Re: why C++ cannot alias an inline function, C can ?

[Jason Vas Dias <jason dot vas dot dias at gmail dot com>]

On 01/04/2018, Max Filippov <jcmvbkbc@gmail.com> wrote:
> On Sun, Apr 1, 2018 at 4:34 AM, Jason Vas Dias <jason.vas.dias@gmail.com>
> wrote:
>> In C++ however, it does not compile :
>>
>>   $ g++ -g -std=gnu++11 -x c++ -c t.c
>> t.c:5:8: error: 'void bar()' aliased to undefined symbol 'foo'
>>    void bar(void);
>>         ^
>>
>> Even though, if I compile the object with the declaration
>> of bar() commented out , I can see C++ is not mangling
>> the name "foo" :
>


> gcc manual says the follwing about the alias attribute:
>  "In C++, the mangled name for the target must be used."
>
> so with the following modification your example compiles:
>
>      static inline __attribute__((always_inline,alias("_ZL3foov")))
>      void bar(void);
>
> --
> Thanks.
> -- Max
>


Aha!  But how to determine the mangled name beforehand ?

Even if I compile the object without the alias, then inspect
the object with objdump, there is no mangled symbol
_ZL3foov defined in the object file .

So I have to form the mangled name string beforehand,
even for symbols for which there is no linkage ?

ie. there should never be any symbol table entry for 'foo' -
only in the debuginfo there will be information about it .

So I must run some name mangler /  generator as a
pre-processing step to get the correct mangled name
string ?

Like, if I compile just :

    static inline __attribute__((always_inline))
    void foo(void) {}
    void f(void)
    { foo();
    }

Then both
   $ nm -a t.o | grep  ZL3foov
 and
   $ objdump -t t.o | grep ZL3foov
 and
   $ objdump -g t.o | grep ZL3foov

produce no output. So how does one determine that mangled name
a-priori ? There seems to be no way of getting cpp to do it, anyway ,
which is what it would have to do to be an acceptable solution -
there must be a string literal constant argument to '__attribute__(alias()))'.

I think some kind of new pragma is needed , to allow one to say
something like:

#define _S(_X) #X
#define QUOTED_STRING(_X) _S(_X)

static inline __attribute__((always_inline, alias(
QUOTED_STRING(
#pragma symbol_name(foo)
// insert mangled symbol name here
)
)));

I'm going to investigate patching GCC to support
something like that.

I think it is unacceptable to be forced to used
mangled symbol names for objects which are
not symbols (pure inline functions) .

Thanks & Regards,
Jason

))
         void bar(void) ;