[Bug c++/109047] New: Harmonize __attribute__((target_clones)) requirement in function prototype

["hbucher at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109047

            Bug ID: 109047
           Summary: Harmonize __attribute__((target_clones)) requirement
                    in function prototype
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hbucher at gmail dot com
  Target Milestone: ---

Let's say I have a library that I want to share with multiple targets
```c++
#include <array>
#include <cstdio>

using Vector = std::array<float, 2>;
using Matrix = std::array<float, 4>;

__attribute__((target_clones("default","arch=core2","arch=znver2")))
Vector multiply(const Matrix& m, const Vector& v) {
    Vector r;
    r[0] = v[0] * m[0] + v[1] * m[2];
    r[1] = v[0] * m[1] + v[1] * m[3];
    return r;
}
```
and I want to use that as below
```c++
#include <array>
#include <cstdio>

using Vector = std::array<float, 2>;
using Matrix = std::array<float, 4>;

Vector multiply(const Matrix& m, const Vector& v);

int main() {
    Matrix m{1,2,3,4};
    Vector v{1,2};
    Vector r = multiply(m,v);
    printf( "%f %f\n", r[0], r[1] );
}
```
Godbolt project: https://godbolt.org/z/3hd4MrzsG

GCC will be happy to compile and link the above two files together but clang++
will complain about undefined references. 
```
ld: CMakeFiles/example.dir/example.cpp.o: in function `main':
example.cpp:(.text+0x2a): undefined reference to `multiply(std::array<float,
4ul> const&, std::array<float, 2ul> const&)'
```
To make this work, I have to add all the targets in the prototype as well as in 
```
#include <array>
#include <cstdio>

using Vector = std::array<float, 2>;
using Matrix = std::array<float, 4>;

__attribute__((target_clones("default","arch=core2","arch=znver2")))
Vector multiply(const Matrix& m, const Vector& v);

int main() {
    Matrix m{1,2,3,4};
    Vector v{1,2};
    Vector r = multiply(m,v);
    printf( "%f %f\n", r[0], r[1] );
}
```
Looking at the object files generated, it seeems that the main difference is
that GCC generates an indirect link for the naked prototype while clang
generate an indirect link to .ifunc and that requires knowledge of the target
attributes. 
```
$ diff nm.gcc nm.clang
3,4d2
<                 U _GLOBAL_OFFSET_TABLE_
<                 U __stack_chk_fail
6,13c4,11
< i multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&)
< t multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&)
[clone .arch_cascadelake.3]
< t _Z8multiplyRKSt5arrayIfLm4EERKS_IfLm2EE.arch_core2.0
< t multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&)
[clone .arch_haswell.2]
< t multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&)
[clone .arch_sandybridge.1]
< t _Z8multiplyRKSt5arrayIfLm4EERKS_IfLm2EE.arch_znver1.4
< t _Z8multiplyRKSt5arrayIfLm4EERKS_IfLm2EE.arch_znver2.5
< t multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&)
[clone .default.6]
---
> T multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) [clone .arch_cascadelake.3]
> T _Z8multiplyRKSt5arrayIfLm4EERKS_IfLm2EE.arch_core2.0
> T multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) [clone .arch_haswell.2]
> T multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) [clone .arch_sandybridge.1]
> T _Z8multiplyRKSt5arrayIfLm4EERKS_IfLm2EE.arch_znver1.4
> T _Z8multiplyRKSt5arrayIfLm4EERKS_IfLm2EE.arch_znver2.5
> T multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) [clone .default.6]
> i multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) [clone .ifunc]
20d17
< r std::piecewise_construct
```
is this behavior by design?