[Bug middle-end/115606] New: return slot opt prevents tail calls

[andi-gcc at firstfloor dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115606

            Bug ID: 115606
           Summary: return slot opt prevents tail calls
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andi-gcc at firstfloor dot org
  Target Milestone: ---

For the following test case 

class Foo {
public:
  int a, b;
  Foo(int a, int b) : a(a), b(b) {}
};

Foo __attribute__((noinline,noclone,noipa))
callee (int i)
{
  return Foo(i, i+1);
}

Foo __attribute__((noinline,noclone,noipa))
caller (int i)
{
  return callee (i + 1);
}

x86 can sibling call the callee call, but ARM can't.

The difference seems to be that ARM ends up with 

  <retval> = callee (_1); [return slot optimization]

while x86_64 just has

  D.2926 = callee (_1);

and then in tree-tailcall this test fails with return slot optimization:

  tree result_decl = DECL_RESULT (cfun->decl);
  if (result_decl
      && may_be_aliased (result_decl)
      && ref_maybe_used_by_stmt_p (call, result_decl, false))

resulting in no sibling call.

This causes a test suite failure in the musttail patchkit, but only on ARM.

Of course the test could be disabled on ARM but I wonder if there is a missing
optimization bug here. With more musttail usage it might be a bigger problem if
not even simple structures are supported.

I'm not sure:
- why return slot opt at the gimple level is target specific?
- can tree tailcall handle this case?