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?
