Re: [RFC PATCH] diagnose built-in declarations without prototype (PR 83656)

[Jeff Law <law at redhat dot com>]

On 06/29/2018 09:54 AM, Martin Sebor wrote:
> 
> All the warnings I have seen are because of declarations like
> the one in the test below that checks for the presence of symbol
> sin in the library:
> 
>   char sin ();
>   int main () { return sin (); }
> 
> GCC has warned for this code by default since at least 4.1 so if
> it is, in fact, a problem it has been with us for over a decade.
> 
> There's a comment in the test that explains that the char return
> type is deliberate to prevent GCC from treating the function as
> a built-in.  (That, of course, relies on undefined behavior
> because names of extern library functions are reserved and
> conforming compilers could simply avoid emitting the call or
> replace it with a trap.)
As you noted, by doing this the test can verify if there's a sin()
function in the library or not, regardless of whether or not the
compiler has a builtin for sin().


> 
>> I wonder if stepping forward to a more modern version of autoconf is
>> going to help here and if we should be feeding them updates to make this
>> kind of stuff less pervasive, at least in standard autoconf tests.
> 
> That would make sense to me.  The tests should not rely on
> undefined behavior.  They should declare standard functions with
> the right prototypes.  IMO, for GCC and compatible compilers they
> should disable built-in expansion instead via -fno-builtin.  For
> all other compilers, they could store the address of each function
> in a (perhaps volatile) pointer and use it to make the call instead.
I think the problem is they can't rely on the compiler having the
-fno-builtin flag.  Of course they're relying on other compilers having
the same kind of behavior as GCC WRT which is possibly worse.

I guess there's a reason why they didn't extract the set of symbols from
the library. :-)

> 
> But since the number of warnings here hasn't changed, the ones
> in GCC logs predate my changes.  So updating the tests seems
> like an improvement to consider independently of the patch.
Agreed.  I'm still wary of proceeding given the general concerns about
configure tests.  It's good that GCC's configury bits aren't affected,
but I'm not sure we can generalize a whole lot from that.

jeff