Re: PATCH: Add ifunc attribute

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Sun, Jun 21, 2009 at 2:23 AM, Jakub Jelinek<jakub@redhat.com> wrote:
> On Fri, Jun 19, 2009 at 03:00:04PM -0700, H.J. Lu wrote:
>> > Yeah. ?I would have proposed that
>> >
>> > void *foo (void) __attribute__((__ifunc__))
>> > {
>> > ?return zero;
>> > }
>> >
>> > would simply generate foo with indirect function type.
>> >
>> > This of course causes a problem if you have a declaration for
>> > the real foo available - but then you should better not have that,
>> > as the compiler will then derive wrong properties for the real foo
>> > from the ifunc wrapper body.
>> >
>> > That would at least not introduce the confusion with the extra name ...
>> >
>>
>> Here is the updated patch. I extended the ifunc attribute to
>> static functions and added testcases to show that we need
>> argument for ifunc attribute:
>
> I agree with Richard here, ifunc attribute shouldn't have any arguments
> and should just mean the function has gnu_indirect_function type instead
> of function, nothing else. ?That way users can combine it with other
> extensions to get the behavior they want, rather than having weird
> unexpected effects (in your case foo_ifunc is for some surprisingly
> not defined at all).
>
> If you don't have foo's prorotype in the current translation unit, you can
> just use ifunc attribute alone, if you do have the prototype, it is not
> much harder, e.g.
>
> __typeof (foo) *foo_ifunc (void) __asm ("foo") __attribute__((ifunc))
> {
> ?return foo_impl1;
> }
>
> will work just fine. ?In this case you don't need to teach GCC about special
> properties of ifunc attribute, foo for GCC isn't defined in the current
> translation unit (it doesn't look at DECL_ASSEMBLER_NAME for inlining and
> similar purposes).

Using

static __typeof (foo) *foo_ifunc (void) __asm ("foo") __attribute__((ifunc))

is not much better than

static __typeof (foo) * foo_ifunc (void) __asm__ ("foo");
__asm__(".type foo, %gnu_indirect_function");

Also, I am getting

ifunc-2.c:3:12: warning: 'foo' used but never defined

on static ifunc.  It is not a new problem:

[hjl@gnu-6 tmp]$ cat foo.c
static int foo ();
static int foo_ifunc (void) __asm__ ("foo");
static int
foo_ifunc (void)
{
  return 0;
}
int
bar ()
{
  return foo ();
}
[hjl@gnu-6 tmp]$ gcc foo.c -S
foo.c:1: warning: âfooâ used but never defined
[hjl@gnu-6 tmp]$

How can we solve this?

-- 
H.J.