Re: User directed Function Multiversioning via Function Overloading (issue5752064)

[Sriraman Tallam <tmsriram at google dot com>]

On Fri, Apr 27, 2012 at 7:38 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Fri, Apr 27, 2012 at 7:35 AM, Sriraman Tallam <tmsriram@google.com> wrote:
>> On Fri, Apr 27, 2012 at 6:38 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Thu, Apr 26, 2012 at 10:08 PM, Sriraman Tallam <tmsriram@google.com> wrote:
>>>> Hi,
>>>>
>>>> ? I have made the following changes in this new patch which is attached:
>>>>
>>>> * Use target attribute itself to create function versions.
>>>> * Handle any number of ISA names and arch= ?args to target attribute,
>>>> generating the right dispatchers.
>>>> * Integrate with the CPU runtime detection checked in this week.
>>>> * Overload resolution: If the caller's target matches any of the
>>>> version function's target, then a direct call to the version is
>>>> generated, no need to go through the dispatching.
>>>>
>>>> Patch also available for review here:
>>>> http://codereview.appspot.com/5752064
>>>>
>>>
>>> Does it work with
>>>
>>> int foo ();
>>> int foo () __attribute__ ((targetv("arch=corei7")));
>>>
>>> int (*foo_p) () = foo?
>>
>> Yes, this will work. foo_p will be the address of the dispatcher
>> function and hence doing (*foo_p)() will call the right version.
>
> Even when foo_p is a global variable and compiled with -fPIC?

I am not sure I understand what the complication is here, but FWIW, I
tried this example and it works

int foo ()
{
 return 0;
}

int  __attribute__ ((target ("arch=corei7)))
foo ()
{
 return 1;
}

int (*foo_p)() = foo;
int main ()
{
 return (*foo_p)();
}

g++ -fPIC -O2 example.cc


Did you have something else in mind? Could you please elaborate if you
a have a particular case in mind.

The way I handle function pointers is straightforward. When the
front-end sees a pointer to a function that is versioned, it returns
the pointer to the dispatcher instead.

Thanks,
-Sri.

>
> Thanks.
>
> --
> H.J.