[[clang::musttail]] unnecessarily errors when tail-calling non-member function from member function

[michael-kenzel]

The following example fails to compile:

struct A
{
    void fun(int x);
};

void blub(A*, int);

void A::fun(int x)
{
    [[clang::musttail]] return blub(this, x);  // error: non-static member function cannot perform a tail call to non-member function 'blub'
}

This is, however, unnecessary. On many targets, a tail call would be perfectly possible here. In fact, clang will happily generate a tail call if the attribute is removed: https://godbolt.org/z/xKbcf71EM. gcc accepts this example: https://godbolt.org/z/d3f6b1b5c.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Michael Kenzel (michael-kenzel)

The following example fails to compile: ```cpp struct A { void fun(int x); };

void blub(A*, int);

void A::fun(int x)
{
[[clang::musttail]] return blub(this, x); // error: non-static member function cannot perform a tail call to non-member function 'blub'
}

This is, however, unnecessary. On many targets, a tail call would be perfectly possible here. In fact, clang will happily generate a tail call if the attribute is removed: https://godbolt.org/z/xKbcf71EM. gcc accepts this example: https://godbolt.org/z/d3f6b1b5c.

</details>

[pinskia]

Note the way GCC handles musttail is NOT in the front-ends but rather in the middle-end so it will accept more cases that clang will reject.

For an example even this:

void f(int a, int b);
void g(int a, int b, int c)
{
  [[clang::musttail]] return f(a, b);
}

[shafik]

Adding to the previous comment based on the commit: 8344675 it seems like we need to be more conservative b/c we can't know on the frontend.

CC @erichkeane

[erichkeane]

@haberman : should probably help us here as the implementer :)

I don't understand the necessity of the diagnostic there, why we think a non-member function can't be tail-called into a member function.

I'm hoping this is just a case of us being overly conservative and a rule we can loosen in some situations. Perhaps the review https://reviews.llvm.org/D99517 mentions it in discussion? Its a little long for me to review right now, but is probably worth looking into to figure out the reasoning for this limit.