Re: [RFC][PATCH][X86_64] Eliminate PLT stubs for specified external functions via -fno-plt=

[Pedro Alves <palves@redhat.com>]

On 05/21/2015 11:02 PM, Sriraman Tallam wrote:
> On Thu, May 21, 2015 at 2:51 PM, Pedro Alves <palves@redhat.com> wrote:
>> On 05/21/2015 10:12 PM, Sriraman Tallam wrote:
>>>
>>> My original proposal, for x86_64 only, was to add
>>> -fno-plt=<function-name>. This lets the user decide for which
>>> functions PLT must be avoided.  Let the compiler always generate an
>>> indirect call using call *func@GOTPCREL(%rip).  We could do this for
>>> non-PIC code too.  No need for linker fixups since this relies on the
>>> user to know that func is from a shared object.
>>
>> Having to pass function names on the command line seems like an odd
>> interface.  E.g, you'll need to pass the mangled name for
>> C++ functions.  Any reason this isn't a function attribute?
> 
> It is not clear to me where I would stick the attribute.  Example
> usage in foo.cc:
> 
> #include<string.h>
> 
> int main() {
>   int n = memcmp(....);
> }
> 
> I want memcmp to not go through PLT, do you propose explicitly
> re-declaring it in foo.cc with the attribute?

I guess you'd do:

#include<string.h>

__attribute__((no_plt)) typeof (memcpy) memcpy;

int main() {
  int n = memcmp(....);
}

or even:

#include<string.h>

int main() {
  if (hotpath) {
    __attribute__((no_plt)) typeof (memcpy) memcpy;
    for (..) {
      int n = memcmp(....);
    }
  } else {
      int n = memcmp(....);
  }
}

or globally:

$ cat no-plt/string.h:
#include_next <string.h>
__attribute__((no_plt)) typeof (memcpy) memcpy;

$ gcc -I no-plt/ ...

Thanks,
Pedro Alves