https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91287
--- Comment #32 from rguenther at suse dot de <rguenther at suse dot de> --- 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
