On Wed, 16 Nov 2022, Michael Matz via Gcc wrote:
> I sympathize, and I would think a compiler emitting an error (not a
> warning) in the situation at hand (in absence of -Werror) is overly
> pedantic. But, could autoconf perhaps avoid the problem? AFAICS the
> ac_fn_c_check_func really does only a link test to check for symbol
> existence, and the perceived problem is that the call statement in main()
> invokes UB. So, let's avoid the call then while retaining the access to
> the symbol? Like:
>
> -----
> char foobar(void);
> int main(void) {
> return &foobar != 0;
> }
> -----
>
> No call involved: no reason for compiler to complain. The prototype decl
> itself will still be "wrong", but compilers complaining about that (in
> absence of a pre-existing different prototype, which is avoided by
> autoconf) seem unlikely.
>
> Obviously this program will also say "foobar exists" if it's a data
> symbol, but that's the same with the variant using the call on most
> platforms (after all it's not run).
>
> The idea is so obvious that I'm probably missing something, why autoconf
> can't use that idiom instead. But perhaps the (historic?) reasons why it
> couldn't be used are gone now?
Ironically, modern GCC and LLVM optimize '&foobar != 0' to '1' even at -O0,
and thus no symbol reference remains in the resulting assembly.
Alexander
