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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/28831] [8/9/10/11 Regression] Aggregate copy not
 elided when using a return value as a pass-by-value parameter
Date: Mon, 22 Mar 2021 11:05:32 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D28831

--- Comment #36 from Richard Biener <rguenth at gcc dot gnu.org> ---
Note that if we'd want to "preallocate" (or re-use) variables for argument
slots we have to properly arrange them according to the ABI.  Consider
a function taking more than just a single argument [passed on stack] which
we might still able to handle optimally [with greater difficulty].

Jasons C++ example expands from

  D.2368 =3D {};
  D.2368.i[0] =3D 1;
  f (D.2368);

if you modify that to

struct A { int i[100]; };
void f(int i, struct A, int j);
int main()
{
  f(0, (struct A){1}, 2);
}

then on i?86 both extra arguments are passed on the stack as well but
RTL expansion sees again

  D.2370 =3D {};
  D.2370.i[0] =3D 1;
  f (0, D.2370, 2);

but clearly we cannot just allocate D.2370 in a random place if it is
passed on the stack (on i?86 it's passed by reference).  For example

struct A { int i[2]; };
void f(struct A);
int main()
{
  f((struct A){1});
}

expanded as

  MEM <char[4]> [(struct A *)&D.1960 + 4B] =3D {};
  D.1960.i[0] =3D 1;
  f (D.1960);

is passed on the stack though.  So if we want to expose anything on GIMPLE
then we have to make the argument space on the stack distinct from random
stack variables.=