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Re: PATCH: Add ifunc attribute


Richard Guenther wrote:
On Tue, Jun 23, 2009 at 7:19 PM, H.J. Lu<hjl.tools@gmail.com> wrote:
Hi,

I opened a bug report:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40528

with the third proposal:

3. Ifunc attribute with prototype, but without argument:

int
__attribute__ ((ifunc))
foo (...)
{
 <return address of foo_{1,2,3}>
}

We can call foo in the same file. Gcc checks the return value for
foo.

Any comments?

I don't understand this third proposal.

"int foo (...)" is only declared as a prototype and with the ifunc attribute dropped. The body of the function is actually compiled to .ifunc.foo or something like that, with a signature


__typeof(foo) __attribute__ ((ifunc)) .ifunc.foo(void)

and with ".gnu.indirect.function .ifunc.foo" emitted in the assembly instead of ".function".

Via some magic mechanism, a further redeclaration of foo is prohibited.

Looks a bit too like DWIM and too little like a specification. I think I prefer option 1, in some cases you may want to call the trampoline and having it accessible can help. With

__typeof (foo) *
__attribute__ ((ifunc ("foo")))
foo_ifunc (void)
{
  <return address of foo_{1,2,3}>
}

you could call foo_ifunc if you wish in principle, and the header file only has the prototype of foo as expected.

Paolo


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