https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91287
--- Comment #37 from rguenther at suse dot de <rguenther at suse dot de> --- On August 8, 2019 3:35:26 AM GMT+02:00, "luoxhu at cn dot ibm.com" <gcc-bugzi...@gcc.gnu.org> wrote: >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 That shows the bug. The builtin may have an implementation in libgcc but not with this symbol. Thus an LTO link would fail. >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.
