresponding to real functions probably should 
> > > > > have
> > > > >       symbol table entries.  */
> > > > > -  if (TREE_CODE (decl) == FUNCTION_DECL && fndecl_built_in_p (decl))
> > > > > -    return false;
> > > > > +  if (TREE_CODE (decl) == FUNCTION_DECL
> > > > > +      && !definition
> > > > > +      && fndecl_built_in_p (decl))
> > > > > +    {
> > > > > +      if (DECL_BUILT_IN_CLASS (decl) == BUILT_IN_NORMAL)
> > > > > +       switch (DECL_FUNCTION_CODE (decl))
> > > > > +         {
> > > > > +         CASE_FLT_FN (BUILT_IN_ATAN2):
> > > > > +         CASE_FLT_FN (BUILT_IN_SIN):
> > > > > +           return true;
> > > > > +         default:
> > > > > +           break;
> > > > > +         }
> > > > > +      return false;
> > > > > +    }
> > > > >  
> > > > >    /* We have real symbol that should be in symbol table.  However 
> > > > > try to
> > > > > trim
> > > > >       down the refernces to libraries bit more because linker will 
> > > > > otherwise
> > > > 
> > > > Hi Richard, no undefineds generated with below code, what's your 
> > > > opinion about
> > > > the updated code, please? Thanks.
> > > 
> > > It will break code calling __builtin_unreachable for example since
> > > we'll emit an UNDEF that cannot be satisfied.
> > 
> > Thanks. I tried to add __builtin_unreachable() in the test case, it can also
> > works. As BUILT_IN_UNREACHABLE is defined in buitins.def instead of
> > internal-fn.def, so associated_internal_fn will return IFN_LAST for it, 
> > then no
> > UNDEF of __builtin_unreachable will be emitted to object file.
> > 
> > Most of functions in internal-fn.def are math functions, I am not sure 
> > whether
> > you mean the BUILT_IN_NOP or something else?
> 
> OK, so a specific example woul dbe __builtin_clz.  IIRC the
> DECL_ASSEMBLER_NAME of the functions which have a libgcc fallback is
> _not_ the symbol in libgcc (you'd have to double-check).
> 
> That said, using associated_internal_fn is probably mostly safe but
> not a complete fix since we have builtins like __builtin_strcpy
> as well (but of course the C library is always linked).
> 
> But I'm fine with an approach that incrementally improves things
> here, but without possibly causing link-failures due to bogus
> UNDEFs

Add x = __builtin_clz(x); and __builtin_strcpy(str, "test\n") to the test code,
the object file's symbol will be:
luoxhu@genoa lto $ ~/workspace/gcc-git/gcc-master_debug/gcc/gcc-nm
-B/home/luoxhu/workspace/gcc-git/gcc-master_debug/gcc/ atan2bashzowie.o
         U atan2
         U __builtin_clz
00000000 T main
         U rand
00000000 T rand_finite_double
00000000 C str
00000000 T zowie

_builtin_clz is also defined in internal-fn.def, it could be linked
successfully with libgcc.
__builtin_strcpy will be lowered to __builtin_memcpy in .008t.lower and linked
successfully with C library. But no UNDEFs for it as associated_internal_fn
return IFN_LAST.
Shall I send a patch to the mail-list using the associated_internal_fn method?
Thanks.

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