On Fri, May 22, 2015 at 2:00 AM, Pedro Alves <pal...@redhat.com> wrote:
> On 05/21/2015 11:02 PM, Sriraman Tallam wrote:
>> On Thu, May 21, 2015 at 2:51 PM, Pedro Alves <pal...@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/ ...
That looks good, thanks.
Sri
>
> Thanks,
> Pedro Alves
>
