https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63215
--- Comment #5 from Jan Hubicka <hubicka at ucw dot cz> --- > Hmm, we can easily distinguish them by seeing whether a definition is > available. Well, what happens in this testcase is that we see ABS at LTO time, we promote it to static (because no one uses it outside) and we optimize out. Later in queue we pattern match ABS and we are screwed. To fix this, we need to keep builtin function definitions around specially and see after all the function bodies are produces what builtin functions are used. This is just part of problem. Consider case where ABS is provided by LTO compiled static library. Because the use of ABS appears at link time, the linker will not see any use of ABS and willnot bring it in. Later we synthetize it and we are screwed. For this we need linker plugin to know what functions we can possibly syntehtize in the backend and pretend them to be used by every LTO body. This however may be quite bad thing, since we can synthetize quite fancy things like STDIO functions. So ful handling of those seems bit difficult to me. I think it may need iteration - i.e. if linker discovers that use of function provided by LTO symbol tables was introduced during linktime it will repreat the whole circus to get object file implementing that function... Honza
